llvm dev - Feb 2019 - [RFC] Adding thread group semantics to LangRef (motivated by GPUs)

On Wed, Jan 30, 2019 at 4:20 PM Jan Sjodin <jan_sjodin at yahoo.com>
wrote:
>
> >
> > > > for (int i = 0; i < 2; i++) {
> > > >  foo = ballot(true); // ballot 1
> > > >
> > > >    if (threadID /* ID of the thread within a wavefront/warp
*/ % 2
> == 0) continue;
> > > >
> > > >    bar = ballot(true); // ballot 2
> > > > }
> > > >
> > > > versus:
> > > >
> > > > int i = 0;
> > > > while (true) {
> > > >    do {
> > > >        if (i == 2) break;
> > > >        foo = ballot(true); // ballot 1
> > > >        i++;
> > > >    } while (threadID % 2 == 0);
> > > >
> > > >    if (i == 2) break;
> > > >    bar = ballot(true); // ballot 2
> > > >    i++;
> > > > }
> > >
> > > I think you can remove the second "i++", otherwise we
can increment
> "i" twice
> > > if threadID % 2 != 0, but I see the issue. Using the equivalence
> classes would
> > > prevent this transform, since we have re-arranged the control
flow in
> that way,
> >
> > What do you mean? Note that the transforms that lead from the first
> > example to the second are actually desirable if there aren't any
> > ballots or other convergent operations, so banning them entirely is a
> > no-go. Otherwise, note that the ballot here can be nested arbitrarily
> > deep, which means that jump forwarding/tail duplication has to be
> > aware of the entire loop unless we have some kind of merge intrinsic.
>
> Yes, if there weren't any calls to a ballot, then the transform would
> be legal. What I was saying in the beginning was that ballot() would
> have some special rules attached to it. It is of course undesirable to
> have flags to enforce correctness, but that is the point we are at
> right now.
>
> > Also, I should say that while interesting, control equivalence classes
> > can be part of the *implementation*, but not the *specification*. We
> > need
> > to define the semantics of the IR -- that is, how a program
> > compiled from any given IR is allowed to do when executed (and *not*
> > what it looks like when compiled) -- *first*, and then which
> > transforms are allowed/not allowed will fall out of that. We can't
> > start by listing disallowed transforms, because then when someone
> > comes along and writes a new optimization, it might be technically
> > "allowed" even though it breaks something in practice.
>
> I think you misunderstand me if you think I was listing disallowed
> transforms. My question was if it is possible to have multi-threaded
> semantics in the source language, but in the compiler have a
> single-threaded view, where some properties of the CFG would determine
> what is legal and not for some functions with a special flag.
>
But that's practically the same as listing allowed and disallowed
transforms -- you're defining what the final IR is allowed to look like,
not how it's allowed to execute. If at all possible, the former should be
derived from the latter.

> I agree
> it is more desirable to have the the semantics specified in the
> IR. However, I am exploring this from a practical point of view, since
> ballot() is very rare compared to all code that is being compiled for
> all targets. These intrinsics would always have to be considered when
> writing a pass. They seem to me harder to think about, and
> test, for someone who is working on a single-threaded target, compared
> to looking at a flag. If we allow ourselves to look at which
> transforms that might violate these properties, we could would free up
> the rest of the compiler (and developers) to not have to worry about
> these things. Intrinsics would have to be maintained throughout the
> entire compilation process in every pass.
>
> > The only way to
> > conclusively prove that transforms will never break code that's
> > supposed to work (except for bugs in the transforms) is to define the
> > semantics of the IR, and then to make sure that it refines the
> > semantics of the source language. This is why the LangRef almost never
> > talks about allowed/disallowed transforms except as examples to
> > explain some given semantics, and if you don't follow that rule,
your
> > patch will probably be rejected.
>
> I'm trying to figure out if we are in the "almost never"
territory
> here or not.
>
I don't think so at all. In addition to being much, much harder to reason
about and prove that the approach is sound, I think it'll be more intrusive.

>
> > Now, to define a semantics for ballot(), we need to define what
it's
> > allowed to return for any given execution of any given program, and to
> > do that, we need to define which threads must be active together when
> > it's reached, which in turns means we need to define when control
flow
> > can re-converge somehow. Any proposal for how to handle ballot() must
> > start here.
>
> > > I'm not sure if using these rules will be easier or harder
than
> dealing with
> > > intrinsics. One problem is that the rules for single-threaded
code
> might seem
> > > arbitrary, and it would be hard to reason about them in a larger
> context.
>
> > > What happens to new control flow created by transforms, and what
will
> guide
> > > the insertion of intrinsics in the new code? Code structurization
is
> one example
> > > were this could happen.
>
> > Hopefully, it's clear from the above how this all falls out. The
> > frontend for e.g. GLSL or SPIR-V would have to insert the merge
> > intrinsics to preserve the semantics of the source language. Any
> > transforms must refine the semantics of the IR, although I can't
think
> > of a scenario where that would involve emitting any new merge
> > intrinsics. Usually, structurized IR's have their own semantics
about
> > when control flow converges, so a code structurizer should respect the
> > original semantics. AMDGPU has its own code structurizer that runs
> > very late in the pipeline (although there are plans to remove it), so
> > we might have to change that to make it respect the merge intrinsic
> > intrinsics, and then we'll correctly implement them "for
free".
>
> Any transform that re-arranges control flow would potentially have to
> know about the properties of ballot(), and the rules with respect to
> the CFG (and maybe consider the target) to know where to insert the
> intrinsics.
>
But the same is true for basically any approach to handling this. In fact,
adding the merge intrinsics makes this much easier. Beyond the usual
problems with hoisting ballots, which passes currently avoid via the
current convergent + speculatable attributes, we'd only have to add
awareness to passes that they can't duplicate/combine merge intrinsics or
move them past convergent intrinsics, which is a local property and hence
easily checkable. In example I explained, without some kind of merge
intrinsic, tail duplication has to look at the entire loop to know whether
the transform is legal. Of course, you could have some kind of "no
convergent calls" flag on a function, but that doesn't eliminate the
nastyness when there are convergent calls.

> I had the impression that the control flow convergence was
> in part specified by what the target architecture can handle.
>
This is true, although hopefully we can agree on something that everyone
can implement.

> One of
> the more complicated cases would be linearization where the control
> flow is completely rewritten, and is encoded in a variable that says
> which basic block is the next one to execute.
>
It's certainly interesting to think about how to maintain correctness in
the face of ballots() with such a pass, but a) it's certainly no harder
with merge intrinsics than merges being implicit and b) I doubt that's
useful for anything you'd want to do with a GPU.

> Another case is DCE,
> where a ballot() could be eliminated, and it would potentially have to
> remove a number of intrinsics to enable later optimizations (unless it
> would affect performance?), so the intrinsics will require some
> non-local updates.
>
Removing merge intrinsics if it's profitable and allowed is a separate
optimization, that can be done relatively quickly in a single pass without
impacting the performance of other optimizations. It's requiring expensive
non-local checks in optimizations which modify control flow that's a
problem.

>
> > > > Would they only be present if ballot and similar functions
are used,
> or do they
> > > > have to be present everywhere?
> > >
> > > They'd only have to be present when ballot or other
convergent
> > > functions are called, since otherwise it doesn't matter when
control
> > flow re-converges. However, we may want to keep them around for
> > performance reasons (usually destroying convergence points is bad for
> > performance).
>
> So we might have them without a ballot(), which would seem to make it
> more difficult for structurizers or other transforms to maintain the
> semantics and insert intrinsics.
>
It's not any more difficult code-wise, since the case where there is a
ballot needs to be handled anyways. And while it might take longer to
process stuff when this hypothetical pass encounters a merge intrinsic (I
can't think of a real-world case where it would matter), it should result
in better code generated.

>
> > > How are uniform branches handled? Do they affect the convergence
model?
> > >
> > We may be able to remove convergence points if branches are uniform.
> > In Nicolai's proposal, I believe we'd want to remove a merge
intrinsic
> > when all the branches that it post-dominates that aren't also
> > post-dominated by some other merge intrinsic are uniform.
>
> I couldn't quite understand the last sentence, but I assume the
> conditions would prevent removing convergence points that help
> performance. Post-domination might not be adequate if there are loops
> btw.
>
It should be -- Nicolai's proposal is that a merge intrinsic merges all the
divergences caused by branches post dominated by the intrinsic, so if all
the divergent branches are merged by some other intrinsic earlier, then
there's no divergence and the merge intrinsic is a no-op.

