llvm dev - Mar 2016 - RFC: Pass to prune redundant profiling instrumentation

> On Mar 11, 2016, at 5:28 PM, Sean Silva <chisophugis at gmail.com>
wrote:
> 
> 
> 
> On Fri, Mar 11, 2016 at 12:47 PM, Vedant Kumar <vsk at apple.com>
wrote:
> There have been a lot of responses. I'll try to summarize the thread
and respond
> to some of the questions/feedback.
> 
> 
> Summary
> ======> 
> 1. We should teach GlobalDCE to strip out instrumented functions which the
>    inliner cannot delete.
> 
> 2. Sean suggests adding metadata to loads/stores of counters to convey that
>    they do not alias normal program data.
> 
>    I'm not familiar with AA, but off-hand this seems like a good idea.
> 
> 3. Sean also suggests improving the optimizer to registerize counters when
>    possible.
> 
>    Ditto, seems like a good idea.
> 
> 4. Sean has an alternate proposal for counter pruning which works by
remapping
>    FE-generated counters to counters generated by the IR instrumentation
pass.
>    David likes this proposal.
> 
>    I have some questions about this but am willing to try it. IMO, the
pruning
>    pass I've suggested could complement this nicely -- I don't view
them as
>    mutually exclusive (though I could be wrong).
> 
> 5. There seems to be consensus over the need to improve performance of
>    FE-instrumented programs.
> 
> 6. David is concerned that the proposed pruning pass is too narrow in
scope,
>    and would like to see results on a larger benchmark. I'll try to get
some
>    numbers.
> 
> 7. David mentioned some optimizations that are possible if we make coverage
>    information "yes or no" (per line). Justin pointed out that
this doesn't
>    work for a few of Apple's use cases.
> 
> 
> Clarifications
> =============> 
> 1. In my original email, two of my numbers were incorrect. I listed the
>    sizes of the __llvm_prf_alias sections, not the number of actual
aliases.
>    The corrected lines are:
> 
> O3 + PGOInstr + Pruning:      0.606s (8.6% performance win, _42_ aliases)
> O3 + CoverageInstr + Pruning: 0.610s (11.6% performance win, _44_ aliases)
> 
> 2.
> 
> >>  Determine which profile counters are essential.
> >
> > What is an "essential counter"?
> 
> The pass I proposed works like this:
> 
>   for (PC : ProfileCounters) {
>     if (Update-Sites-Of(PC) == Update-Sites-Of(Other-PC)) {
>       mark PC as "essential"
>       mark Other-PC as "redundant" (i.e, alias of PC)
>     }
>   }
> 
> Where Update-Sites-Of(PC) is the set of basic blocks in which PC is
updated.
> If there are multiple counters updated in each update site for PC, they are
> all marked as aliases in one pass.
> 
> 
> 3.
> 
> >> Erase all updates to redundant profile counters.
> >
> > When you are saying "erase", you need to actually replace
the multiple counter increment with *a new* counter, right? I.e. when replacing:
> >
> >     instrprof_increment(profc_f2);
> >     instrprof_increment(profc_f1);
> >
> > you need to emit:
> >
> >     instrprof_increment(profc_f2f1_fused);
> 
> 
> No. The pass only erases updates to counters which it can prove are
> truly redundant. There is no need to create a new (or fused) counter
> because profc_f1 == profc_f2 in all possible profiles.
> 
> Oh, this makes it substantially less useful I would think (and I
misunderstood your original proposal the same way that Mehdi did here).
> I guess I'm not sure what situations your proposed transformation would
catch.
> The only one I can think of is something like:
> - function foo() has an entry counter
> - function bar() calls foo() from a BB that has a counter
> - function bar() is the only caller of foo() and calls it in only one BB.
> Can you add some debug dumping to your pass and give some real-world
examples of counters that it merges?
I still have the pruned IR from the example I brought up at the start
of the thread:

  https://ghostbin.com/paste/qus2s

Look for the __llvm_prf_alias section.

> The fused counter approach (or the one I suggested which is just a
generalization of that (see below)) seems like it can reduce the number of
counter updates much more.
>  
> 
> 4.
> 
> >> The complexity of this pass is O(N*M), where N is the number of
profile
> >> counters, and M is the average number of updates per counter.
> >
> > What you're describing here is not clear to me? But I may have
been totally misunderstanding the whole concept :)
> 
> Hopefully the pseudo-code helps? Alternatively I can put a WIP patch up
> on Phab.
> 
> > It should be just O(N) where N is the number of instructions.
> 
> The complexity of the pass is not upper-bounded by the number of
> instructions in the program because certain loads and stores can be visited
> more than once.
> 
> 
> FE to IR Counter Remapping
> =========================> 
> I have a question about this plan:
> 
> > for each CFG edge:
> >     record which FE counters have ended up associated with it
> > remove FE counters
> > run IR instrumentation pass
> > emit a side table mapping IR instr counters to FE counters
> 
> Currently, -instrprof happens early in the pipeline. IIUC this is done to
> allow the optimizer to work with load+add+stores, instead of profile update
> intrinsics.
> 
> Say we introduce a counter remapping pass like the one Sean suggested. It
> should be run before -instrprof so that we don't waste time lowering a
bunch
> of instrprof_increment intrinsics which we'll have to throw away later.
> 
> But that means that the CFGs that the counter remapping pass operates on
won't
> reflect changes made by the inliner (or any other optimizations which alter
the
> CFG), right?
> 
> ISTM the pruning pass I've proposed is useful whether we're doing
FE-based
> instrumentation _or_ late instrumentation. Since it operates on
loads+stores
> directly, it can clean up redundant counter increments at any point in the
> pipeline (after -instrprof).
> 
> 
> What I suggested would operate on loads/stores and could be used for
cleanup at any point in the pipeline too.
> 
> It would just be a generic transformation "okay, we have some set of
counter updates chosen earlier in the pipeline that are now mangled / made
redundant by further optimization; let's re-run counter placement and emit
some side tables mapping the newly chosen counters to the original
counters". I'm not sure how complicated it is to reconstruct one set of
counters from the other (just linear combinations, I think? So hopefully not too
complicated).
I think I understand what you're proposing.

