llvm dev - Jan 2018 - RFC: attribute synthetic("reason")

(Forwarding response to llvm-dev with Dave's permission.)
> On Jan 10, 2018, at 11:19 AM, David Blaikie <dblaikie at gmail.com>
wrote:
> Optnone stops these things, doesnt it? But then, as you say, you'd have
to find some way to tunnels true optnone through to the coroutine expansion
pass.
> 
Right.  More importantly, we definitely want to allow internal optimization of
the coroutine function — mem2reg, GVN, etc. — prior to lowering.  That should
include inlining *into* it.  The attribute should only block propagating
information *out of* the function implementation.

John.
> 
> On Tue., 9 Jan. 2018, 10:20 pm John McCall via llvm-dev, <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Summary
> 
> I would like to propose that we add the following function attribute to
LLVM:
> 
>   synthetic(<string>)
> 
> This attribute can only be applied to functions.  It is not a semantic
statement about the function it decorates.  It is, instead, an explicit
directive to LLVM to not attempt to propagate information about the function
body outside of the function, including by changing the attributes of the
function.  The expectation is that some special pass will eventually remove the
attribute and enable normal optimization.
> 
> So, why should we add this?
> 
> Problem: coroutine structure
> 
> I've recently been working on implementing coroutines for Swift.  This
involves embracing and extending Gor's excellent work on LLVM coroutines
with an alternate code-generation pattern and ABI. (*)  LLVM doesn't
natively support coroutines, which means that a pre-split coroutine represented
in IR looks rather artificial.
> 
> (* This work has been happening out-of-tree while I qualify the design with
Swift frontend support.  That doesn't mean it's closed-source; if
you're curious, you can see the current documentation update here:
>  
https://github.com/apple/swift-llvm/blob/upstream-with-swift/docs/Coroutines.rst
<https://github.com/apple/swift-llvm/blob/upstream-with-swift/docs/Coroutines.rst>
> or just check out the implementation:
>  
https://github.com/apple/swift-llvm/tree/upstream-with-swift/lib/Transforms/Coroutines
<https://github.com/apple/swift-llvm/tree/upstream-with-swift/lib/Transforms/Coroutines>
> I promise it will be submitted to LLVM when we've proven the design to
our satisfaction by starting to use it in the frontend.  That should be within
the next few months.)
> 
> For example, here's a simple coroutine:
> 
> define i8* @f(i8* %buffer, i32 %n) {
> entry:
>   %id = call token @llvm.coro.id.retcon(i32 8, i32 4, i8* %buffer, i8*
bitcast (i8* (i8*, i1)* @prototype to i8*), i8* bitcast (i8* (i32)* @allocate to
i8*), i8* bitcast (void (i8*)* @deallocate to i8*))
>   %hdl = call i8* @llvm.coro.begin(token %id, i8* null)
>   br label %loop
> 
> loop:
>   %n.val = phi i32 [ %n, %entry ], [ %inc, %resume ]
>   call void @print(i32 %n.val)
>   %unwind0 = call i1 (...) @llvm.coro.suspend.retcon.i1()
>   br i1 %unwind0, label %cleanup, label %resume
> 
> resume:
>   %inc = add i32 %n.val, 1
>   br label %loop
> 
> cleanup:
>   call i1 @llvm.coro.end(i8* %hdl, i1 0)
>   unreachable
> }
> 
> In this function body, the coroutine intrinsics are structurally special:
they are tied to the identity of this function and cannot be independently
understood.  This is true in the same general sense that "ret" and
"unwind" are structurally special in an ordinary function.  The
control flow and semantics of this function are *internally* consistent and
correct, and accordingly most intra-procedural analyses will not pose a problem.
However, its interaction with other functions, and especially with its caller,
cannot be understood by a naive function-wide analysis which does not know about
these intrinsics.  Such an analysis is likely to take actions that would cause a
miscompile.