>
> - Jan
>
>
>
> On Wednesday, January 30, 2019, 6:29:52 AM EST, Connor Abbott <
> cwabbott0 at gmail.com> wrote:
>
>
> On Mon, Jan 28, 2019 at 9:09 PM Jan Sjodin <jan_sjodin at yahoo.com>
wrote:
> >
> > > for (int i = 0; i < 2; i++) {
> > >  foo = ballot(true); // ballot 1
> > >
> > >    if (threadID /* ID of the thread within a wavefront/warp */ %
2 => 0) continue;
> > >
> > >    bar = ballot(true); // ballot 2
> > > }
> > >
> > > versus:
> > >
> > > int i = 0;
> > > while (true) {
> > >    do {
> > >        if (i == 2) break;
> > >        foo = ballot(true); // ballot 1
> > >        i++;
> > >    } while (threadID % 2 == 0);
> > >
> > >    if (i == 2) break;
> > >    bar = ballot(true); // ballot 2
> > >    i++;
> > > }
> >
> > I think you can remove the second "i++", otherwise we can
increment "i"
> twice
> > if threadID % 2 != 0, but I see the issue. Using the equivalence
classes
> would
> > prevent this transform, since we have re-arranged the control flow in
> that way,
>
> What do you mean? Note that the transforms that lead from the first
> example to the second are actually desirable if there aren't any
> ballots or other convergent operations, so banning them entirely is a
> no-go. Otherwise, note that the ballot here can be nested arbitrarily
> deep, which means that jump forwarding/tail duplication has to be
> aware of the entire loop unless we have some kind of merge intrinsic.
>
> Also, I should say that while interesting, control equivalence classes
> can be part of the *implementation*, but not the *specification*. We
> need to define the semantics of the IR -- that is, how a program
> compiled from any given IR is allowed to do when executed (and *not*
> what it looks like when compiled) -- *first*, and then which
> transforms are allowed/not allowed will fall out of that. We can't
> start by listing disallowed transforms, because then when someone
> comes along and writes a new optimization, it might be technically
> "allowed" even though it breaks something in practice. The only
way to
> conclusively prove that transforms will never break code that's
> supposed to work (except for bugs in the transforms) is to define the
> semantics of the IR, and then to make sure that it refines the
> semantics of the source language. This is why the LangRef almost never
> talks about allowed/disallowed transforms except as examples to
> explain some given semantics, and if you don't follow that rule, your
> patch will probably be rejected.
>
> Now, to define a semantics for ballot(), we need to define what it's
> allowed to return for any given execution of any given program, and to
> do that, we need to define which threads must be active together when
> it's reached, which in turns means we need to define when control flow
> can re-converge somehow. Any proposal for how to handle ballot() must
> start here.
>
> > I'm not sure if using these rules will be easier or harder than
dealing
> with
> > intrinsics. One problem is that the rules for single-threaded code
might
> seem
> > arbitrary, and it would be hard to reason about them in a larger
context.
> >
> > > Nicolai's proposal solves this by having the frontend emit a
merge
> intrinsic
> > > before the i++ is emitted. This prevents the jump forwarding from
> occurring.
> >
> > I was thinking about getting through the single-thread view and the
> issues with
> > that first, then I will think more about the multi-thread and explicit
> convergence.
> >
> > If we are done with the single-thread stuff for now these are the
> question that I
> > have been thinking about with the multi-threaded view:
> >
> > What happens to new control flow created by transforms, and what will
> guide
> > the insertion of intrinsics in the new code? Code structurization is
one
> example
> > were this could happen.
>
> Hopefully, it's clear from the above how this all falls out. The
> frontend for e.g. GLSL or SPIR-V would have to insert the merge
> intrinsics to preserve the semantics of the source language. Any
> transforms must refine the semantics of the IR, although I can't think
> of a scenario where that would involve emitting any new merge
> intrinsics. Usually, structurized IR's have their own semantics about
> when control flow converges, so a code structurizer should respect the
> original semantics. AMDGPU has its own code structurizer that runs
> very late in the pipeline (although there are plans to remove it), so
> we might have to change that to make it respect the merge intrinsic
> intrinsics, and then we'll correctly implement them "for
free".
>
> >
> > Would they only be present if ballot and similar functions are used,
or
> do they
> > have to be present everywhere?
>
> They'd only have to be present when ballot or other convergent
> functions are called, since otherwise it doesn't matter when control
> flow re-converges. However, we may want to keep them around for
> performance reasons (usually destroying convergence points is bad for
> performance).
>
> >
> > How are uniform branches handled? Do they affect the convergence
model?
>
> We may be able to remove convergence points if branches are uniform.
> In Nicolai's proposal, I believe we'd want to remove a merge
intrinsic
> when all the branches that it post-dominates that aren't also
> post-dominated by some other merge intrinsic are uniform.
>
>
>
> >
> >
> > - Jan
> >
> >
> > On Monday, January 28, 2019, 11:16:36 AM EST, Connor Abbott <
> cwabbott0 at gmail.com> wrote:
> >
> >
> >
> > On Fri, Jan 25, 2019 at 3:05 AM Jan Sjodin <jan_sjodin at
yahoo.com> wrote:
> > >
> > > > for (...) {
> > > >    ballot();
> > > >    if (... /* non-uniform */) continue;
> > > > }
> > > >
> > > > into
> > > >
> > > > for (...) {
> > > >    do {
> > > >        ballot();
> > > >    } while (... /* non-uniform */);
> > > > }
> > >
> > > I'm not sure if I follow this example, could you and explain
a bit
> more?
> > > It looks to me that the condition in the "if" must be
false (if the
> > > same condition is used in the while), or we would
> > > call ballot the wrong number of times.
> >
> > Yes, the idea is that the same condition is used in the if and the
> do-while. I think I messed up the example a little... in the second
> snippet, we're supposed to break out of the inner loop if the outer
loop's
> exit condition is true. Here's a more concrete example:
> >
> > for (int i = 0; i < 2; i++) {
> >    foo = ballot(true); // ballot 1
> >
> >    if (threadID /* ID of the thread within a wavefront/warp */ % 2 ==
0)
> continue;
> >
> >    bar = ballot(true); // ballot 2
> > }
> >
> > versus:
> >
> > int i = 0;
> > while (true) {
> >    do {
> >        if (i == 2) break;
> >        foo = ballot(true); // ballot 1
> >        i++;
> >    } while (threadID % 2 == 0);
> >
> >    if (i == 2) break;
> >    bar = ballot(true); // ballot 2
> >    i++;
> > }
> >
> > From a single-threaded perspective, these two snippets are identical,
> even if ballot() writes to arbitrary memory. The first can easily get
> transformed to something like the second when LLVM decides to duplicate the
> final i++ through jump forwarding, and then re-interprets the loop as two
> nested loops and splits the loop header in two. This is what currently
> happens with DOOM when we try to enable subgroup operations with it.
Let's
> say there are two threads in a wavefront. Then the execution trace mandated
> by SPIR-V for the first looks like:
> >
> > thread 0        | thread 1
> > ballot 1 = 0b11 | ballot 1 = 0b11
> > skipped        | ballot 2 = 0b10
> > ballot 1 = 0b11 | ballot 1 = 0b11
> > skipped        | ballot 2 = 0b10
> >
> > Now, contrast this with the execution trace that programmers would
> expect for the second example:
> >
> > thread 0        | thread 1
> > ballot 1 = 0b11 | ballot 1 = 0b11
> > ballot 1 = 0b01 | skipped
> > skipped        | ballot 2 = 0b10
> > skipped        | ballot 1 = 0b10
> > skipped        | ballot 2 = 0b10
> >
> > Nicolai's proposal solves this by having the frontend emit a merge
> intrinsic before the i++ is emitted. This prevents the jump forwarding from
> occurring.
> >
> >
> > >
> > > About the CSE, when would that be legal? I can imagine with
uniform
> > > branches that it could work, but would like to see an example to
> > > fully understand this.
> > >
> > > I agree that it would be more conservative than if we model the
> threading,
> > > but I'm not sure about the cost/benefit. I am mostly curious
if it is
> > > possible to have a single-thread view or not. Then we would have
to
> > > see if it is adequate.
> > >
> > > - Jan
> > >
> > > On Thursday, January 24, 2019, 10:31:47 AM EST, Connor Abbott
<
> cwabbott0 at gmail.com> wrote:
> > >
> > >
> > > I don't see how this would fix the continue vs. nested loop
problem I
> > > explained earlier. That is, how would this prevent turning:
> > >
> > > for (...) {
> > >    ballot();
> > >    if (... /* non-uniform */) continue;
> > > }
> > >
> > > into
> > >
> > > for (...) {
> > >    do {
> > >        ballot();
> > >    } while (... /* non-uniform */);
> > > }
> > >
> > > and vice versa? Note that there's no duplication going on
here, and
> > > the single-threaded flow of control is exactly the same.
> > >
> > > Another reason this isn't so great is that it prevents e.g.
CSE on
> > > ballots that actually should be combined, since you're
modelling it as
> > > a write. It seems like the optimizer is going to need some
special
> > > knowledge of convergent things that fake memory constraints
can't give
> > > us.
> > >
> > > On Thu, Jan 24, 2019 at 4:06 PM Jan Sjodin <jan_sjodin at
yahoo.com>
> wrote:
> > > >
> > > >
> > > > I was looking into ballot() and how if it is possible to
keep a
> single-threaded
> > > > view of the code, but add some extra conditions that must
hold after
> the
> > > > transformation. I had the initial idea that each call to
ballot() in
> a
> > > > single-threaded program can be seen as a partial write to a
memory
> > > > location, and each location memory location is unique for
every call
> site,
> > > > plus there some externally observable side effect. We can
abstract
> this
> > > > away by tagging the calls, e.g. by using aliases.
> > > >
> > > > For example:
> > > >
> > > > if (...) {
> > > >      foo1 = ballot();
> > > > } else {
> > > >      foo2 = ballot();
> > > > }
> > > >
> > > > simply becomes:
> > > >
> > > > if (...) {
> > > >      foo1 = ballot_1();
> > > > } else {
> > > >      foo2 = ballot_2();
> > > > }
> > > >
> > > >
> > > > and
> > > >
> > > > if (...) {
> > > > } else {
> > > > }
> > > > ballot();
> > > >
> > > > becomes
> > > >
> > > > if (...) {
> > > > } else {
> > > > }
> > > > ballot_1();
> > > >
> > > > In the first case it would prevent combining the two calls
into one
> > > > after the if. In the second example there is generally
nothing that
> > > > says it could not be transformed into the first example with
two
> > > > calls to ballot_1(), which should not be allowed.
> > > >
> > > > Another form of duplication that we must allow are loop
transforms,
> > > > like unrolling or peeling. These might seem similar to the
example
> > > > above, since we are cloning code and with conditions etc.
But
> > > > they are different since they calls are in different loop
iterations.
> > > >
> > > > The condition that needs to be met is that:
> > > >
> > > > There must be a single path between all cloned ballot_n()
functions.
> > > >
> > > > The reason for this condition is that if we clone the same
call, then
> > > > the copies must be mutually exclusive, but if they are
cloned from
> > > > a loop, there must be a path, or we would skip iterations.
> > > >
> > > > If we want to be more sophisticated we can add:
> > > >
> > > > If there is no such path, the calls must be separated by
uniform
> branches.
> > > >
> > > > After the transform everything should be re-tagged, since we
already
> > > > checked the calls and we don't want to check them again.
Also, not
> all
> > > > transforms need (or should) have the tagging checked. One
example is
> > > > inlining, where multiple copies are created, but they are
clearly
> different
> > > > calls. The tagging can be done temporarily for a single
pass, and
> then
> > > > eliminated. This could be a good tool for debugging as well,
since
> it can
> > > > detect if a transform is suspect.
> > > >
> > > > The code would of course have to make sense as far as
control flow.
> If
> > > > we have:
> > > >
> > > > for(;;) {
> > > >    if(...) {
> > > >      ballot();
> > > >      break;
> > > >    }
> > > > }
> > > >
> > > > This would have to be translated to:
> > > >
> > > > for(;;) {
> > > >    if(...) {
> > > >      ballot();
> > > >      if(UnknownConstant) {  // Not a uniform condition, but
later on
> translated to "true"
> > > >        break;
> > > >      }
> > > > }
> > > >
> > > > However, I think that this is the way the code is generated
today
> anyway.
> > > > There would have to be some attribute that indicate that
these calls
> (or functions)
> > > > contain ballot or other cross-lane operations so they could
be
> tagged and
> > > > checked. The attribute would be used by the passes to know
that the
> special
> > > > conditions exist for those calls.
> > > >
> > > > As far as what it means to have a path, it could be
complicated.
> > > > For example:
> > > >
> > > > x = ...
> > > > ballot_1();
> > > >
> > > > could be transformed to:
> > > >
> > > > if (x < 4711) {
> > > >  ballot_1();
> > > >
> > > > if(x >= 4711) {
> > > >  ballot_1();
> > > > }
> > > >
> > > > So a simple path check would say there is a path, and the
transform
> is legal,
> > > > but if we examine the conditions, there is no path, and the
> transform should not be legal.
> > > > It could be made even more obscure of course, but I
don't see any
> optimizations really
> > > > doing this kind of thing,
> > > >
> > > > - Jan
> > > >
> > > > On Saturday, December 29, 2018, 11:32:25 AM EST, Nicolai
Hähnle via
> llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > > >
> > > >
> > > > On 20.12.18 18:03, Connor Abbott wrote:
> > > > >    We already have the notion of "convergent"
functions like
> > > > >    syncthreads(), to which we cannot add control-flow
dependencies.
> > > > >    That is, it's legal to hoist syncthreads out of
an "if", but
> it's
> > > > >    not legal to sink it into an "if". 
It's not clear to me why we
> > > > >    can't have "anticonvergent" (terrible
name) functions which
> cannot
> > > > >    have control-flow dependencies removed from them? 
ballot()
> would be
> > > > >    both convergent and anticonvergent.
> > > > >
> > > > >    Would that solve your problem?
> > > > >
> > > > >
> > > > > I think it's important to note that we already have
such an
> attribute,
> > > > > although with the opposite sense - it's impossible
to remove
> control
> > > > > flow dependencies from a call unless you mark it as
"speculatable".
> > > >
> > > > This isn't actually true. If both sides of an if/else
have the same
> > > > non-speculative function call, it can still be moved out of
control
> flow.
> > > >
> > > > That's because doing so doesn't change anything at
all from a
> > > > single-threaded perspective. Hence why I think we should
model the
> > > > communication between threads honestly.
> > > >
> > > >
> > > > > However, this doesn't prevent
> > > > >
> > > > > if (...) {
> > > > > } else {
> > > > > }
> > > > > foo = ballot();
> > > > >
> > > > > from being turned into
> > > > >
> > > > > if (...) {
> > > > >      foo1 = ballot();
> > > > > } else {
> > > > >      foo2 = ballot();
> > > > > }
> > > > > foo = phi(foo1, foo2)
> > > > >
> > > > > and vice versa. We have a "noduplicate"
attribute which prevents
> > > > > transforming the first into the second, but not the
other way
> around. Of
> > > > > course we could keep going this way and add a
"nocombine"
> attribute to
> > > > > complement noduplicate. But even then, there are even
still
> problematic
> > > > > transforms. For example, take this program, which is
simplified
> from a
> > > > > real game that doesn't work with the AMDGPU
backend:
> > > > >
> > > > > while (cond1 /* uniform */) {
> > > > >      ballot();
> > > > >      ...
> > > > >      if (cond2 /* non-uniform */) continue;
> > > > >      ...
> > > > > }
> > > > >
> > > > > In SPIR-V, when using structured control flow, the
semantics of
> this are
> > > > > pretty clearly defined. In particular, there's a
continue block
> after
> > > > > the body of the loop where control flow re-converges,
and the only
> back
> > > > > edge is from the continue block, so the ballot is in
uniform
> control
> > > > > flow. But LLVM will get rid of the continue block since
it's
> empty, and
> > > > > re-analyze the loop as two nested loops, splitting the
loop header
> in
> > > > > two, producing a CFG which corresponds to this:
> > > > >
> > > > > while (cond1 /* uniform */) {
> > > > >      do {
> > > > >          ballot();
> > > > >          ...
> > > > >      } while (cond2 /* non-uniform */);
> > > > >      ...
> > > > > }
> > > > >
> > > > > Now, in an implementation where control flow
re-converges at the
> > > > > immediate post-dominator, this won't do the right
thing anymore. In
> > > > > order to handle it correctly, you'd effectively
need to always
> flatten
> > > > > nested loops, which will probably be really bad for
performance if
> the
> > > > > programmer actually wanted the second thing. It also
makes it
> impossible
> > > > > when translating a high-level language to LLVM to get
the "natural"
> > > > > behavior which game developers actually expect. This is
exactly
> the sort
> > > > > of "spooky action at a distance" which makes
me think that
> everything
> > > > > we've done so far is really insufficient, and we
need to add an
> explicit
> > > > > notion of control-flow divergence and reconvergence to
the IR. We
> need a
> > > > > way to say that control flow re-converges at the
continue block,
> so that
> > > > > LLVM won't eliminate it, and we can vectorize it
correctly without
> > > > > penalizing cases where it's better for control flow
not to
> re-converge.
> > > >
> > > > Well said!
> > > >
> > > > Cheers,
> > > >
> > > > Nicolai
> > > > --
> > > > Lerne, wie die Welt wirklich ist,
> > > > Aber vergiss niemals, wie sie sein sollte.
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > llvm-dev at lists.llvm.org
> > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> >    I think you misunderstand me if you think I was listing disallowed
> >    transforms. My question was if it is possible to have
multi-threaded
> >    semantics in the source language, but in the compiler have a
> >    single-threaded view, where some properties of the CFG would
determine
> >    what is legal and not for some functions with a special flag.
> 
>
> But that's practically the same as listing allowed and disallowed
> transforms -- you're defining what the final IR is allowed to look
> like, not how it's allowed to execute. If at all possible, the former
> should be derived from the latter.
It does at least say something about transforms that haven't been written
yet, but from that viewpoint, sure you can say that.
> > > The only way to
> > > conclusively prove that transforms will never break code
that's
> > > supposed to work (except for bugs in the transforms) is to define
the
> > > semantics of the IR, and then to make sure that it refines the
> > > semantics of the source language. This is why the LangRef almost
never
> > > talks about allowed/disallowed transforms except as examples to
> > > explain some given semantics, and if you don't follow that
rule, your
> > > patch will probably be rejected.
> >
> > I'm trying to figure out if we are in the "almost never"
territory
> >  here or not.
> > 
> I don't think so at all. In addition to being much, much harder to
> reason about and prove that the approach is sound, I think it'll be
> more intrusive.
It is much harder to come up with rules that are correct, and will not
be too restrictive for sure. I am not convinced that it is more
intrusive since it will only affect some transforms.
> >  Any transform that re-arranges control flow would potentially have to
> >  know about the properties of ballot(), and the rules with respect to
> >  the CFG (and maybe consider the target) to know where to insert the
> >  intrinsics.
> But the same is true for basically any approach to handling this. In
> fact, adding the merge intrinsics makes this much easier. Beyond the
> usual problems with hoisting ballots, which passes currently avoid via
> the current convergent + speculatable attributes, we'd only have to
> add awareness to passes that they can't duplicate/combine merge
> intrinsics or move them past convergent intrinsics, which is a local
> property and hence easily checkable. In example I explained, without
> some kind of merge intrinsic, tail duplication has to look at the
> entire loop to know whether the transform is legal. Of course, you
> could have some kind of "no convergent calls" flag on a function,
but
> that doesn't eliminate the nastyness when there are convergent calls.
We will have to determine if the intrinsics are worth more compared to
scanning the code.
> >   I had the impression that the control flow convergence was
> >   in part specified by what the target architecture can handle.
> This is true, although hopefully we can agree on something that
> everyone can implement.
Yes, hopefully there is an execution model that is works for everyone.
> >  One of
> >  the more complicated cases would be linearization where the control
> >  flow is completely rewritten, and is encoded in a variable that says
> >  which basic block is the next one to execute.
> It's certainly interesting to think about how to maintain correctness
> in the face of ballots() with such a pass, but a) it's certainly no
> harder with merge intrinsics than merges being implicit and b) I doubt
> that's useful for anything you'd want to do with a GPU.
Irreducible control flow has to be handled somehow, and linearization
is the only transform I know of, that will handle everything. I'm not
sure what the execution model says about irreducible control flow.
> >  Another case is DCE,
> >  where a ballot() could be eliminated, and it would potentially have
to
> >  remove a number of intrinsics to enable later optimizations (unless
it
> >  would affect performance?), so the intrinsics will require some
> >  non-local updates.
> Removing merge intrinsics if it's profitable and allowed is a
> separate optimization, that can be done relatively quickly in a single
> pass without impacting the performance of other optimizations. It's
> requiring expensive non-local checks in optimizations which modify
> control flow that's a problem.
There are certainly a lot of tradeoffs. My point was simply that the
intrinsics are not strictly local.
> >  So we might have them without a ballot(), which would seem to make it
> >  more difficult for structurizers or other transforms to maintain the
> >  semantics and insert intrinsics.
> It's not any more difficult code-wise, since the case where there is
> a ballot needs to be handled anyways. And while it might take longer
> to process stuff when this hypothetical pass encounters a merge
> intrinsic (I can't think of a real-world case where it would matter),
> it should result in better code generated.
I was thinking of linearization or something similar, where an
instrinsic by itself might be hard to preserve, compared to looking at
a ballot() and derive where intrinsics should be inserted.
> > > > How are uniform branches handled? Do they affect the
convergence model?
> > > >
> > > We may be able to remove convergence points if branches are
uniform.
> > > In Nicolai's proposal, I believe we'd want to remove a
merge intrinsic
> > > when all the branches that it post-dominates that aren't also
> > > post-dominated by some other merge intrinsic are uniform.
> >
> > I couldn't quite understand the last sentence, but I assume the
> > conditions would prevent removing convergence points that help
> > performance. Post-domination might not be adequate if there are loops
> > btw.
> 
> It should be -- Nicolai's proposal is that a merge intrinsic merges
> all the divergences caused by branches post dominated by the
> intrinsic, so if all the divergent branches are merged by some other
> intrinsic earlier, then there's no divergence and the merge intrinsic
> is a no-op.
Makes sense.