To not break code coverage, we would need to emit the 'side table' you
described
into the binary (so that we can correctly evaluate counter expressions).

While I think this is feasible and can get better results, it might not *also*
be
simpler than running pass I described. IMO what you've described would take
more
work to implement and would put a higher burden on the compiler (the pipeline
looks
something like -instrprof, -remove-counters, -pgo-instr-gen, -remap-counters).
That's not a bad thing -- just something to be aware of.

vedant

> In other words, what I was suggesting is basically a generalization of
Mehdi's "fused" counter transformation.
> 
> Ideally this would be a reusable transformation. We could run it one or
multiple times throughout the optimization pipeline, and probably want to always
run it at least once at the very end before codegen. (note that you only ever
need at most a single side-table mapping the "current" counters to the
"original" counters no matter how many times you re-run counter
placement, since every time you re-run counter placement you are emitting a
completely new side-table and deleting the old one, and the side table always
reconstructs the original counters).
> 
> The availability of this facility would enable frontends to be as naive as
they want about their counter placement, which is probably a net win for LLVM as
a toolkit. It would be analogous to the alloca trick that frontends use to avoid
having to construct SSA themselves: the existence of mem2reg allows them to not
have to worry about it.
> 
> -- Sean Silva
>  
> 
> 
> vedant
> 
> 
> > On Mar 10, 2016, at 7:48 PM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
> >
> >>
> >> On Mar 10, 2016, at 7:21 PM, Vedant Kumar via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi,
> >>
> >> I'd like to add a new pass to LLVM which removes redundant
profile counter
> >> updates. The goal is to speed up code coverage testing and profile
generation
> >> for PGO.
> >>
> >> I'm sending this email out to describe my approach, share some
early results,
> >> and gather feedback.
> >>
> >>
> >> Problem Overview
> >> ===============> >>
> >> A profile counter is redundant if it's incremented in exactly
the same basic
> >> blocks as some other profile counter. Consider the following
module:
> >>
> >>   local void f1() {
> >>     instrprof_increment(profc_f1);
> >>   }
> >>
> >>   void f2() {
> >>     instrprof_increment(profc_f2);
> >>     f1();
> >>   }
> >>
> >> Once the inliner runs and deletes f1, we're left with:
> >>
> >>   void f2() {
> >>     instrprof_increment(profc_f2);
> >>     instrprof_increment(profc_f1);
> >>   }
> >>
> >> Now we can say profc_f1 is redundant (or, an alias for profc_f2).
> >>
> >> I've noticed that frontend-based instrumentation can generate
many redundant
> >> profile counters. This degrades performance and increases code
size.  We can
> >> address the problem by getting rid of redundant counter updates.
The trick is
> >> to make sure we get back the same profiles.
> >>
> >>
> >> Proposed Solution
> >> ================> >>
> >> I propose a pruning pass which takes the following steps:
> >>
> >> 1. Delete functions with local linkage and only one use, if that
use is in
> >>    a profile data record.
> >>
> >>    These functions are left around by the inliner (it doesn't
know that
> >>    they're safe to delete). Deleting them reduces code size
and simplifies
> >>    subsequent analysis of profile counters.
> >
> > I think that this is something global-DCE should be teached about (if
it does not know already).
> >
> >
> >>
> >> 2. Determine which profile counters are essential.
> >
> > What is an "essential counter"?
> >
> >>
> >> 3. Erase all updates to redundant profile counters.
> >
> > When you are saying "erase", you need to actually replace
the multiple counter increment with *a new* counter, right? I.e. when replacing:
> >
> >     instrprof_increment(profc_f2);
> >     instrprof_increment(profc_f1);
> >
> > you need to emit:
> >
> >     instrprof_increment(profc_f2f1_fused);
> >
> >
> >> 4. Emit the aliases into a new section in the binary.
> >>
> >>    Aliases are represented as { Dst: i64*, Src: i64* } tuples.
Some changes
> >>    in compiler-rt are required to walk the alias section and fill
in the
> >>    correct execution counts at program exit time.
> >>
> >> This pass needs to be run after the inliner in order to be
effective.
> >>
> >> The complexity of this pass is O(N*M), where N is the number of
profile
> >> counters, and M is the average number of updates per counter.
> >
> > What you're describing here is not clear to me? But I may have
been totally misunderstanding the whole concept :)
> >
> >
> > --
> > Mehdi
> >
> >
> >> In practice it is
> >> a bit faster, since we can skip the analysis of counters which are
discovered to
> >> be redundant early on in the process.
> >>
> >>
> >> Early Results
> >> ============> >>
> >> The pruning pass results in 25% speed improvement in the example
program above
> >> (where f2 is called in a loop 10^8 times).
> >>
> >> Here is a slightly less contrived example:
> >>
> >> #include <vector>
> >> #include <algorithm>
> >> #include <cstdlib>
> >>
> >> static void escape(void *p) {
> >>   asm volatile("" : : "g"(p) :
"memory");
> >> }
> >>
> >> int main(int argc, char **argv) {
> >>   std::vector<int> V(atoi(argv[1]));
> >>   escape(reinterpret_cast<void *>(V.data()));
> >>   std::sort(V.begin(), V.end());
> >>   return V[0];
> >> }
> >>
> >> I get the following results on my desktop (10^8 elements, 5 runs
each):
> >>
> >> O3:                           0.262s
> >> O3 + PGOInstr:                0.663s
> >> O3 + PGOInstr + Pruning:      0.606s (8.6% performance win, 672
aliases)
> >> O3 + CoverageInstr:           0.690s
> >> O3 + CoverageInstr + Pruning: 0.610s (11.6% performance win, 688
aliases)
> >>
> >>
> >> Next Steps?
> >> ==========> >>
> >> Is the performance of instrumented code something we think we need
to fix?
> >> What's an acceptable compile-time overhead for running this
pruning pass? Is
> >> the general approach a non-starter for anybody?
> >>
> >> I'd like to get some feedback and gauge interest before
pursuing this further.
> >> Possible next steps include benchmarking instrumented versions of
clang and
> >> swift on the relevant test suites, running performance tests from
lnt, running
> >> compile-time tests, and measuring any code size differences.
> >>
> >>
> >> thanks
> >> vedant
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
>

