[llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default

Thu Feb 21 00:23:23 PST 2019

Hi Roman,

As Alina has pointed out, we do MSSA update wrong in general in this patch. I’m going to prepare a fix in ~3 hours from now. I’ll send it to you for review when it’s ready.

--Max

From: rtereshin at apple.com <rtereshin at apple.com>
Sent: Thursday, February 21, 2019 2:41 PM
To: Maxim Kazantsev <max.kazantsev at azul.com>
Cc: Alina Sbirlea <alina.sbirlea at gmail.com>; dberlin at dberlin.org; llvm-commits <llvm-commits at lists.llvm.org>; asbirlea at google.com
Subject: Re: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default

Hi Max,

Thank you for replying so quickly!

I’ve applied the change locally and the original IR module I used to bugpoint the reproducer I’ve sent you earlier failed anyway, this time for a different reason.
I can’t send the module as it is for IP reasons, but here’s another bugpoint from the same source (with some manual tidying up):

define void @main(i32* %a, i32* %b) {
entry:
  br label %for.body

for.body:
  %i = phi i32 [ 0, %entry ], [ %i.inc, %latch ]
  br label %switch.bb

switch.bb:
  switch i2 1, label %default [
    i2 1, label %case
  ]

case:
  br label %latch

default:
  unreachable

latch:
  store i32 %i, i32* %a
  store i32 %i, i32* %b
  %i.inc = add nsw i32 %i, 1
  %exitcond = icmp eq i32 %i.inc, 4
  br i1 %exitcond, label %exit, label %for.body

exit:
  ret void
}

./bin/opt -loop-simplifycfg loop-simplifycfg-term-folding-AND-mssa-loop-dependency-2.ll -enable-mssa-loop-dependency=true -verify-memoryssa=true -enable-loop-simplifycfg-term-folding=true -S -o -
Assertion failed: (dominates(MD, U) && "Memory Def does not dominate it's uses"), function verifyDomination, file ../lib/Analysis/MemorySSA.cpp, line 1912.

If I don’t apply https://reviews.llvm.org/rL354547 it fails the same way the previous reproducer did, so I guess this test is strictly better. Sorry for overbugpointing the first time around!

Thanks,
Roman

On Feb 20, 2019, at 10:12 PM, Alina Sbirlea <alina.sbirlea at gmail.com<mailto:alina.sbirlea at gmail.com>> wrote:

Are you sure your update is complete?
It should theoretically match the DT update, i.e. include insert edges not just the delete. All updates should be done together using the same list of updates used for DT, and an up to date DT (see MSSAU->applyUpdates(DTUpdates, DT)).

On Wed, Feb 20, 2019 at 9:53 PM Maxim Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:

Should be fixed by https://reviews.llvm.org/rL354547

From: Maxim Kazantsev
Sent: Thursday, February 21, 2019 11:49 AM
To: 'rtereshin at apple.com<mailto:rtereshin at apple.com>' <rtereshin at apple.com<mailto:rtereshin at apple.com>>
Cc: llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>>; dberlin at dberlin.org<mailto:dberlin at dberlin.org>; asbirlea at google.com<mailto:asbirlea at google.com>
Subject: RE: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default

Hi Roman,

Thanks for notification! I’ll fix it asap.

--Max

From: rtereshin at apple.com<mailto:rtereshin at apple.com> <rtereshin at apple.com<mailto:rtereshin at apple.com>>
Sent: Thursday, February 21, 2019 10:25 AM
To: Maxim Kazantsev <max.kazantsev at azul.com<mailto:max.kazantsev at azul.com>>
Cc: llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>>; dberlin at dberlin.org<mailto:dberlin at dberlin.org>; asbirlea at google.com<mailto:asbirlea at google.com>
Subject: Re: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default

Hi Max,

Looks like this doesn’t collaborate with MemorySSA very well:

./bin/opt -loop-simplifycfg loop-simplifycfg-term-folding-AND-mssa-loop-dependency.ll -enable-mssa-loop-dependency=true -verify-memoryssa=true -enable-loop-simplifycfg-term-folding=true -S -o -
Assertion failed: (find(predecessors(&B), Phi->getIncomingBlock(I)) != pred_end(&B) && "Incoming phi block not a block predecessor"), function verifyDefUses, file ../lib/Analysis/MemorySSA.cpp, line 1947.

The IR is:

define void @main() {
entry:
  br label %for.body

for.body:                                         ; preds = %exit, %entry
  br label %switch.bb<http://switch.bb/>

switch.bb<http://switch.bb/>:                                        ; preds = %for.body
  switch i2 1, label %default.bb<http://default.bb/> [
    i2 1, label %case.bb<http://case.bb/>
  ]

case.bb<http://case.bb/>:                                          ; preds = %switch
  br label %exit

default.bb<http://default.bb/>:                                       ; preds = %switch
  unreachable

exit:                                             ; preds = %case.bb<http://case.bb/>
  call void @foo()
  br label %for.body
}

declare void @foo()

Thanks,
Roman

> On Feb 12, 2019, at 10:12 PM, Max Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
>
> Author: mkazantsev
> Date: Tue Feb 12 22:12:48 2019
> New Revision: 353911
>
> URL: http://llvm.org/viewvc/llvm-project?rev=353911&view=rev
> Log:
> [LoopSimplifyCFG] Re-enable const branch folding by default
>
> Known underlying bugs have been fixed, intensive fuzz testing did not
> find any new problems. Re-enabling by default. Feel free to revert if
> it causes any functional failures.
>
> Modified:
>    llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp?rev=353911&r1=353910&r2=353911&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Tue Feb 12 22:12:48 2019
> @@ -41,7 +41,7 @@ using namespace llvm;
> #define DEBUG_TYPE "loop-simplifycfg"
>
> static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
> -                                       cl::init(false));
> +                                       cl::init(true));
>
> STATISTIC(NumTerminatorsFolded,
>           "Number of terminators folded to unconditional branches");
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>
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