- Jan
    On Wednesday, January 30, 2019, 11:41:29 AM EST, Connor Abbott <cwabbott0
at gmail.com> wrote:
 
 On Wed, Jan 30, 2019 at 4:20 PM Jan Sjodin <jan_sjodin at yahoo.com>
wrote:

 
>
> > > for (int i = 0; i < 2; i++) {
> > >  foo = ballot(true); // ballot 1
> > >
> > >    if (threadID /* ID of the thread within a wavefront/warp */ %
2 == 0) continue;
> > >
> > >    bar = ballot(true); // ballot 2
> > > }
> > >
> > > versus:
> > >
> > > int i = 0;
> > > while (true) {
> > >    do {
> > >        if (i == 2) break;
> > >        foo = ballot(true); // ballot 1
> > >        i++;
> > >    } while (threadID % 2 == 0);
> > >
> > >    if (i == 2) break;
> > >    bar = ballot(true); // ballot 2
> > >    i++;
> > > }
> >
> > I think you can remove the second "i++", otherwise we can
increment "i" twice
> > if threadID % 2 != 0, but I see the issue. Using the equivalence
classes would
> > prevent this transform, since we have re-arranged the control flow in
that way,
>
> What do you mean? Note that the transforms that lead from the first
> example to the second are actually desirable if there aren't any
> ballots or other convergent operations, so banning them entirely is a
> no-go. Otherwise, note that the ballot here can be nested arbitrarily
> deep, which means that jump forwarding/tail duplication has to be
> aware of the entire loop unless we have some kind of merge intrinsic.
Yes, if there weren't any calls to a ballot, then the transform would
be legal. What I was saying in the beginning was that ballot() would
have some special rules attached to it. It is of course undesirable to
have flags to enforce correctness, but that is the point we are at
right now.
> Also, I should say that while interesting, control equivalence classes
> can be part of the *implementation*, but not the *specification*. We
> need
> to define the semantics of the IR -- that is, how a program
> compiled from any given IR is allowed to do when executed (and *not*
> what it looks like when compiled) -- *first*, and then which
> transforms are allowed/not allowed will fall out of that. We can't
> start by listing disallowed transforms, because then when someone
> comes along and writes a new optimization, it might be technically
> "allowed" even though it breaks something in practice.
I think you misunderstand me if you think I was listing disallowed
transforms. My question was if it is possible to have multi-threaded
semantics in the source language, but in the compiler have a
single-threaded view, where some properties of the CFG would determine
what is legal and not for some functions with a special flag.