On Fri, Mar 11, 2016 at 6:00 PM, Vedant Kumar <vsk at apple.com> wrote:
>
> > On Mar 11, 2016, at 5:28 PM, Sean Silva <chisophugis at
gmail.com> wrote:
> >
> >
> >
> > On Fri, Mar 11, 2016 at 12:47 PM, Vedant Kumar <vsk at
apple.com> wrote:
> > There have been a lot of responses. I'll try to summarize the
thread and
> respond
> > to some of the questions/feedback.
> >
> >
> > Summary
> > ======> >
> > 1. We should teach GlobalDCE to strip out instrumented functions which
> the
> >    inliner cannot delete.
> >
> > 2. Sean suggests adding metadata to loads/stores of counters to convey
> that
> >    they do not alias normal program data.
> >
> >    I'm not familiar with AA, but off-hand this seems like a good
idea.
> >
> > 3. Sean also suggests improving the optimizer to registerize counters
> when
> >    possible.
> >
> >    Ditto, seems like a good idea.
> >
> > 4. Sean has an alternate proposal for counter pruning which works by
> remapping
> >    FE-generated counters to counters generated by the IR
instrumentation
> pass.
> >    David likes this proposal.
> >
> >    I have some questions about this but am willing to try it. IMO, the
> pruning
> >    pass I've suggested could complement this nicely -- I don't
view them
> as
> >    mutually exclusive (though I could be wrong).
> >
> > 5. There seems to be consensus over the need to improve performance of
> >    FE-instrumented programs.
> >
> > 6. David is concerned that the proposed pruning pass is too narrow in
> scope,
> >    and would like to see results on a larger benchmark. I'll try
to get
> some
> >    numbers.
> >
> > 7. David mentioned some optimizations that are possible if we make
> coverage
> >    information "yes or no" (per line). Justin pointed out
that this
> doesn't
> >    work for a few of Apple's use cases.
> >
> >
> > Clarifications
> > =============> >
> > 1. In my original email, two of my numbers were incorrect. I listed
the
> >    sizes of the __llvm_prf_alias sections, not the number of actual
> aliases.
> >    The corrected lines are:
> >
> > O3 + PGOInstr + Pruning:      0.606s (8.6% performance win, _42_
aliases)
> > O3 + CoverageInstr + Pruning: 0.610s (11.6% performance win, _44_
> aliases)
> >
> > 2.
> >
> > >>  Determine which profile counters are essential.
> > >
> > > What is an "essential counter"?
> >
> > The pass I proposed works like this:
> >
> >   for (PC : ProfileCounters) {
> >     if (Update-Sites-Of(PC) == Update-Sites-Of(Other-PC)) {
> >       mark PC as "essential"
> >       mark Other-PC as "redundant" (i.e, alias of PC)
> >     }
> >   }
> >
> > Where Update-Sites-Of(PC) is the set of basic blocks in which PC is
> updated.
> > If there are multiple counters updated in each update site for PC,
they
> are
> > all marked as aliases in one pass.
> >
> >
> > 3.
> >
> > >> Erase all updates to redundant profile counters.
> > >
> > > When you are saying "erase", you need to actually
replace the multiple
> counter increment with *a new* counter, right? I.e. when replacing:
> > >
> > >     instrprof_increment(profc_f2);
> > >     instrprof_increment(profc_f1);
> > >
> > > you need to emit:
> > >
> > >     instrprof_increment(profc_f2f1_fused);
> >
> >
> > No. The pass only erases updates to counters which it can prove are
> > truly redundant. There is no need to create a new (or fused) counter
> > because profc_f1 == profc_f2 in all possible profiles.
> >
> > Oh, this makes it substantially less useful I would think (and I
> misunderstood your original proposal the same way that Mehdi did here).
> > I guess I'm not sure what situations your proposed transformation
would
> catch.
> > The only one I can think of is something like:
> > - function foo() has an entry counter
> > - function bar() calls foo() from a BB that has a counter
> > - function bar() is the only caller of foo() and calls it in only one
BB.
> > Can you add some debug dumping to your pass and give some real-world
> examples of counters that it merges?
>
> I still have the pruned IR from the example I brought up at the start
> of the thread:
>
>   https://ghostbin.com/paste/qus2s
>
> Look for the __llvm_prf_alias section.
>
Could you maybe digest that a bit for us to give source-level examples of
what those counters are that end up aliased?

Also, in the linked IR there is still a tremendous amount of counter
redundancy being left on the table that a "counter fusion" approach
would
avoid.
E.g.
https://ghostbin.com/paste/qus2s#L306 - 6 counter updates in the same BB
https://ghostbin.com/paste/qus2s#L286 - 3 counter updates in the same BB
https://ghostbin.com/paste/qus2s#L261 - 2 counter updates in the same BB
https://ghostbin.com/paste/qus2s#L191 - 5 counter updates in the same BB
https://ghostbin.com/paste/qus2s#L435 - 9 counter updates in the same BB
https://ghostbin.com/paste/qus2s#L373 - 2 counter updates in the same BB

It seems like the "identical counter merge" leaves a lot on the table
that
even a simple BB-at-a-time counter fusion approach would get right.

