[llvm-dev] [InstCombine] rL292492 affected LoopVectorizer and caused 17.30%/11.37% perf regressions on Cortex-A53/Cortex-A15 LNT machines

Sun Jan 22 12:06:24 PST 2017

Thank you for information.
I’ll build clang without the hack and re-run the benchmark tomorrow.

-Evgeny

From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Sunday, January 22, 2017 8:00 PM
To: Evgeny Astigeevich
Cc: llvm-dev; nd
Subject: Re: [InstCombine] rL292492 affected LoopVectorizer and caused 17.30%/11.37% perf regressions on Cortex-A53/Cortex-A15 LNT machines

> Do you mean to remove the hack in InstCombiner::visitICmpInst()?

Yes. Although (this just came up in D28625 too) we might need to remove multiple versions of that in order to unlock optimization:
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCompares.cpp#L4338
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCasts.cpp#L470
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstructionCombining.cpp#L803
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L409

Similar for FP:
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCompares.cpp#L4780
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCasts.cpp#L1376

On Sun, Jan 22, 2017 at 12:40 PM, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com<mailto:Evgeny.Astigeevich at arm.com>> wrote:
Hi Sanjay,

The benchmark source file: http://www.llvm.org/viewvc/llvm-project/test-suite/trunk/SingleSource/Benchmarks/Shootout/sieve.c?view=markup
Clang options used to produce the initial IR: clang -DNDEBUG  -O3 -DNDEBUG -mcpu=cortex-a53 -fomit-frame-pointer -O3 -DNDEBUG   -w -Werror=date-time -c sieve.c -S -emit-llvm -mllvm -disable-llvm-optzns --target=aarch64-arm-linux
Opt options: opt -O3 -o /dev/null -print-before-all -print-after-all sieve.ll >& sieve.log

I used the IR (in attached sieve.zip) created with the r292487 version.
The attached sieve contains the output of ‘-print-before-all -print-after-all’  for r292487 and rL292492.

> If it's possible, can you remove that check locally, rebuild,
> and try the benchmark again on your system? I'd love to know
> if that change alone would solve the problem.

Do you mean to remove the hack in InstCombiner::visitICmpInst()?

Kind regards,
Evgeny Astigeevich
Senior Compiler Engineer
Compilation Tools
ARM

From: Sanjay Patel [mailto:spatel at rotateright.com<mailto:spatel at rotateright.com>]
Sent: Friday, January 20, 2017 6:16 PM
To: Evgeny Astigeevich
Cc: llvm-dev; Renato Golin; t.p.northover at gmail.com<mailto:t.p.northover at gmail.com>; hfinkel at anl.gov<mailto:hfinkel at anl.gov>
Subject: Re: [InstCombine] rL292492 affected LoopVectorizer and caused 17.30%/11.37% perf regressions on Cortex-A53/Cortex-A15 LNT machines

Thanks for letting me know about this problem!

There's no 'shl nsw' visible in the earlier (r292487) code, so it would be better to see exactly what the IR looks like before that added transform fires.

But I see a red flag:
  %smax = select i1 %11, i64 %10, i64 8193
The new icmp transform allowed us to create an smax, but we have this hack in InstCombiner::visitICmpInst():

  // Test if the ICmpInst instruction is used exclusively by a select as
  // part of a minimum or maximum operation. If so, refrain from doing
  // any other folding. This helps out other analyses which understand
  // non-obfuscated minimum and maximum idioms, such as ScalarEvolution
  // and CodeGen. And in this case, at least one of the comparison
  // operands has at least one user besides the compare (the select),
  // which would often largely negate the benefit of folding anyway.
...so that prevented folding the icmp into the earlier math.
I am actively working on trying to get rid of that bail-out by improving min/max value tracking and icmp/select folding. In fact, we might be able to remove it right now, but I don't know the history of that code or what cases it was supposed to help.

If it's possible, can you remove that check locally, rebuild, and try the benchmark again on your system? I'd love to know if that change alone would solve the problem.