But that's practically the same as listing allowed and disallowed transforms
-- you're defining what the final IR is allowed to look like, not how
it's allowed to execute. If at all possible, the former should be derived
from the latter.
 
 I agree
it is more desirable to have the the semantics specified in the
IR. However, I am exploring this from a practical point of view, since
ballot() is very rare compared to all code that is being compiled for
all targets. These intrinsics would always have to be considered when
writing a pass. They seem to me harder to think about, and
test, for someone who is working on a single-threaded target, compared
to looking at a flag. If we allow ourselves to look at which
transforms that might violate these properties, we could would free up
the rest of the compiler (and developers) to not have to worry about
these things. Intrinsics would have to be maintained throughout the
entire compilation process in every pass.
> The only way to
> conclusively prove that transforms will never break code that's
> supposed to work (except for bugs in the transforms) is to define the
> semantics of the IR, and then to make sure that it refines the
> semantics of the source language. This is why the LangRef almost never
> talks about allowed/disallowed transforms except as examples to
> explain some given semantics, and if you don't follow that rule, your
> patch will probably be rejected.
I'm trying to figure out if we are in the "almost never" territory
here or not.


I don't think so at all. In addition to being much, much harder to reason
about and prove that the approach is sound, I think it'll be more intrusive.
 
> Now, to define a semantics for ballot(), we need to define what it's
> allowed to return for any given execution of any given program, and to
> do that, we need to define which threads must be active together when
> it's reached, which in turns means we need to define when control flow
> can re-converge somehow. Any proposal for how to handle ballot() must
> start here.
> > I'm not sure if using these rules will be easier or harder than
dealing with
> > intrinsics. One problem is that the rules for single-threaded code
might seem
> > arbitrary, and it would be hard to reason about them in a larger
context.
> > What happens to new control flow created by transforms, and what will
guide
> > the insertion of intrinsics in the new code? Code structurization is
one example
> > were this could happen.
> Hopefully, it's clear from the above how this all falls out. The
> frontend for e.g. GLSL or SPIR-V would have to insert the merge
> intrinsics to preserve the semantics of the source language. Any
> transforms must refine the semantics of the IR, although I can't think
> of a scenario where that would involve emitting any new merge
> intrinsics. Usually, structurized IR's have their own semantics about
> when control flow converges, so a code structurizer should respect the
> original semantics. AMDGPU has its own code structurizer that runs
> very late in the pipeline (although there are plans to remove it), so
> we might have to change that to make it respect the merge intrinsic
> intrinsics, and then we'll correctly implement them "for
free".
Any transform that re-arranges control flow would potentially have to
know about the properties of ballot(), and the rules with respect to
the CFG (and maybe consider the target) to know where to insert the
intrinsics.

But the same is true for basically any approach to handling this. In fact,
adding the merge intrinsics makes this much easier. Beyond the usual problems
with hoisting ballots, which passes currently avoid via the current convergent +
speculatable attributes, we'd only have to add awareness to passes that they
can't duplicate/combine merge intrinsics or move them past convergent
intrinsics, which is a local property and hence easily checkable. In example I
explained, without some kind of merge intrinsic, tail duplication has to look at
the entire loop to know whether the transform is legal. Of course, you could
have some kind of "no convergent calls" flag on a function, but that
doesn't eliminate the nastyness when there are convergent calls. 
 I had the impression that the control flow convergence was
in part specified by what the target architecture can handle.

This is true, although hopefully we can agree on something that everyone can
implement.
 
 One of
the more complicated cases would be linearization where the control
flow is completely rewritten, and is encoded in a variable that says
which basic block is the next one to execute.

It's certainly interesting to think about how to maintain correctness in the
face of ballots() with such a pass, but a) it's certainly no harder with
merge intrinsics than merges being implicit and b) I doubt that's useful for
anything you'd want to do with a GPU.
 
Another case is DCE,
where a ballot() could be eliminated, and it would potentially have to
remove a number of intrinsics to enable later optimizations (unless it
would affect performance?), so the intrinsics will require some
non-local updates.


Removing merge intrinsics if it's profitable and allowed is a separate
optimization, that can be done relatively quickly in a single pass without
impacting the performance of other optimizations. It's requiring expensive
non-local checks in optimizations which modify control flow that's a
problem.
 
> > > Would they only be present if ballot and similar functions are
used, or do they
> > > have to be present everywhere?
> >
> > They'd only have to be present when ballot or other convergent
> > functions are called, since otherwise it doesn't matter when
control
> flow re-converges. However, we may want to keep them around for
> performance reasons (usually destroying convergence points is bad for
> performance).
So we might have them without a ballot(), which would seem to make it
more difficult for structurizers or other transforms to maintain the
semantics and insert intrinsics.


It's not any more difficult code-wise, since the case where there is a
ballot needs to be handled anyways. And while it might take longer to process
stuff when this hypothetical pass encounters a merge intrinsic (I can't
think of a real-world case where it would matter), it should result in better
code generated.
 
> > How are uniform branches handled? Do they affect the convergence
model?
> >
> We may be able to remove convergence points if branches are uniform.
> In Nicolai's proposal, I believe we'd want to remove a merge
intrinsic
> when all the branches that it post-dominates that aren't also
> post-dominated by some other merge intrinsic are uniform.
I couldn't quite understand the last sentence, but I assume the
conditions would prevent removing convergence points that help
performance. Post-domination might not be adequate if there are loops
btw.


It should be -- Nicolai's proposal is that a merge intrinsic merges all the
divergences caused by branches post dominated by the intrinsic, so if all the
divergent branches are merged by some other intrinsic earlier, then there's
no divergence and the merge intrinsic is a no-op.
 

- Jan



    On Wednesday, January 30, 2019, 6:29:52 AM EST, Connor Abbott <cwabbott0
at gmail.com> wrote:
 
 On Mon, Jan 28, 2019 at 9:09 PM Jan Sjodin <jan_sjodin at yahoo.com>
wrote:
>
> > for (int i = 0; i < 2; i++) {
> >  foo = ballot(true); // ballot 1
> >
> >    if (threadID /* ID of the thread within a wavefront/warp */ % 2 ==
0) continue;
> >
> >    bar = ballot(true); // ballot 2
> > }
> >
> > versus:
> >
> > int i = 0;
> > while (true) {
> >    do {
> >        if (i == 2) break;
> >        foo = ballot(true); // ballot 1
> >        i++;
> >    } while (threadID % 2 == 0);
> >
> >    if (i == 2) break;
> >    bar = ballot(true); // ballot 2
> >    i++;
> > }
>
> I think you can remove the second "i++", otherwise we can
increment "i" twice
> if threadID % 2 != 0, but I see the issue. Using the equivalence classes
would
> prevent this transform, since we have re-arranged the control flow in that
way,
What do you mean? Note that the transforms that lead from the first
example to the second are actually desirable if there aren't any
ballots or other convergent operations, so banning them entirely is a
no-go. Otherwise, note that the ballot here can be nested arbitrarily
deep, which means that jump forwarding/tail duplication has to be
aware of the entire loop unless we have some kind of merge intrinsic.

Also, I should say that while interesting, control equivalence classes
can be part of the *implementation*, but not the *specification*. We
need to define the semantics of the IR -- that is, how a program
compiled from any given IR is allowed to do when executed (and *not*
what it looks like when compiled) -- *first*, and then which
transforms are allowed/not allowed will fall out of that. We can't
start by listing disallowed transforms, because then when someone
comes along and writes a new optimization, it might be technically
"allowed" even though it breaks something in practice. The only way to
conclusively prove that transforms will never break code that's
supposed to work (except for bugs in the transforms) is to define the
semantics of the IR, and then to make sure that it refines the
semantics of the source language. This is why the LangRef almost never
talks about allowed/disallowed transforms except as examples to
explain some given semantics, and if you don't follow that rule, your
patch will probably be rejected.

Now, to define a semantics for ballot(), we need to define what it's
allowed to return for any given execution of any given program, and to
do that, we need to define which threads must be active together when
it's reached, which in turns means we need to define when control flow
can re-converge somehow. Any proposal for how to handle ballot() must
start here.
> I'm not sure if using these rules will be easier or harder than dealing
with
> intrinsics. One problem is that the rules for single-threaded code might
seem
> arbitrary, and it would be hard to reason about them in a larger context.
>
> > Nicolai's proposal solves this by having the frontend emit a merge
intrinsic
> > before the i++ is emitted. This prevents the jump forwarding from
occurring.
>
> I was thinking about getting through the single-thread view and the issues
with
> that first, then I will think more about the multi-thread and explicit
convergence.
>
> If we are done with the single-thread stuff for now these are the question
that I
> have been thinking about with the multi-threaded view:
>
> What happens to new control flow created by transforms, and what will guide
> the insertion of intrinsics in the new code? Code structurization is one
example
> were this could happen.
Hopefully, it's clear from the above how this all falls out. The
frontend for e.g. GLSL or SPIR-V would have to insert the merge
intrinsics to preserve the semantics of the source language. Any
transforms must refine the semantics of the IR, although I can't think
of a scenario where that would involve emitting any new merge
intrinsics. Usually, structurized IR's have their own semantics about
when control flow converges, so a code structurizer should respect the
original semantics. AMDGPU has its own code structurizer that runs
very late in the pipeline (although there are plans to remove it), so
we might have to change that to make it respect the merge intrinsic
intrinsics, and then we'll correctly implement them "for free".
>
> Would they only be present if ballot and similar functions are used, or do
they
> have to be present everywhere?
They'd only have to be present when ballot or other convergent
functions are called, since otherwise it doesn't matter when control
flow re-converges. However, we may want to keep them around for
performance reasons (usually destroying convergence points is bad for
performance).
>
> How are uniform branches handled? Do they affect the convergence model?
We may be able to remove convergence points if branches are uniform.
In Nicolai's proposal, I believe we'd want to remove a merge intrinsic
when all the branches that it post-dominates that aren't also
post-dominated by some other merge intrinsic are uniform.