Also, notice that in that IR there are 21 BB's but 65 counters (even after
identical counter merge). This indicates that actually eliminating the
original counters entirely (besides recording a way to reconstruct them,
but that can be compressed) might actually reduce the total amount of data
in the final binary.

>
>
> > The fused counter approach (or the one I suggested which is just a
> generalization of that (see below)) seems like it can reduce the number of
> counter updates much more.
> >
> >
> > 4.
> >
> > >> The complexity of this pass is O(N*M), where N is the number
of
> profile
> > >> counters, and M is the average number of updates per counter.
> > >
> > > What you're describing here is not clear to me? But I may
have been
> totally misunderstanding the whole concept :)
> >
> > Hopefully the pseudo-code helps? Alternatively I can put a WIP patch
up
> > on Phab.
> >
> > > It should be just O(N) where N is the number of instructions.
> >
> > The complexity of the pass is not upper-bounded by the number of
> > instructions in the program because certain loads and stores can be
> visited
> > more than once.
> >
> >
> > FE to IR Counter Remapping
> > =========================> >
> > I have a question about this plan:
> >
> > > for each CFG edge:
> > >     record which FE counters have ended up associated with it
> > > remove FE counters
> > > run IR instrumentation pass
> > > emit a side table mapping IR instr counters to FE counters
> >
> > Currently, -instrprof happens early in the pipeline. IIUC this is done
to
> > allow the optimizer to work with load+add+stores, instead of profile
> update
> > intrinsics.
> >
> > Say we introduce a counter remapping pass like the one Sean suggested.
It
> > should be run before -instrprof so that we don't waste time
lowering a
> bunch
> > of instrprof_increment intrinsics which we'll have to throw away
later.
> >
> > But that means that the CFGs that the counter remapping pass operates
on
> won't
> > reflect changes made by the inliner (or any other optimizations which
> alter the
> > CFG), right?
> >
> > ISTM the pruning pass I've proposed is useful whether we're
doing
> FE-based
> > instrumentation _or_ late instrumentation. Since it operates on
> loads+stores
> > directly, it can clean up redundant counter increments at any point in
> the
> > pipeline (after -instrprof).
> >
> >
> > What I suggested would operate on loads/stores and could be used for
> cleanup at any point in the pipeline too.
> >
> > It would just be a generic transformation "okay, we have some set
of
> counter updates chosen earlier in the pipeline that are now mangled / made
> redundant by further optimization; let's re-run counter placement and
emit
> some side tables mapping the newly chosen counters to the original
> counters". I'm not sure how complicated it is to reconstruct one
set of
> counters from the other (just linear combinations, I think? So hopefully
> not too complicated).
>
> I think I understand what you're proposing.
>
> To not break code coverage, we would need to emit the 'side table'
you
> described
> into the binary (so that we can correctly evaluate counter expressions).
>
Yes. Otherwise the original counters cannot be reconstructed. This is no
different from emitting the aliases needed by the "identical counter
merge"
approach.

>
> While I think this is feasible and can get better results, it might not
> *also* be
> simpler than running pass I described. IMO what you've described would
> take more
> work to implement and would put a higher burden on the compiler (the
> pipeline looks
> something like -instrprof, -remove-counters, -pgo-instr-gen,
> -remap-counters).
> That's not a bad thing -- just something to be aware of.
>
FWIW, I was imagining that this would be a single pass that would call into
the IR instr gen as a utility (some refactoring would probably be needed,
but that's doable).

I expect it to be more work to implement the fully general thing I
described. But implementing a BB-at-a-time counter fusion I think would
likely be not much more complicated than "identical counter merge". It
would just be:

for each BB:
    collect all counter updates in this BB, together with their
multiplicities
    create new counter
    emit side-table data that relates the new counter to an array of (other
counter, multiplicity of update)

The runtime just emits the side-table and then llvm-profdata does:

for each counter C:
    for (otherCounter, multiplicity) in side-table[C]:
        counters[otherCounter] += multiplicity * counters[C]