On Fri, Jan 20, 2017 at 10:11 AM, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com<mailto:Evgeny.Astigeevich at arm.com>> wrote:
Hi,

We found that today's 17.30%/11.37% performance regressions in LNT SingleSource/Benchmarks/Shootout/sieve on LNT-AArch64-A53-O3__clang_DEV__aarch64 and LNT-Thumb2v7-A15-O3__clang_DEV__thumbv7 (http://llvm.org/perf/db_default/v4/nts/daily_report/2017/1/20?filter-machine-regex=aarch64%7Carm%7Cthumb%7Cgreen) are caused by changes [rL292492] in InstCombine:

https://reviews.llvm.org/D28406 "[InstCombine] icmp sgt (shl nsw X, C1), C0 --> icmp sgt X, C0 >> C1"

The Loop Vectorizer generates code with more instructions:

==== Loop Vectorizer from rL292492  ====
for.body5:                                        ; preds = %for.inc16.for.body5_crit_edge, %for.cond.preheader
  %indvar = phi i64 [ %indvar.next, %for.inc16.for.body5_crit_edge ], [ 0, %for.cond.preheader ]
  %1 = phi i8 [ %.pre, %for.inc16.for.body5_crit_edge ], [ 1, %for.cond.preheader ]
  %count.122 = phi i32 [ %count.2, %for.inc16.for.body5_crit_edge ], [ 0, %for.cond.preheader ]
  %i.119 = phi i64 [ %inc17, %for.inc16.for.body5_crit_edge ], [ 2, %for.cond.preheader ]
  %2 = add i64 %indvar, 2
  %3 = shl i64 %indvar, 1
  %4 = add i64 %3, 4
  %5 = add i64 %indvar, 2
  %6 = shl i64 %indvar, 1
  %7 = add i64 %6, 4
  %8 = add i64 %indvar, 2
  %9 = mul i64 %indvar, 3
  %10 = add i64 %9, 6
  %11 = icmp sgt i64 %10, 8193
  %smax = select i1 %11, i64 %10, i64 8193
  %12 = mul i64 %indvar, -2
  %13 = add i64 %12, -5
  %14 = add i64 %smax, %13
  %15 = add i64 %indvar, 2
  %16 = udiv i64 %14, %15
  %17 = add i64 %16, 1
  %tobool7 = icmp eq i8 %1, 0
  br i1 %tobool7, label %for.inc16, label %if.then
================================

The code generated by the Loop Vectorizer before the changes:

==== Loop Vectorizer from rL292487 ====
for.body5:                                        ; preds = %for.inc16.for.body5_crit_edge, %for.cond.preheader
  %indvar = phi i64 [ %indvar.next, %for.inc16.for.body5_crit_edge ], [ 0, %for.cond.preheader ]
  %1 = phi i8 [ %.pre, %for.inc16.for.body5_crit_edge ], [ 1, %for.cond.preheader ]
  %count.122 = phi i32 [ %count.2, %for.inc16.for.body5_crit_edge ], [ 0, %for.cond.preheader ]
  %i.119 = phi i64 [ %inc17, %for.inc16.for.body5_crit_edge ], [ 2, %for.cond.preheader ]
  %2 = add i64 %indvar, 2
  %3 = shl i64 %indvar, 1
  %4 = add i64 %3, 4
  %5 = add i64 %indvar, 2
  %6 = shl i64 %indvar, 1
  %7 = add i64 %6, 4
  %8 = add i64 %indvar, 2
  %9 = mul i64 %indvar, -2
  %10 = add i64 %9, 8188
  %11 = add i64 %indvar, 2
  %12 = udiv i64 %10, %11
  %13 = add i64 %12, 1
  %tobool7 = icmp eq i8 %1, 0
  br i1 %tobool7, label %for.inc16, label %if.then
================================

I have not investigated yet why the behaviour of the Vectorizer is changed.

Kind regards,
Evgeny Astigeevich
Senior Compiler Engineer
Compilation Tools
ARM

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