>
>
> - Jan
>
>
> On Monday, January 28, 2019, 11:16:36 AM EST, Connor Abbott <cwabbott0
at gmail.com> wrote:
>
>
>
> On Fri, Jan 25, 2019 at 3:05 AM Jan Sjodin <jan_sjodin at yahoo.com>
wrote:
> >
> > > for (...) {
> > >    ballot();
> > >    if (... /* non-uniform */) continue;
> > > }
> > >
> > > into
> > >
> > > for (...) {
> > >    do {
> > >        ballot();
> > >    } while (... /* non-uniform */);
> > > }
> >
> > I'm not sure if I follow this example, could you and explain a bit
more?
> > It looks to me that the condition in the "if" must be false
(if the
> > same condition is used in the while), or we would
> > call ballot the wrong number of times.
>
> Yes, the idea is that the same condition is used in the if and the
do-while. I think I messed up the example a little... in the second snippet,
we're supposed to break out of the inner loop if the outer loop's exit
condition is true. Here's a more concrete example:
>
> for (int i = 0; i < 2; i++) {
>    foo = ballot(true); // ballot 1
>
>    if (threadID /* ID of the thread within a wavefront/warp */ % 2 == 0)
continue;
>
>    bar = ballot(true); // ballot 2
> }
>
> versus:
>
> int i = 0;
> while (true) {
>    do {
>        if (i == 2) break;
>        foo = ballot(true); // ballot 1
>        i++;
>    } while (threadID % 2 == 0);
>
>    if (i == 2) break;
>    bar = ballot(true); // ballot 2
>    i++;
> }
>
> From a single-threaded perspective, these two snippets are identical, even
if ballot() writes to arbitrary memory. The first can easily get transformed to
something like the second when LLVM decides to duplicate the final i++ through
jump forwarding, and then re-interprets the loop as two nested loops and splits
the loop header in two. This is what currently happens with DOOM when we try to
enable subgroup operations with it. Let's say there are two threads in a
wavefront. Then the execution trace mandated by SPIR-V for the first looks like:
>
> thread 0        | thread 1
> ballot 1 = 0b11 | ballot 1 = 0b11
> skipped        | ballot 2 = 0b10
> ballot 1 = 0b11 | ballot 1 = 0b11
> skipped        | ballot 2 = 0b10
>
> Now, contrast this with the execution trace that programmers would expect
for the second example:
>
> thread 0        | thread 1
> ballot 1 = 0b11 | ballot 1 = 0b11
> ballot 1 = 0b01 | skipped
> skipped        | ballot 2 = 0b10
> skipped        | ballot 1 = 0b10
> skipped        | ballot 2 = 0b10
>
> Nicolai's proposal solves this by having the frontend emit a merge
intrinsic before the i++ is emitted. This prevents the jump forwarding from
occurring.
>
>
> >
> > About the CSE, when would that be legal? I can imagine with uniform
> > branches that it could work, but would like to see an example to
> > fully understand this.
> >
> > I agree that it would be more conservative than if we model the
threading,
> > but I'm not sure about the cost/benefit. I am mostly curious if it
is
> > possible to have a single-thread view or not. Then we would have to
> > see if it is adequate.
> >
> > - Jan
> >
> > On Thursday, January 24, 2019, 10:31:47 AM EST, Connor Abbott
<cwabbott0 at gmail.com> wrote:
> >
> >
> > I don't see how this would fix the continue vs. nested loop
problem I
> > explained earlier. That is, how would this prevent turning:
> >
> > for (...) {
> >    ballot();
> >    if (... /* non-uniform */) continue;
> > }
> >
> > into
> >
> > for (...) {
> >    do {
> >        ballot();
> >    } while (... /* non-uniform */);
> > }
> >
> > and vice versa? Note that there's no duplication going on here,
and
> > the single-threaded flow of control is exactly the same.
> >
> > Another reason this isn't so great is that it prevents e.g. CSE on
> > ballots that actually should be combined, since you're modelling
it as
> > a write. It seems like the optimizer is going to need some special
> > knowledge of convergent things that fake memory constraints can't
give
> > us.
> >
> > On Thu, Jan 24, 2019 at 4:06 PM Jan Sjodin <jan_sjodin at
yahoo.com> wrote:
> > >
> > >
> > > I was looking into ballot() and how if it is possible to keep a
single-threaded
> > > view of the code, but add some extra conditions that must hold
after the
> > > transformation. I had the initial idea that each call to ballot()
in a
> > > single-threaded program can be seen as a partial write to a
memory
> > > location, and each location memory location is unique for every
call site,
> > > plus there some externally observable side effect. We can
abstract this
> > > away by tagging the calls, e.g. by using aliases.
> > >
> > > For example:
> > >
> > > if (...) {
> > >      foo1 = ballot();
> > > } else {
> > >      foo2 = ballot();
> > > }
> > >
> > > simply becomes:
> > >
> > > if (...) {
> > >      foo1 = ballot_1();
> > > } else {
> > >      foo2 = ballot_2();
> > > }
> > >
> > >
> > > and
> > >
> > > if (...) {
> > > } else {
> > > }
> > > ballot();
> > >
> > > becomes
> > >
> > > if (...) {
> > > } else {
> > > }
> > > ballot_1();
> > >
> > > In the first case it would prevent combining the two calls into
one
> > > after the if. In the second example there is generally nothing
that
> > > says it could not be transformed into the first example with two
> > > calls to ballot_1(), which should not be allowed.
> > >
> > > Another form of duplication that we must allow are loop
transforms,
> > > like unrolling or peeling. These might seem similar to the
example
> > > above, since we are cloning code and with conditions etc. But
> > > they are different since they calls are in different loop
iterations.
> > >
> > > The condition that needs to be met is that:
> > >
> > > There must be a single path between all cloned ballot_n()
functions.
> > >
> > > The reason for this condition is that if we clone the same call,
then
> > > the copies must be mutually exclusive, but if they are cloned
from
> > > a loop, there must be a path, or we would skip iterations.
> > >
> > > If we want to be more sophisticated we can add:
> > >
> > > If there is no such path, the calls must be separated by uniform
branches.
> > >
> > > After the transform everything should be re-tagged, since we
already
> > > checked the calls and we don't want to check them again.
Also, not all
> > > transforms need (or should) have the tagging checked. One example
is
> > > inlining, where multiple copies are created, but they are clearly
different
> > > calls. The tagging can be done temporarily for a single pass, and
then
> > > eliminated. This could be a good tool for debugging as well,
since it can
> > > detect if a transform is suspect.
> > >
> > > The code would of course have to make sense as far as control
flow. If
> > > we have:
> > >
> > > for(;;) {
> > >    if(...) {
> > >      ballot();
> > >      break;
> > >    }
> > > }
> > >
> > > This would have to be translated to:
> > >
> > > for(;;) {
> > >    if(...) {
> > >      ballot();
> > >      if(UnknownConstant) {  // Not a uniform condition, but later
on translated to "true"
> > >        break;
> > >      }
> > > }
> > >
> > > However, I think that this is the way the code is generated today
anyway.
> > > There would have to be some attribute that indicate that these
calls (or functions)
> > > contain ballot or other cross-lane operations so they could be
tagged and
> > > checked. The attribute would be used by the passes to know that
the special
> > > conditions exist for those calls.
> > >
> > > As far as what it means to have a path, it could be complicated.
> > > For example:
> > >
> > > x = ...
> > > ballot_1();
> > >
> > > could be transformed to:
> > >
> > > if (x < 4711) {
> > >  ballot_1();
> > >
> > > if(x >= 4711) {
> > >  ballot_1();
> > > }
> > >
> > > So a simple path check would say there is a path, and the
transform is legal,
> > > but if we examine the conditions, there is no path, and the
transform should not be legal.
> > > It could be made even more obscure of course, but I don't see
any optimizations really
> > > doing this kind of thing,
> > >
> > > - Jan
> > >
> > > On Saturday, December 29, 2018, 11:32:25 AM EST, Nicolai Hähnle
via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > >
> > >
> > > On 20.12.18 18:03, Connor Abbott wrote:
> > > >    We already have the notion of "convergent"
functions like
> > > >    syncthreads(), to which we cannot add control-flow
dependencies.
> > > >    That is, it's legal to hoist syncthreads out of an
"if", but it's
> > > >    not legal to sink it into an "if".  It's
not clear to me why we
> > > >    can't have "anticonvergent" (terrible name)
functions which cannot
> > > >    have control-flow dependencies removed from them? 
ballot() would be
> > > >    both convergent and anticonvergent.
> > > >
> > > >    Would that solve your problem?
> > > >
> > > >
> > > > I think it's important to note that we already have such
an attribute,
> > > > although with the opposite sense - it's impossible to
remove control
> > > > flow dependencies from a call unless you mark it as
"speculatable".
> > >
> > > This isn't actually true. If both sides of an if/else have
the same
> > > non-speculative function call, it can still be moved out of
control flow.
> > >
> > > That's because doing so doesn't change anything at all
from a
> > > single-threaded perspective. Hence why I think we should model
the
> > > communication between threads honestly.
> > >
> > >
> > > > However, this doesn't prevent
> > > >
> > > > if (...) {
> > > > } else {
> > > > }
> > > > foo = ballot();
> > > >
> > > > from being turned into
> > > >
> > > > if (...) {
> > > >      foo1 = ballot();
> > > > } else {
> > > >      foo2 = ballot();
> > > > }
> > > > foo = phi(foo1, foo2)
> > > >
> > > > and vice versa. We have a "noduplicate" attribute
which prevents
> > > > transforming the first into the second, but not the other
way around. Of
> > > > course we could keep going this way and add a
"nocombine" attribute to
> > > > complement noduplicate. But even then, there are even still
problematic
> > > > transforms. For example, take this program, which is
simplified from a
> > > > real game that doesn't work with the AMDGPU backend:
> > > >
> > > > while (cond1 /* uniform */) {
> > > >      ballot();
> > > >      ...
> > > >      if (cond2 /* non-uniform */) continue;
> > > >      ...
> > > > }
> > > >
> > > > In SPIR-V, when using structured control flow, the semantics
of this are
> > > > pretty clearly defined. In particular, there's a
continue block after
> > > > the body of the loop where control flow re-converges, and
the only back
> > > > edge is from the continue block, so the ballot is in uniform
control
> > > > flow. But LLVM will get rid of the continue block since
it's empty, and
> > > > re-analyze the loop as two nested loops, splitting the loop
header in
> > > > two, producing a CFG which corresponds to this:
> > > >
> > > > while (cond1 /* uniform */) {
> > > >      do {
> > > >          ballot();
> > > >          ...
> > > >      } while (cond2 /* non-uniform */);
> > > >      ...
> > > > }
> > > >
> > > > Now, in an implementation where control flow re-converges at
the
> > > > immediate post-dominator, this won't do the right thing
anymore. In
> > > > order to handle it correctly, you'd effectively need to
always flatten
> > > > nested loops, which will probably be really bad for
performance if the
> > > > programmer actually wanted the second thing. It also makes
it impossible
> > > > when translating a high-level language to LLVM to get the
"natural"
> > > > behavior which game developers actually expect. This is
exactly the sort
> > > > of "spooky action at a distance" which makes me
think that everything
> > > > we've done so far is really insufficient, and we need to
add an explicit
> > > > notion of control-flow divergence and reconvergence to the
IR. We need a
> > > > way to say that control flow re-converges at the continue
block, so that
> > > > LLVM won't eliminate it, and we can vectorize it
correctly without
> > > > penalizing cases where it's better for control flow not
to re-converge.
> > >
> > > Well said!
> > >
> > > Cheers,
> > >
> > > Nicolai
> > > --
> > > Lerne, wie die Welt wirklich ist,
> > > Aber vergiss niemals, wie sie sein sollte.
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev  
  
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On 31.01.19 15:59, Jan Sjodin wrote:
>> >  Any transform that re-arranges control flow would potentially
have to
>> >  know about the properties of ballot(), and the rules with respect
to
>> >  the CFG (and maybe consider the target) to know where to insert
the
>> >  intrinsics.
> 
>> But the same is true for basically any approach to handling this. In
>> fact, adding the merge intrinsics makes this much easier. Beyond the
>> usual problems with hoisting ballots, which passes currently avoid via
>> the current convergent + speculatable attributes, we'd only have to
>> add awareness to passes that they can't duplicate/combine merge
>> intrinsics or move them past convergent intrinsics, which is a local
>> property and hence easily checkable. In example I explained, without
>> some kind of merge intrinsic, tail duplication has to look at the
>> entire loop to know whether the transform is legal. Of course, you
>> could have some kind of "no convergent calls" flag on a
function, but
>> that doesn't eliminate the nastyness when there are convergent
calls.
> 
> We will have to determine if the intrinsics are worth more compared to
> scanning the code.
The reason I'm looking for solutions that can work without "scanning
the
code" or "spooky action at a distance" is that we should have a
solution
that's easily digestible by folks who are not aware of GPU execution models.