-- Sean Silva

>
> vedant
>
>
> > In other words, what I was suggesting is basically a generalization of
> Mehdi's "fused" counter transformation.
> >
> > Ideally this would be a reusable transformation. We could run it one
or
> multiple times throughout the optimization pipeline, and probably want to
> always run it at least once at the very end before codegen. (note that you
> only ever need at most a single side-table mapping the "current"
counters
> to the "original" counters no matter how many times you re-run
counter
> placement, since every time you re-run counter placement you are emitting a
> completely new side-table and deleting the old one, and the side table
> always reconstructs the original counters).
> >
> > The availability of this facility would enable frontends to be as
naive
> as they want about their counter placement, which is probably a net win for
> LLVM as a toolkit. It would be analogous to the alloca trick that frontends
> use to avoid having to construct SSA themselves: the existence of mem2reg
> allows them to not have to worry about it.
> >
> > -- Sean Silva
> >
> >
> >
> > vedant
> >
> >
> > > On Mar 10, 2016, at 7:48 PM, Mehdi Amini <mehdi.amini at
apple.com>
> wrote:
> > >
> > >>
> > >> On Mar 10, 2016, at 7:21 PM, Vedant Kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > >>
> > >> Hi,
> > >>
> > >> I'd like to add a new pass to LLVM which removes
redundant profile
> counter
> > >> updates. The goal is to speed up code coverage testing and
profile
> generation
> > >> for PGO.
> > >>
> > >> I'm sending this email out to describe my approach, share
some early
> results,
> > >> and gather feedback.
> > >>
> > >>
> > >> Problem Overview
> > >> ===============> > >>
> > >> A profile counter is redundant if it's incremented in
exactly the
> same basic
> > >> blocks as some other profile counter. Consider the following
module:
> > >>
> > >>   local void f1() {
> > >>     instrprof_increment(profc_f1);
> > >>   }
> > >>
> > >>   void f2() {
> > >>     instrprof_increment(profc_f2);
> > >>     f1();
> > >>   }
> > >>
> > >> Once the inliner runs and deletes f1, we're left with:
> > >>
> > >>   void f2() {
> > >>     instrprof_increment(profc_f2);
> > >>     instrprof_increment(profc_f1);
> > >>   }
> > >>
> > >> Now we can say profc_f1 is redundant (or, an alias for
profc_f2).
> > >>
> > >> I've noticed that frontend-based instrumentation can
generate many
> redundant
> > >> profile counters. This degrades performance and increases
code size.
> We can
> > >> address the problem by getting rid of redundant counter
updates. The
> trick is
> > >> to make sure we get back the same profiles.
> > >>
> > >>
> > >> Proposed Solution
> > >> ================> > >>
> > >> I propose a pruning pass which takes the following steps:
> > >>
> > >> 1. Delete functions with local linkage and only one use, if
that use
> is in
> > >>    a profile data record.
> > >>
> > >>    These functions are left around by the inliner (it
doesn't know
> that
> > >>    they're safe to delete). Deleting them reduces code
size and
> simplifies
> > >>    subsequent analysis of profile counters.
> > >
> > > I think that this is something global-DCE should be teached about
(if
> it does not know already).
> > >
> > >
> > >>
> > >> 2. Determine which profile counters are essential.
> > >
> > > What is an "essential counter"?
> > >
> > >>
> > >> 3. Erase all updates to redundant profile counters.
> > >
> > > When you are saying "erase", you need to actually
replace the multiple
> counter increment with *a new* counter, right? I.e. when replacing:
> > >
> > >     instrprof_increment(profc_f2);
> > >     instrprof_increment(profc_f1);
> > >
> > > you need to emit:
> > >
> > >     instrprof_increment(profc_f2f1_fused);
> > >
> > >
> > >> 4. Emit the aliases into a new section in the binary.
> > >>
> > >>    Aliases are represented as { Dst: i64*, Src: i64* }
tuples. Some
> changes
> > >>    in compiler-rt are required to walk the alias section and
fill in
> the
> > >>    correct execution counts at program exit time.
> > >>
> > >> This pass needs to be run after the inliner in order to be
effective.
> > >>
> > >> The complexity of this pass is O(N*M), where N is the number
of
> profile
> > >> counters, and M is the average number of updates per counter.
> > >
> > > What you're describing here is not clear to me? But I may
have been
> totally misunderstanding the whole concept :)
> > >
> > >
> > > --
> > > Mehdi
> > >
> > >
> > >> In practice it is
> > >> a bit faster, since we can skip the analysis of counters
which are
> discovered to
> > >> be redundant early on in the process.
> > >>
> > >>
> > >> Early Results
> > >> ============> > >>
> > >> The pruning pass results in 25% speed improvement in the
example
> program above
> > >> (where f2 is called in a loop 10^8 times).
> > >>
> > >> Here is a slightly less contrived example:
> > >>
> > >> #include <vector>
> > >> #include <algorithm>
> > >> #include <cstdlib>
> > >>
> > >> static void escape(void *p) {
> > >>   asm volatile("" : : "g"(p) :
"memory");
> > >> }
> > >>
> > >> int main(int argc, char **argv) {
> > >>   std::vector<int> V(atoi(argv[1]));
> > >>   escape(reinterpret_cast<void *>(V.data()));
> > >>   std::sort(V.begin(), V.end());
> > >>   return V[0];
> > >> }
> > >>
> > >> I get the following results on my desktop (10^8 elements, 5
runs
> each):
> > >>
> > >> O3:                           0.262s
> > >> O3 + PGOInstr:                0.663s
> > >> O3 + PGOInstr + Pruning:      0.606s (8.6% performance win,
672
> aliases)
> > >> O3 + CoverageInstr:           0.690s
> > >> O3 + CoverageInstr + Pruning: 0.610s (11.6% performance win,
688
> aliases)
> > >>
> > >>
> > >> Next Steps?
> > >> ==========> > >>
> > >> Is the performance of instrumented code something we think we
need to
> fix?
> > >> What's an acceptable compile-time overhead for running
this pruning
> pass? Is
> > >> the general approach a non-starter for anybody?
> > >>
> > >> I'd like to get some feedback and gauge interest before
pursuing this
> further.
> > >> Possible next steps include benchmarking instrumented
versions of
> clang and
> > >> swift on the relevant test suites, running performance tests
from
> lnt, running
> > >> compile-time tests, and measuring any code size differences.
> > >>
> > >>
> > >> thanks
> > >> vedant
> > >>
> > >> _______________________________________________
> > >> LLVM Developers mailing list
> > >> llvm-dev at lists.llvm.org
> > >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160311/6b7801e3/attachment.html>