> 
> For example:
> 
>   - It is not legal to clone these intrinsics into another function, e.g.
during inlining.  A function is a coroutine if (and only if) it contains an
@llvm.coro.id.* call.  Cloning the 'id' call into the caller would turn
that function into a coroutine, which is not correct.  Similarly, cloning a
'suspend' into the caller would probably create malformed IR and, if
not, would be semantically incorrect in the same way as cloning a
"ret" into the caller would be.
> 
>   - Some of these intrinsics will be expanded to things that are not
ordinarily legal for functions to expand to.  For example, the llvm.coro.suspend
call will expand into a code sequence that contains a return.  The llvm.coro.end
call also expands to contain a return, but with different semantics.  An
analysis that does not know that might add the 'noreturn' attribute to
this function, which will then be incorrect after expansion.  If a caller
propagates that information to its call site, it will be badly miscompiled.
> 
> We need some way to stop these things from happening.
> 
> Should we just update existing passes to know about these intrinsics?
> 
> This is a reasonable question, but I am quite convinced the answer is
"no".
> 
> Even just considering coroutines, I think this is not really a reasonable
request.  There may be many different places that would need to updated to know
about most of these intrinsics.  Moreover, it is likely that the set of
intrinsics will grow and change over time, because the details of LLVM coroutine
lowering are very much in flux, for a pair of reasons:
> 
>   - First, there isn't a standard, widely-accepted ABI for coroutines. 
For ordinary functions, there's general agreement on concepts like
"return addresses", "return registers", "argument
registers", "argument areas", and "the stack", even if
there's a lot of target-specific variation in how to apply them.  But with
coroutines, there's a lot of room for debate about how to do even basic
things like allocating space for the execution record, yielding and resuming
control, and passing values in and out of the coroutine.  I'm already adding
a second ABI for Swift (in fact, I'm really adding two closely-related
ABIs), and more will come after that.  These ABIs have trade-offs with each
other, and I don't think that any one of them will eventually
"win"; I think we'll just have to support them all, as they get
adopted in different languages.
> 
>   - Even if there were agreement about the ABI, it's not clear to me
that there's agreement about how best to compile and optimize them.  As we
gain more experience with this, it is not unlikely that we will want to evolve
the intrinsics.  I don't think we want to encumber all this experimentation
with the need to go update a ton of analyses, especially when the right answer
across all of them is generally just "ignore the function for now".
> 
> Moreover, I doubt that coroutines are the only feature with a similar set
of problems.  My experience with compilers that have tried to use LLVM IR for
higher-level analyses and optimizations is that they often want some way of
explicitly limiting LLVM's ability to analyze their functions until
they've gotten to a certain stage in the lowering.  This becomes more and
more likely the more structurally interesting the IR gets.
> 
> Aren't there already ways to make a function opaque?
> 
> Well, yes, but they're all independently meaningful, which is a
problem.  You can block inter-procedural analysis by giving a function weak
linkage, but at the end of the day, it's not okay to make all coroutine
functions weak!  Coroutines ought to be able to have the same spectrum of
linkages, visibilities, etc. as normal functions, so if we play linkage tricks
to block analysis, we have to also find a way to tunnel the real linkage all the
way to the coroutine lowering pass.  The same thing applies for
'noinline' — there might be good reasons to make the coroutine function
'noinline' even after lowering, so we have to tunnel the real value
through.
> 
> It's possible to make the call indirect, e.g. by hiding the function
pointer behind an intrinsic just in case it gets devirtualized, but this
requires a lot of awkward casting (all of which needs to be removed in order to
make the call properly direct if possible later), and it creates a bunch of
extra IR that then needs to be removed at the right point in a careful
phase-order with the lowering.  Still, it works, which is why it's how
I'm currently trying to solve the problem.  However:
> 
> None of those approaches stops passes from changing the function attributes
based on the function body, like the (very real) example above of an analysis
that decides that the coroutine cannot return.  As far as I know, there's no
way to stop this; my current pass just has explicit code to remove
'noreturn' when doing the lowering, but that does not seem like a
satisfactory solution.
> 
> That is why I think we should have a dedicated attribute whose sole purpose
is to block analysis.
> 
> John.
> _______________________________________________
> 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
<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/20180110/4994c3a6/attachment.html>