The fallback model for such folks should be: if your pass doesn't touch 
specially-marked calls (whether that marker is "convergent" or
something
else), then you don't have to worry. Which, as a corollary, means that 
transforms can be conservatively correct by not touching such calls.

I don't see how to achieve this goal while forcing passes to do specific 
code scans.

>> >   I had the impression that the control flow convergence was
>> >   in part specified by what the target architecture can handle.
> 
>> This is true, although hopefully we can agree on something that
>> everyone can implement.
> 
> Yes, hopefully there is an execution model that is works for everyone.
> 
>> >  One of
>> >  the more complicated cases would be linearization where the
control
>> >  flow is completely rewritten, and is encoded in a variable that
says
>> >  which basic block is the next one to execute.
> 
>> It's certainly interesting to think about how to maintain
correctness
>> in the face of ballots() with such a pass, but a) it's certainly no
>> harder with merge intrinsics than merges being implicit and b) I doubt
>> that's useful for anything you'd want to do with a GPU.
> 
> Irreducible control flow has to be handled somehow, and linearization
> is the only transform I know of, that will handle everything. I'm not
> sure what the execution model says about irreducible control flow.
For what it's worth, the reconverging CFG approach can also handle 
arbitrary irreducible control flow.

Whether (or how) it can do this while being compatible with whatever 
semantics we come up with for cross-lane operations remains to be seen.

>> >  Another case is DCE,
>> >  where a ballot() could be eliminated, and it would potentially
have to
>> >  remove a number of intrinsics to enable later optimizations
(unless it
>> >  would affect performance?), so the intrinsics will require some
>> >  non-local updates.
> 
>> Removing merge intrinsics if it's profitable and allowed is a
>> separate optimization, that can be done relatively quickly in a single
>> pass without impacting the performance of other optimizations. It's
>> requiring expensive non-local checks in optimizations which modify
>> control flow that's a problem.
> 
> There are certainly a lot of tradeoffs. My point was simply that the
> intrinsics are not strictly local.
Right. I have some ideas for how to fix this, but wasn't able to work on 
them for the last weeks.