On Fri, Mar 11, 2016 at 7:04 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Fri, Mar 11, 2016 at 6:00 PM, Vedant Kumar <vsk at apple.com>
wrote:
>
>>
>> > On Mar 11, 2016, at 5:28 PM, Sean Silva <chisophugis at
gmail.com> wrote:
>> >
>> >
>> >
>> > On Fri, Mar 11, 2016 at 12:47 PM, Vedant Kumar <vsk at
apple.com> wrote:
>> > There have been a lot of responses. I'll try to summarize the
thread
>> and respond
>> > to some of the questions/feedback.
>> >
>> >
>> > Summary
>> > ======>> >
>> > 1. We should teach GlobalDCE to strip out instrumented functions
which
>> the
>> >    inliner cannot delete.
>> >
>> > 2. Sean suggests adding metadata to loads/stores of counters to
convey
>> that
>> >    they do not alias normal program data.
>> >
>> >    I'm not familiar with AA, but off-hand this seems like a
good idea.
>> >
>> > 3. Sean also suggests improving the optimizer to registerize
counters
>> when
>> >    possible.
>> >
>> >    Ditto, seems like a good idea.
>> >
>> > 4. Sean has an alternate proposal for counter pruning which works
by
>> remapping
>> >    FE-generated counters to counters generated by the IR
>> instrumentation pass.
>> >    David likes this proposal.
>> >
>> >    I have some questions about this but am willing to try it. IMO,
the
>> pruning
>> >    pass I've suggested could complement this nicely -- I
don't view
>> them as
>> >    mutually exclusive (though I could be wrong).
>> >
>> > 5. There seems to be consensus over the need to improve
performance of
>> >    FE-instrumented programs.
>> >
>> > 6. David is concerned that the proposed pruning pass is too narrow
in
>> scope,
>> >    and would like to see results on a larger benchmark. I'll
try to get
>> some
>> >    numbers.
>> >
>> > 7. David mentioned some optimizations that are possible if we make
>> coverage
>> >    information "yes or no" (per line). Justin pointed
out that this
>> doesn't
>> >    work for a few of Apple's use cases.
>> >
>> >
>> > Clarifications
>> > =============>> >
>> > 1. In my original email, two of my numbers were incorrect. I
listed the
>> >    sizes of the __llvm_prf_alias sections, not the number of
actual
>> aliases.
>> >    The corrected lines are:
>> >
>> > O3 + PGOInstr + Pruning:      0.606s (8.6% performance win, _42_
>> aliases)
>> > O3 + CoverageInstr + Pruning: 0.610s (11.6% performance win, _44_
>> aliases)
>> >
>> > 2.
>> >
>> > >>  Determine which profile counters are essential.
>> > >
>> > > What is an "essential counter"?
>> >
>> > The pass I proposed works like this:
>> >
>> >   for (PC : ProfileCounters) {
>> >     if (Update-Sites-Of(PC) == Update-Sites-Of(Other-PC)) {
>> >       mark PC as "essential"
>> >       mark Other-PC as "redundant" (i.e, alias of PC)
>> >     }
>> >   }
>> >
>> > Where Update-Sites-Of(PC) is the set of basic blocks in which PC
is
>> updated.
>> > If there are multiple counters updated in each update site for PC,
they
>> are
>> > all marked as aliases in one pass.
>> >
>> >
>> > 3.
>> >
>> > >> Erase all updates to redundant profile counters.
>> > >
>> > > When you are saying "erase", you need to actually
replace the
>> multiple counter increment with *a new* counter, right? I.e. when
replacing:
>> > >
>> > >     instrprof_increment(profc_f2);
>> > >     instrprof_increment(profc_f1);
>> > >
>> > > you need to emit:
>> > >
>> > >     instrprof_increment(profc_f2f1_fused);
>> >
>> >
>> > No. The pass only erases updates to counters which it can prove
are
>> > truly redundant. There is no need to create a new (or fused)
counter
>> > because profc_f1 == profc_f2 in all possible profiles.
>> >
>> > Oh, this makes it substantially less useful I would think (and I
>> misunderstood your original proposal the same way that Mehdi did here).
>> > I guess I'm not sure what situations your proposed
transformation would
>> catch.
>> > The only one I can think of is something like:
>> > - function foo() has an entry counter
>> > - function bar() calls foo() from a BB that has a counter
>> > - function bar() is the only caller of foo() and calls it in only
one
>> BB.
>> > Can you add some debug dumping to your pass and give some
real-world
>> examples of counters that it merges?
>>
>> I still have the pruned IR from the example I brought up at the start
>> of the thread:
>>
>>   https://ghostbin.com/paste/qus2s
>>
>> Look for the __llvm_prf_alias section.
>>
>
> Could you maybe digest that a bit for us to give source-level examples of
> what those counters are that end up aliased?
>
> Also, in the linked IR there is still a tremendous amount of counter
> redundancy being left on the table that a "counter fusion"
approach would
> avoid.
> E.g.
> https://ghostbin.com/paste/qus2s#L306 - 6 counter updates in the same BB
> https://ghostbin.com/paste/qus2s#L286 - 3 counter updates in the same BB
> https://ghostbin.com/paste/qus2s#L261 - 2 counter updates in the same BB
> https://ghostbin.com/paste/qus2s#L191 - 5 counter updates in the same BB
> https://ghostbin.com/paste/qus2s#L435 - 9 counter updates in the same BB