Cheers,
Nicolai

>> >  So we might have them without a ballot(), which would seem to
make it
>> >  more difficult for structurizers or other transforms to maintain
the
>> >  semantics and insert intrinsics.
> 
>> It's not any more difficult code-wise, since the case where there
is
>> a ballot needs to be handled anyways. And while it might take longer
>> to process stuff when this hypothetical pass encounters a merge
>> intrinsic (I can't think of a real-world case where it would
matter),
>> it should result in better code generated.
> 
> I was thinking of linearization or something similar, where an
> instrinsic by itself might be hard to preserve, compared to looking at
> a ballot() and derive where intrinsics should be inserted.
> 
>> > > > How are uniform branches handled? Do they affect the
convergence model?
>> > > >
>> > > We may be able to remove convergence points if branches are
uniform.
>> > > In Nicolai's proposal, I believe we'd want to remove
a merge intrinsic
>> > > when all the branches that it post-dominates that aren't
also
>> > > post-dominated by some other merge intrinsic are uniform.
>> >
>> > I couldn't quite understand the last sentence, but I assume
the
>> > conditions would prevent removing convergence points that help
>> > performance. Post-domination might not be adequate if there are
loops
>> > btw.
>> 
>> It should be -- Nicolai's proposal is that a merge intrinsic merges
>> all the divergences caused by branches post dominated by the
>> intrinsic, so if all the divergent branches are merged by some other
>> intrinsic earlier, then there's no divergence and the merge
intrinsic
>> is a no-op.
> 
> Makes sense.
> 
> - Jan
> On Wednesday, January 30, 2019, 11:41:29 AM EST, Connor Abbott 
> <cwabbott0 at gmail.com> wrote:
> 
> 
> On Wed, Jan 30, 2019 at 4:20 PM Jan Sjodin <jan_sjodin at yahoo.com 
> <mailto:jan_sjodin at yahoo.com>> wrote:
> 
> 
>     >
>     > > > for (int i = 0; i < 2; i++) {
>     > > >  foo = ballot(true); // ballot 1
>     > > >
>     > > >    if (threadID /* ID of the thread within a
wavefront/warp */ % 2 == 0) continue;
>     > > >
>     > > >    bar = ballot(true); // ballot 2
>     > > > }
>     > > >
>     > > > versus:
>     > > >
>     > > > int i = 0;
>     > > > while (true) {
>     > > >    do {
>     > > >        if (i == 2) break;
>     > > >        foo = ballot(true); // ballot 1
>     > > >        i++;
>     > > >    } while (threadID % 2 == 0);
>     > > >
>     > > >    if (i == 2) break;
>     > > >    bar = ballot(true); // ballot 2
>     > > >    i++;
>     > > > }
>     > >
>     > > I think you can remove the second "i++", otherwise
we can increment "i" twice
>     > > if threadID % 2 != 0, but I see the issue. Using the
equivalence classes would
>     > > prevent this transform, since we have re-arranged the control
flow in that way,
>     >
>     > What do you mean? Note that the transforms that lead from the
first
>     > example to the second are actually desirable if there aren't
any
>     > ballots or other convergent operations, so banning them entirely
is a
>     > no-go. Otherwise, note that the ballot here can be nested
arbitrarily
>     > deep, which means that jump forwarding/tail duplication has to be
>     > aware of the entire loop unless we have some kind of merge
intrinsic.
> 
>     Yes, if there weren't any calls to a ballot, then the transform
would
>     be legal. What I was saying in the beginning was that ballot() would
>     have some special rules attached to it. It is of course undesirable to
>     have flags to enforce correctness, but that is the point we are at
>     right now.
> 
>     > Also, I should say that while interesting, control equivalence
classes
>     > can be part of the *implementation*, but not the *specification*.
We
>     > need
>     > to define the semantics of the IR -- that is, how a program
>     > compiled from any given IR is allowed to do when executed (and
*not*
>     > what it looks like when compiled) -- *first*, and then which
>     > transforms are allowed/not allowed will fall out of that. We
can't
>     > start by listing disallowed transforms, because then when someone
>     > comes along and writes a new optimization, it might be technically
>     > "allowed" even though it breaks something in practice.
> 
>     I think you misunderstand me if you think I was listing disallowed
>     transforms. My question was if it is possible to have multi-threaded
>     semantics in the source language, but in the compiler have a
>     single-threaded view, where some properties of the CFG would determine
>     what is legal and not for some functions with a special flag.
> 
> 
> But that's practically the same as listing allowed and disallowed 
> transforms -- you're defining what the final IR is allowed to look
like,
> not how it's allowed to execute. If at all possible, the former should 
> be derived from the latter.
> 
>     I agree
>     it is more desirable to have the the semantics specified in the
>     IR. However, I am exploring this from a practical point of view, since
>     ballot() is very rare compared to all code that is being compiled for
>     all targets. These intrinsics would always have to be considered when
>     writing a pass. They seem to me harder to think about, and
>     test, for someone who is working on a single-threaded target, compared
>     to looking at a flag. If we allow ourselves to look at which
>     transforms that might violate these properties, we could would free up
>     the rest of the compiler (and developers) to not have to worry about
>     these things. Intrinsics would have to be maintained throughout the
>     entire compilation process in every pass.
> 
>     > The only way to
>     > conclusively prove that transforms will never break code
that's
>     > supposed to work (except for bugs in the transforms) is to define
the
>     > semantics of the IR, and then to make sure that it refines the
>     > semantics of the source language. This is why the LangRef almost
never
>     > talks about allowed/disallowed transforms except as examples to
>     > explain some given semantics, and if you don't follow that
rule, your
>     > patch will probably be rejected.
> 
>     I'm trying to figure out if we are in the "almost never"
territory
>     here or not.
> 
> 
> I don't think so at all. In addition to being much, much harder to 
> reason about and prove that the approach is sound, I think it'll be
more
> intrusive.
> 
> 
>     > Now, to define a semantics for ballot(), we need to define what
it's
>     > allowed to return for any given execution of any given program,
and to
>     > do that, we need to define which threads must be active together
when
>     > it's reached, which in turns means we need to define when
control flow
>     > can re-converge somehow. Any proposal for how to handle ballot()
must
>     > start here.
> 
>     > > I'm not sure if using these rules will be easier or
harder than dealing with
>     > > intrinsics. One problem is that the rules for single-threaded
code might seem
>     > > arbitrary, and it would be hard to reason about them in a
larger context.
> 
>     > > What happens to new control flow created by transforms, and
what will guide
>     > > the insertion of intrinsics in the new code? Code
structurization is one example
>     > > were this could happen.
> 
>     > Hopefully, it's clear from the above how this all falls out.
The
>     > frontend for e.g. GLSL or SPIR-V would have to insert the merge
>     > intrinsics to preserve the semantics of the source language. Any
>     > transforms must refine the semantics of the IR, although I
can't think
>     > of a scenario where that would involve emitting any new merge
>     > intrinsics. Usually, structurized IR's have their own
semantics about
>     > when control flow converges, so a code structurizer should respect
the
>     > original semantics. AMDGPU has its own code structurizer that runs
>     > very late in the pipeline (although there are plans to remove it),
so
>     > we might have to change that to make it respect the merge
intrinsic
>     > intrinsics, and then we'll correctly implement them "for
free".
> 
>     Any transform that re-arranges control flow would potentially have to
>     know about the properties of ballot(), and the rules with respect to
>     the CFG (and maybe consider the target) to know where to insert the
>     intrinsics.
> 
> 
> But the same is true for basically any approach to handling this. In 
> fact, adding the merge intrinsics makes this much easier. Beyond the 
> usual problems with hoisting ballots, which passes currently avoid via 
> the current convergent + speculatable attributes, we'd only have to add
> awareness to passes that they can't duplicate/combine merge intrinsics 
> or move them past convergent intrinsics, which is a local property and 
> hence easily checkable. In example I explained, without some kind of 
> merge intrinsic, tail duplication has to look at the entire loop to know 
> whether the transform is legal. Of course, you could have some kind of 
> "no convergent calls" flag on a function, but that doesn't
eliminate the
> nastyness when there are convergent calls.
> 
>     I had the impression that the control flow convergence was
>     in part specified by what the target architecture can handle.
> 
> 
> This is true, although hopefully we can agree on something that everyone 
> can implement.
> 
>     One of
>     the more complicated cases would be linearization where the control
>     flow is completely rewritten, and is encoded in a variable that says
>     which basic block is the next one to execute.
> 
> 
> It's certainly interesting to think about how to maintain correctness
in
> the face of ballots() with such a pass, but a) it's certainly no harder
> with merge intrinsics than merges being implicit and b) I doubt that's 
> useful for anything you'd want to do with a GPU.
> 
>     Another case is DCE,
>     where a ballot() could be eliminated, and it would potentially have to
>     remove a number of intrinsics to enable later optimizations (unless it
>     would affect performance?), so the intrinsics will require some
>     non-local updates.
> 
> 
> Removing merge intrinsics if it's profitable and allowed is a separate 
> optimization, that can be done relatively quickly in a single pass 
> without impacting the performance of other optimizations. It's
requiring
> expensive non-local checks in optimizations which modify control flow 
> that's a problem.
> 
> 
>     > > > Would they only be present if ballot and similar
functions are used, or do they
>     > > > have to be present everywhere?
>     > >
>     > > They'd only have to be present when ballot or other
convergent
>     > > functions are called, since otherwise it doesn't matter
when control
>     > flow re-converges. However, we may want to keep them around for
>     > performance reasons (usually destroying convergence points is bad
for
>     > performance).
> 
>     So we might have them without a ballot(), which would seem to make it
>     more difficult for structurizers or other transforms to maintain the
>     semantics and insert intrinsics.
> 
> 
> It's not any more difficult code-wise, since the case where there is a 
> ballot needs to be handled anyways. And while it might take longer to 
> process stuff when this hypothetical pass encounters a merge intrinsic 
> (I can't think of a real-world case where it would matter), it should 
> result in better code generated.
> 
> 
>     > > How are uniform branches handled? Do they affect the
convergence model?
>     > >
>     > We may be able to remove convergence points if branches are
uniform.
>     > In Nicolai's proposal, I believe we'd want to remove a
merge intrinsic
>     > when all the branches that it post-dominates that aren't also
>     > post-dominated by some other merge intrinsic are uniform.
> 
>     I couldn't quite understand the last sentence, but I assume the
>     conditions would prevent removing convergence points that help
>     performance. Post-domination might not be adequate if there are loops
>     btw.
> 
> 
> It should be -- Nicolai's proposal is that a merge intrinsic merges all
> the divergences caused by branches post dominated by the intrinsic, so 
> if all the divergent branches are merged by some other intrinsic 
> earlier, then there's no divergence and the merge intrinsic is a no-op.
> 
> 
>     - Jan
> 
> 
> 
>     On Wednesday, January 30, 2019, 6:29:52 AM EST, Connor Abbott
>     <cwabbott0 at gmail.com <mailto:cwabbott0 at gmail.com>>
wrote:
> 
> 
>     On Mon, Jan 28, 2019 at 9:09 PM Jan Sjodin <jan_sjodin at yahoo.com
>     <mailto:jan_sjodin at yahoo.com>> wrote:
>      >
>      > > for (int i = 0; i < 2; i++) {
>      > >  foo = ballot(true); // ballot 1
>      > >
>      > >    if (threadID /* ID of the thread within a wavefront/warp
*/
>     % 2 == 0) continue;
>      > >
>      > >    bar = ballot(true); // ballot 2
>      > > }
>      > >
>      > > versus:
>      > >
>      > > int i = 0;
>      > > while (true) {
>      > >    do {
>      > >        if (i == 2) break;
>      > >        foo = ballot(true); // ballot 1
>      > >        i++;
>      > >    } while (threadID % 2 == 0);
>      > >
>      > >    if (i == 2) break;
>      > >    bar = ballot(true); // ballot 2
>      > >    i++;
>      > > }
>      >
>      > I think you can remove the second "i++", otherwise we
can
>     increment "i" twice
>      > if threadID % 2 != 0, but I see the issue. Using the equivalence
>     classes would
>      > prevent this transform, since we have re-arranged the control
>     flow in that way,
> 
>     What do you mean? Note that the transforms that lead from the first
>     example to the second are actually desirable if there aren't any
>     ballots or other convergent operations, so banning them entirely is a
>     no-go. Otherwise, note that the ballot here can be nested arbitrarily
>     deep, which means that jump forwarding/tail duplication has to be
>     aware of the entire loop unless we have some kind of merge intrinsic.
> 
>     Also, I should say that while interesting, control equivalence classes
>     can be part of the *implementation*, but not the *specification*. We
>     need to define the semantics of the IR -- that is, how a program
>     compiled from any given IR is allowed to do when executed (and *not*
>     what it looks like when compiled) -- *first*, and then which
>     transforms are allowed/not allowed will fall out of that. We can't
>     start by listing disallowed transforms, because then when someone
>     comes along and writes a new optimization, it might be technically
>     "allowed" even though it breaks something in practice. The
only way to
>     conclusively prove that transforms will never break code that's
>     supposed to work (except for bugs in the transforms) is to define the
>     semantics of the IR, and then to make sure that it refines the
>     semantics of the source language. This is why the LangRef almost never
>     talks about allowed/disallowed transforms except as examples to
>     explain some given semantics, and if you don't follow that rule,
your
>     patch will probably be rejected.
> 
>     Now, to define a semantics for ballot(), we need to define what
it's
>     allowed to return for any given execution of any given program, and to
>     do that, we need to define which threads must be active together when
>     it's reached, which in turns means we need to define when control
flow
>     can re-converge somehow. Any proposal for how to handle ballot() must
>     start here.
> 
>      > I'm not sure if using these rules will be easier or harder
than
>     dealing with
>      > intrinsics. One problem is that the rules for single-threaded
>     code might seem
>      > arbitrary, and it would be hard to reason about them in a larger
>     context.
>      >
>      > > Nicolai's proposal solves this by having the frontend
emit a
>     merge intrinsic
>      > > before the i++ is emitted. This prevents the jump forwarding
>     from occurring.
>      >
>      > I was thinking about getting through the single-thread view and
>     the issues with
>      > that first, then I will think more about the multi-thread and
>     explicit convergence.
>      >
>      > If we are done with the single-thread stuff for now these are the
>     question that I
>      > have been thinking about with the multi-threaded view:
>      >
>      > What happens to new control flow created by transforms, and what
>     will guide
>      > the insertion of intrinsics in the new code? Code structurization
>     is one example
>      > were this could happen.
> 
>     Hopefully, it's clear from the above how this all falls out. The
>     frontend for e.g. GLSL or SPIR-V would have to insert the merge
>     intrinsics to preserve the semantics of the source language. Any
>     transforms must refine the semantics of the IR, although I can't
think
>     of a scenario where that would involve emitting any new merge
>     intrinsics. Usually, structurized IR's have their own semantics
about
>     when control flow converges, so a code structurizer should respect the
>     original semantics. AMDGPU has its own code structurizer that runs
>     very late in the pipeline (although there are plans to remove it), so
>     we might have to change that to make it respect the merge intrinsic
>     intrinsics, and then we'll correctly implement them "for
free".
> 
>      >
>      > Would they only be present if ballot and similar functions are
>     used, or do they
>      > have to be present everywhere?
> 
>     They'd only have to be present when ballot or other convergent
>     functions are called, since otherwise it doesn't matter when
control
>     flow re-converges. However, we may want to keep them around for
>     performance reasons (usually destroying convergence points is bad for
>     performance).
> 
>      >
>      > How are uniform branches handled? Do they affect the convergence
>     model?
> 
>     We may be able to remove convergence points if branches are uniform.
>     In Nicolai's proposal, I believe we'd want to remove a merge
intrinsic
>     when all the branches that it post-dominates that aren't also
>     post-dominated by some other merge intrinsic are uniform.
> 
> 
> 
>      >
>      >
>      > - Jan
>      >
>      >
>      > On Monday, January 28, 2019, 11:16:36 AM EST, Connor Abbott
>     <cwabbott0 at gmail.com <mailto:cwabbott0 at gmail.com>>
wrote:
>      >
>      >
>      >
>      > On Fri, Jan 25, 2019 at 3:05 AM Jan Sjodin <jan_sjodin at
yahoo.com
>     <mailto:jan_sjodin at yahoo.com>> wrote:
>      > >
>      > > > for (...) {
>      > > >    ballot();
>      > > >    if (... /* non-uniform */) continue;
>      > > > }
>      > > >
>      > > > into
>      > > >
>      > > > for (...) {
>      > > >    do {
>      > > >        ballot();
>      > > >    } while (... /* non-uniform */);
>      > > > }
>      > >
>      > > I'm not sure if I follow this example, could you and
explain a
>     bit more?
>      > > It looks to me that the condition in the "if" must
be false (if the
>      > > same condition is used in the while), or we would
>      > > call ballot the wrong number of times.
>      >
>      > Yes, the idea is that the same condition is used in the if and
>     the do-while. I think I messed up the example a little... in the
>     second snippet, we're supposed to break out of the inner loop if
the
>     outer loop's exit condition is true. Here's a more concrete
example:
>      >
>      > for (int i = 0; i < 2; i++) {
>      >    foo = ballot(true); // ballot 1
>      >
>      >    if (threadID /* ID of the thread within a wavefront/warp */ %
>     2 == 0) continue;
>      >
>      >    bar = ballot(true); // ballot 2
>      > }
>      >
>      > versus:
>      >
>      > int i = 0;
>      > while (true) {
>      >    do {
>      >        if (i == 2) break;
>      >        foo = ballot(true); // ballot 1
>      >        i++;
>      >    } while (threadID % 2 == 0);
>      >
>      >    if (i == 2) break;
>      >    bar = ballot(true); // ballot 2
>      >    i++;
>      > }
>      >
>      > From a single-threaded perspective, these two snippets are
>     identical, even if ballot() writes to arbitrary memory. The first
>     can easily get transformed to something like the second when LLVM
>     decides to duplicate the final i++ through jump forwarding, and then
>     re-interprets the loop as two nested loops and splits the loop
>     header in two. This is what currently happens with DOOM when we try
>     to enable subgroup operations with it. Let's say there are two
>     threads in a wavefront. Then the execution trace mandated by SPIR-V
>     for the first looks like:
>      >
>      > thread 0        | thread 1
>      > ballot 1 = 0b11 | ballot 1 = 0b11
>      > skipped        | ballot 2 = 0b10
>      > ballot 1 = 0b11 | ballot 1 = 0b11
>      > skipped        | ballot 2 = 0b10
>      >
>      > Now, contrast this with the execution trace that programmers
>     would expect for the second example:
>      >
>      > thread 0        | thread 1
>      > ballot 1 = 0b11 | ballot 1 = 0b11
>      > ballot 1 = 0b01 | skipped
>      > skipped        | ballot 2 = 0b10
>      > skipped        | ballot 1 = 0b10
>      > skipped        | ballot 2 = 0b10
>      >
>      > Nicolai's proposal solves this by having the frontend emit a
>     merge intrinsic before the i++ is emitted. This prevents the jump
>     forwarding from occurring.
>      >
>      >
>      > >
>      > > About the CSE, when would that be legal? I can imagine with
uniform
>      > > branches that it could work, but would like to see an
example to
>      > > fully understand this.
>      > >
>      > > I agree that it would be more conservative than if we model
the
>     threading,
>      > > but I'm not sure about the cost/benefit. I am mostly
curious if
>     it is
>      > > possible to have a single-thread view or not. Then we would
have to
>      > > see if it is adequate.
>      > >
>      > > - Jan
>      > >
>      > > On Thursday, January 24, 2019, 10:31:47 AM EST, Connor
Abbott
>     <cwabbott0 at gmail.com <mailto:cwabbott0 at gmail.com>>
wrote:
>      > >
>      > >
>      > > I don't see how this would fix the continue vs. nested
loop
>     problem I
>      > > explained earlier. That is, how would this prevent turning:
>      > >
>      > > for (...) {
>      > >    ballot();
>      > >    if (... /* non-uniform */) continue;
>      > > }
>      > >
>      > > into
>      > >
>      > > for (...) {
>      > >    do {
>      > >        ballot();
>      > >    } while (... /* non-uniform */);
>      > > }
>      > >
>      > > and vice versa? Note that there's no duplication going
on here, and
>      > > the single-threaded flow of control is exactly the same.
>      > >
>      > > Another reason this isn't so great is that it prevents
e.g. CSE on
>      > > ballots that actually should be combined, since you're
>     modelling it as
>      > > a write. It seems like the optimizer is going to need some
special
>      > > knowledge of convergent things that fake memory constraints
>     can't give
>      > > us.
>      > >
>      > > On Thu, Jan 24, 2019 at 4:06 PM Jan Sjodin
>     <jan_sjodin at yahoo.com <mailto:jan_sjodin at yahoo.com>>
wrote:
>      > > >
>      > > >
>      > > > I was looking into ballot() and how if it is possible
to keep
>     a single-threaded
>      > > > view of the code, but add some extra conditions that
must
>     hold after the
>      > > > transformation. I had the initial idea that each call
to
>     ballot() in a
>      > > > single-threaded program can be seen as a partial write
to a
>     memory
>      > > > location, and each location memory location is unique
for
>     every call site,
>      > > > plus there some externally observable side effect. We
can
>     abstract this
>      > > > away by tagging the calls, e.g. by using aliases.
>      > > >
>      > > > For example:
>      > > >
>      > > > if (...) {
>      > > >      foo1 = ballot();
>      > > > } else {
>      > > >      foo2 = ballot();
>      > > > }
>      > > >
>      > > > simply becomes:
>      > > >
>      > > > if (...) {
>      > > >      foo1 = ballot_1();
>      > > > } else {
>      > > >      foo2 = ballot_2();
>      > > > }
>      > > >
>      > > >
>      > > > and
>      > > >
>      > > > if (...) {
>      > > > } else {
>      > > > }
>      > > > ballot();
>      > > >
>      > > > becomes
>      > > >
>      > > > if (...) {
>      > > > } else {
>      > > > }
>      > > > ballot_1();
>      > > >
>      > > > In the first case it would prevent combining the two
calls
>     into one
>      > > > after the if. In the second example there is generally
>     nothing that
>      > > > says it could not be transformed into the first example
with two
>      > > > calls to ballot_1(), which should not be allowed.
>      > > >
>      > > > Another form of duplication that we must allow are loop
>     transforms,
>      > > > like unrolling or peeling. These might seem similar to
the
>     example
>      > > > above, since we are cloning code and with conditions
etc. But
>      > > > they are different since they calls are in different
loop
>     iterations.
>      > > >
>      > > > The condition that needs to be met is that:
>      > > >
>      > > > There must be a single path between all cloned
ballot_n()
>     functions.
>      > > >
>      > > > The reason for this condition is that if we clone the
same
>     call, then
>      > > > the copies must be mutually exclusive, but if they are
cloned
>     from
>      > > > a loop, there must be a path, or we would skip
iterations.
>      > > >
>      > > > If we want to be more sophisticated we can add:
>      > > >
>      > > > If there is no such path, the calls must be separated
by
>     uniform branches.
>      > > >
>      > > > After the transform everything should be re-tagged,
since we
>     already
>      > > > checked the calls and we don't want to check them
again.
>     Also, not all
>      > > > transforms need (or should) have the tagging checked.
One
>     example is
>      > > > inlining, where multiple copies are created, but they
are
>     clearly different
>      > > > calls. The tagging can be done temporarily for a single
pass,
>     and then
>      > > > eliminated. This could be a good tool for debugging as
well,
>     since it can
>      > > > detect if a transform is suspect.
>      > > >
>      > > > The code would of course have to make sense as far as
control
>     flow. If
>      > > > we have:
>      > > >
>      > > > for(;;) {
>      > > >    if(...) {
>      > > >      ballot();
>      > > >      break;
>      > > >    }
>      > > > }
>      > > >
>      > > > This would have to be translated to:
>      > > >
>      > > > for(;;) {
>      > > >    if(...) {
>      > > >      ballot();
>      > > >      if(UnknownConstant) {  // Not a uniform condition,
but
>     later on translated to "true"
>      > > >        break;
>      > > >      }
>      > > > }
>      > > >
>      > > > However, I think that this is the way the code is
generated
>     today anyway.
>      > > > There would have to be some attribute that indicate
that
>     these calls (or functions)
>      > > > contain ballot or other cross-lane operations so they
could
>     be tagged and
>      > > > checked. The attribute would be used by the passes to
know
>     that the special
>      > > > conditions exist for those calls.
>      > > >
>      > > > As far as what it means to have a path, it could be
complicated.
>      > > > For example:
>      > > >
>      > > > x = ...
>      > > > ballot_1();
>      > > >
>      > > > could be transformed to:
>      > > >
>      > > > if (x < 4711) {
>      > > >  ballot_1();
>      > > >
>      > > > if(x >= 4711) {
>      > > >  ballot_1();
>      > > > }
>      > > >
>      > > > So a simple path check would say there is a path, and
the
>     transform is legal,
>      > > > but if we examine the conditions, there is no path, and
the
>     transform should not be legal.
>      > > > It could be made even more obscure of course, but I
don't see
>     any optimizations really
>      > > > doing this kind of thing,
>      > > >
>      > > > - Jan
>      > > >
>      > > > On Saturday, December 29, 2018, 11:32:25 AM EST,
Nicolai
>     Hähnle via llvm-dev <llvm-dev at lists.llvm.org
>     <mailto:llvm-dev at lists.llvm.org>> wrote:
>      > > >
>      > > >
>      > > > On 20.12.18 18:03, Connor Abbott wrote:
>      > > > >    We already have the notion of
"convergent" functions like
>      > > > >    syncthreads(), to which we cannot add
control-flow
>     dependencies.
>      > > > >    That is, it's legal to hoist syncthreads
out of an "if",
>     but it's
>      > > > >    not legal to sink it into an "if". 
It's not clear to me
>     why we
>      > > > >    can't have "anticonvergent"
(terrible name) functions
>     which cannot
>      > > > >    have control-flow dependencies removed from
them?
>     ballot() would be
>      > > > >    both convergent and anticonvergent.
>      > > > >
>      > > > >    Would that solve your problem?
>      > > > >
>      > > > >
>      > > > > I think it's important to note that we already
have such an
>     attribute,
>      > > > > although with the opposite sense - it's
impossible to
>     remove control
>      > > > > flow dependencies from a call unless you mark it
as
>     "speculatable".
>      > > >
>      > > > This isn't actually true. If both sides of an
if/else have
>     the same
>      > > > non-speculative function call, it can still be moved
out of
>     control flow.
>      > > >
>      > > > That's because doing so doesn't change anything
at all from a
>      > > > single-threaded perspective. Hence why I think we
should
>     model the
>      > > > communication between threads honestly.
>      > > >
>      > > >
>      > > > > However, this doesn't prevent
>      > > > >
>      > > > > if (...) {
>      > > > > } else {
>      > > > > }
>      > > > > foo = ballot();
>      > > > >
>      > > > > from being turned into
>      > > > >
>      > > > > if (...) {
>      > > > >      foo1 = ballot();
>      > > > > } else {
>      > > > >      foo2 = ballot();
>      > > > > }
>      > > > > foo = phi(foo1, foo2)
>      > > > >
>      > > > > and vice versa. We have a "noduplicate"
attribute which
>     prevents
>      > > > > transforming the first into the second, but not
the other
>     way around. Of
>      > > > > course we could keep going this way and add a
"nocombine"
>     attribute to
>      > > > > complement noduplicate. But even then, there are
even still
>     problematic
>      > > > > transforms. For example, take this program, which
is
>     simplified from a
>      > > > > real game that doesn't work with the AMDGPU
backend:
>      > > > >
>      > > > > while (cond1 /* uniform */) {
>      > > > >      ballot();
>      > > > >      ...
>      > > > >      if (cond2 /* non-uniform */) continue;
>      > > > >      ...
>      > > > > }
>      > > > >
>      > > > > In SPIR-V, when using structured control flow, the
>     semantics of this are
>      > > > > pretty clearly defined. In particular, there's
a continue
>     block after
>      > > > > the body of the loop where control flow
re-converges, and
>     the only back
>      > > > > edge is from the continue block, so the ballot is
in
>     uniform control
>      > > > > flow. But LLVM will get rid of the continue block
since
>     it's empty, and
>      > > > > re-analyze the loop as two nested loops, splitting
the loop
>     header in
>      > > > > two, producing a CFG which corresponds to this:
>      > > > >
>      > > > > while (cond1 /* uniform */) {
>      > > > >      do {
>      > > > >          ballot();
>      > > > >          ...
>      > > > >      } while (cond2 /* non-uniform */);
>      > > > >      ...
>      > > > > }
>      > > > >
>      > > > > Now, in an implementation where control flow
re-converges
>     at the
>      > > > > immediate post-dominator, this won't do the
right thing
>     anymore. In
>      > > > > order to handle it correctly, you'd
effectively need to
>     always flatten
>      > > > > nested loops, which will probably be really bad
for
>     performance if the
>      > > > > programmer actually wanted the second thing. It
also makes
>     it impossible
>      > > > > when translating a high-level language to LLVM to
get the
>     "natural"
>      > > > > behavior which game developers actually expect.
This is
>     exactly the sort
>      > > > > of "spooky action at a distance" which
makes me think that
>     everything
>      > > > > we've done so far is really insufficient, and
we need to
>     add an explicit
>      > > > > notion of control-flow divergence and
reconvergence to the
>     IR. We need a
>      > > > > way to say that control flow re-converges at the
continue
>     block, so that
>      > > > > LLVM won't eliminate it, and we can vectorize
it correctly
>     without
>      > > > > penalizing cases where it's better for control
flow not to
>     re-converge.
>      > > >
>      > > > Well said!
>      > > >
>      > > > Cheers,
>      > > >
>      > > > Nicolai
>      > > > --
>      > > > Lerne, wie die Welt wirklich ist,
>      > > > Aber vergiss niemals, wie sie sein sollte.
>      > > > _______________________________________________
>      > > > LLVM Developers mailing list
>      > > > llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>      > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 

-- 
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.