> https://ghostbin.com/paste/qus2s#L373 - 2 counter updates in the same BB
>
> It seems like the "identical counter merge" leaves a lot on the
table that
> even a simple BB-at-a-time counter fusion approach would get right.
>
> Also, notice that in that IR there are 21 BB's but 65 counters (even
after
> identical counter merge). This indicates that actually eliminating the
> original counters entirely (besides recording a way to reconstruct them,
> but that can be compressed) might actually reduce the total amount of data
> in the final binary.
>
>
>>
>>
>> > The fused counter approach (or the one I suggested which is just a
>> generalization of that (see below)) seems like it can reduce the number
of
>> counter updates much more.
>> >
>> >
>> > 4.
>> >
>> > >> The complexity of this pass is O(N*M), where N is the
number of
>> profile
>> > >> counters, and M is the average number of updates per
counter.
>> > >
>> > > What you're describing here is not clear to me? But I may
have been
>> totally misunderstanding the whole concept :)
>> >
>> > Hopefully the pseudo-code helps? Alternatively I can put a WIP
patch up
>> > on Phab.
>> >
>> > > It should be just O(N) where N is the number of instructions.
>> >
>> > The complexity of the pass is not upper-bounded by the number of
>> > instructions in the program because certain loads and stores can
be
>> visited
>> > more than once.
>> >
>> >
>> > FE to IR Counter Remapping
>> > =========================>> >
>> > I have a question about this plan:
>> >
>> > > for each CFG edge:
>> > >     record which FE counters have ended up associated with it
>> > > remove FE counters
>> > > run IR instrumentation pass
>> > > emit a side table mapping IR instr counters to FE counters
>> >
>> > Currently, -instrprof happens early in the pipeline. IIUC this is
done
>> to
>> > allow the optimizer to work with load+add+stores, instead of
profile
>> update
>> > intrinsics.
>> >
>> > Say we introduce a counter remapping pass like the one Sean
suggested.
>> It
>> > should be run before -instrprof so that we don't waste time
lowering a
>> bunch
>> > of instrprof_increment intrinsics which we'll have to throw
away later.
>> >
>> > But that means that the CFGs that the counter remapping pass
operates
>> on won't
>> > reflect changes made by the inliner (or any other optimizations
which
>> alter the
>> > CFG), right?
>> >
>> > ISTM the pruning pass I've proposed is useful whether
we're doing
>> FE-based
>> > instrumentation _or_ late instrumentation. Since it operates on
>> loads+stores
>> > directly, it can clean up redundant counter increments at any
point in
>> the
>> > pipeline (after -instrprof).
>> >
>> >
>> > What I suggested would operate on loads/stores and could be used
for
>> cleanup at any point in the pipeline too.
>> >
>> > It would just be a generic transformation "okay, we have some
set of
>> counter updates chosen earlier in the pipeline that are now mangled /
made
>> redundant by further optimization; let's re-run counter placement
and emit
>> some side tables mapping the newly chosen counters to the original
>> counters". I'm not sure how complicated it is to reconstruct
one set of
>> counters from the other (just linear combinations, I think? So
hopefully
>> not too complicated).
>>
>> I think I understand what you're proposing.
>>
>> To not break code coverage, we would need to emit the 'side
table' you
>> described
>> into the binary (so that we can correctly evaluate counter
expressions).
>>
>
> Yes. Otherwise the original counters cannot be reconstructed. This is no
> different from emitting the aliases needed by the "identical counter
merge"
> approach.
>
>
>>
>> While I think this is feasible and can get better results, it might not
>> *also* be
>> simpler than running pass I described. IMO what you've described
would
>> take more
>> work to implement and would put a higher burden on the compiler (the
>> pipeline looks
>> something like -instrprof, -remove-counters, -pgo-instr-gen,
>> -remap-counters).
>> That's not a bad thing -- just something to be aware of.
>>
>
> FWIW, I was imagining that this would be a single pass that would call
> into the IR instr gen as a utility (some refactoring would probably be
> needed, but that's doable).
>
> I expect it to be more work to implement the fully general thing I
> described. But implementing a BB-at-a-time counter fusion I think would
> likely be not much more complicated than "identical counter
merge". It
> would just be:
>
> for each BB:
>     collect all counter updates in this BB, together with their
> multiplicities
>     create new counter
>     emit side-table data that relates the new counter to an array of
> (other counter, multiplicity of update)
>
> The runtime just emits the side-table and then llvm-profdata does:
>
> for each counter C:
>     for (otherCounter, multiplicity) in side-table[C]:
>         counters[otherCounter] += multiplicity * counters[C]
>
>
There are other issues that can complicate the matter.

1) The assumption in the algorithm is that the source counter has only one
update site -- but instead it may have more than one sites update due to
inlining.  Only the counts collected in the out-of-line function instance's
update site can be safely propagated to the 'fused' counters.
2) the 'source' BB may not even have a 'source' counter owned by
the caller
function created -- so 'implicit' counters need to be used
3) The side table can be huge for large applications.

IMO, doing post-cleanup (other than enhancing generic optimizations that
can be applied to other globals)  is not the right approach.

David



> -- Sean Silva
>
>
>>
>> vedant
>>
>>
>> > In other words, what I was suggesting is basically a
generalization of
>> Mehdi's "fused" counter transformation.
>> >
>> > Ideally this would be a reusable transformation. We could run it
one or
>> multiple times throughout the optimization pipeline, and probably want
to
>> always run it at least once at the very end before codegen. (note that
you
>> only ever need at most a single side-table mapping the
"current" counters
>> to the "original" counters no matter how many times you
re-run counter
>> placement, since every time you re-run counter placement you are
emitting a
>> completely new side-table and deleting the old one, and the side table
>> always reconstructs the original counters).
>> >
>> > The availability of this facility would enable frontends to be as
naive
>> as they want about their counter placement, which is probably a net win
for
>> LLVM as a toolkit. It would be analogous to the alloca trick that
frontends
>> use to avoid having to construct SSA themselves: the existence of
mem2reg
>> allows them to not have to worry about it.
>> >
>> > -- Sean Silva
>> >
>> >
>> >
>> > vedant
>> >
>> >
>> > > On Mar 10, 2016, at 7:48 PM, Mehdi Amini <mehdi.amini at
apple.com>
>> wrote:
>> > >
>> > >>
>> > >> On Mar 10, 2016, at 7:21 PM, Vedant Kumar via llvm-dev
<
>> llvm-dev at lists.llvm.org> wrote:
>> > >>
>> > >> Hi,
>> > >>
>> > >> I'd like to add a new pass to LLVM which removes
redundant profile
>> counter
>> > >> updates. The goal is to speed up code coverage testing
and profile
>> generation
>> > >> for PGO.
>> > >>
>> > >> I'm sending this email out to describe my approach,
share some early
>> results,
>> > >> and gather feedback.
>> > >>
>> > >>
>> > >> Problem Overview
>> > >> ===============>> > >>
>> > >> A profile counter is redundant if it's incremented in
exactly the
>> same basic
>> > >> blocks as some other profile counter. Consider the
following module:
>> > >>
>> > >>   local void f1() {
>> > >>     instrprof_increment(profc_f1);
>> > >>   }
>> > >>
>> > >>   void f2() {
>> > >>     instrprof_increment(profc_f2);
>> > >>     f1();
>> > >>   }
>> > >>
>> > >> Once the inliner runs and deletes f1, we're left
with:
>> > >>
>> > >>   void f2() {
>> > >>     instrprof_increment(profc_f2);
>> > >>     instrprof_increment(profc_f1);
>> > >>   }
>> > >>
>> > >> Now we can say profc_f1 is redundant (or, an alias for
profc_f2).
>> > >>
>> > >> I've noticed that frontend-based instrumentation can
generate many
>> redundant
>> > >> profile counters. This degrades performance and increases
code
>> size.  We can
>> > >> address the problem by getting rid of redundant counter
updates. The
>> trick is
>> > >> to make sure we get back the same profiles.
>> > >>
>> > >>
>> > >> Proposed Solution
>> > >> ================>> > >>
>> > >> I propose a pruning pass which takes the following steps:
>> > >>
>> > >> 1. Delete functions with local linkage and only one use,
if that use
>> is in
>> > >>    a profile data record.
>> > >>
>> > >>    These functions are left around by the inliner (it
doesn't know
>> that
>> > >>    they're safe to delete). Deleting them reduces
code size and
>> simplifies
>> > >>    subsequent analysis of profile counters.
>> > >
>> > > I think that this is something global-DCE should be teached
about (if
>> it does not know already).
>> > >
>> > >
>> > >>
>> > >> 2. Determine which profile counters are essential.
>> > >
>> > > What is an "essential counter"?
>> > >
>> > >>
>> > >> 3. Erase all updates to redundant profile counters.
>> > >
>> > > When you are saying "erase", you need to actually
replace the
>> multiple counter increment with *a new* counter, right? I.e. when
replacing:
>> > >
>> > >     instrprof_increment(profc_f2);
>> > >     instrprof_increment(profc_f1);
>> > >
>> > > you need to emit:
>> > >
>> > >     instrprof_increment(profc_f2f1_fused);
>> > >
>> > >
>> > >> 4. Emit the aliases into a new section in the binary.
>> > >>
>> > >>    Aliases are represented as { Dst: i64*, Src: i64* }
tuples. Some
>> changes
>> > >>    in compiler-rt are required to walk the alias section
and fill in
>> the
>> > >>    correct execution counts at program exit time.
>> > >>
>> > >> This pass needs to be run after the inliner in order to
be effective.
>> > >>
>> > >> The complexity of this pass is O(N*M), where N is the
number of
>> profile
>> > >> counters, and M is the average number of updates per
counter.
>> > >
>> > > What you're describing here is not clear to me? But I may
have been
>> totally misunderstanding the whole concept :)
>> > >
>> > >
>> > > --
>> > > Mehdi
>> > >
>> > >
>> > >> In practice it is
>> > >> a bit faster, since we can skip the analysis of counters
which are
>> discovered to
>> > >> be redundant early on in the process.
>> > >>
>> > >>
>> > >> Early Results
>> > >> ============>> > >>
>> > >> The pruning pass results in 25% speed improvement in the
example
>> program above
>> > >> (where f2 is called in a loop 10^8 times).
>> > >>
>> > >> Here is a slightly less contrived example:
>> > >>
>> > >> #include <vector>
>> > >> #include <algorithm>
>> > >> #include <cstdlib>
>> > >>
>> > >> static void escape(void *p) {
>> > >>   asm volatile("" : : "g"(p) :
"memory");
>> > >> }
>> > >>
>> > >> int main(int argc, char **argv) {
>> > >>   std::vector<int> V(atoi(argv[1]));
>> > >>   escape(reinterpret_cast<void *>(V.data()));
>> > >>   std::sort(V.begin(), V.end());
>> > >>   return V[0];
>> > >> }
>> > >>
>> > >> I get the following results on my desktop (10^8 elements,
5 runs
>> each):
>> > >>
>> > >> O3:                           0.262s
>> > >> O3 + PGOInstr:                0.663s
>> > >> O3 + PGOInstr + Pruning:      0.606s (8.6% performance
win, 672
>> aliases)
>> > >> O3 + CoverageInstr:           0.690s
>> > >> O3 + CoverageInstr + Pruning: 0.610s (11.6% performance
win, 688
>> aliases)
>> > >>
>> > >>
>> > >> Next Steps?
>> > >> ==========>> > >>
>> > >> Is the performance of instrumented code something we
think we need
>> to fix?
>> > >> What's an acceptable compile-time overhead for
running this pruning
>> pass? Is
>> > >> the general approach a non-starter for anybody?
>> > >>
>> > >> I'd like to get some feedback and gauge interest
before pursuing
>> this further.
>> > >> Possible next steps include benchmarking instrumented
versions of
>> clang and
>> > >> swift on the relevant test suites, running performance
tests from
>> lnt, running
>> > >> compile-time tests, and measuring any code size
differences.
>> > >>
>> > >>
>> > >> thanks
>> > >> vedant
>> > >>
>> > >> _______________________________________________
>> > >> LLVM Developers mailing list
>> > >> llvm-dev at lists.llvm.org
>> > >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> >
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160311/32073d0c/attachment.html>