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

Tue Jan 24 08:56:55 PST 2017

> On Jan 24, 2017, at 8:55 AM, Sanjay Patel <spatel at rotateright.com> wrote:
> 
> 
> 
> On Tue, Jan 24, 2017 at 9:24 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Jan 24, 2017, at 7:18 AM, Sanjay Patel <spatel at rotateright.com <mailto:spatel at rotateright.com>> wrote:
>> 
>> 
>> 
>> On Mon, Jan 23, 2017 at 10:53 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> 
>>> On Jan 23, 2017, at 3:48 PM, Sanjay Patel via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> All targets are likely affected in some way by the icmp+shl fold introduced with r292492. It's a basic pattern that occurs in lots of code. Did you see any perf wins on your targets with this commit?
>>> 
>>> Sadly, it is also likely that many (all?) targets are negatively impacted on the particular test (SingleSource/Benchmarks/Shootout/sieve) that you have pointed out here because the IR is now decidedly worse.
>>> 
>>> IMO, we should not revert the commit because it exposed shortcomings in the optimizer. It's an "obvious" fold/canonicalization, and the related 'nuw' variant of this fold has existed in trunk since:
>>> https://reviews.llvm.org/rL285729 <https://reviews.llvm.org/rL285729>
>>> 
>>> We need to dissect what analysis/folds are missing to restore the IR to the better form that existed before, but this is probably going to be a long process because we treat min/max like an optimization fence. 
>> 
>> If this is gonna be a long process to recover, this looks like something to be reverted in the 4.0 branch (unless I missed that there is a correctness fix involved?).
>> 
>> 
>> Nope - this is just about perf, not correctness. Of course, the intent was that this transform should only improve perf, so I wonder if we can pin any other perf changes from this commit.
>> 
>> I'm new to using the LNT site, but this should be the full set of results for the A53 machine in question with a baseline (r292491) before this patch and current (r292522) :
>> http://llvm.org/perf/db_default/v4/nts/107364 <http://llvm.org/perf/db_default/v4/nts/107364>
>> 
>> If these are reliable results, we have 2 perf wins (puzzle, gramschmidt) on the A53 machine. How do we determine the importance of the sieve benchmark vs. the rest of the suite?
>> 
>> An x86 machine doesn't show any regressions from this change:
>> http://llvm.org/perf/db_default/v4/nts/107353 <http://llvm.org/perf/db_default/v4/nts/107353>
>> 
>> Are there target-scope-based guidelines for when something is bad enough to revert?
> 
> I don’t think we have any guidelines.
> 
> I think my suggestion was more about other regression that we would discover after the release, it was more of a “maturity” call: if we just notice a problem with the commit right before the release, it may not have been in tree long enough to get enough scrutiny.
> 
> That makes sense. I have no stake in any particular branch, so I have no objection to revert from the release branch if that's what people would like to do.

+Hans for advising on what to on clang-4.0.

> My preference is to keep it in trunk though because it should be a win in theory and reverting there would make it harder to find and debug problems like this one.

Right, trunk seems fine to me at this point.

— 
Mehdi

> 
> 
> 
>  
> 
> (Also I thought this thread included a compile time regression, which on re-read it doesn’t).
> 
> — 
> Mehdi
> 
> 
>> 
>> Also, we've absolutely destroyed perf (-48%) on the sieve benchmark on that A53 target since the baseline (r256803). There are multiple things to fix before we can truly recover?
>> 
>> Regardless of whether we revert or not, I am looking at how to clawback the IR from the r292492 regression. Here's one step towards that:
>> https://reviews.llvm.org/D29053 <https://reviews.llvm.org/D29053>
>> 
>> If we get lucky, we may be able to sidestep the min/max problem by folding harder before we reach that point in the optimization pipeline.
>> 
>> 
>>  
>> — 
>> Mehdi
>> 
>> 
>> 
>>> 
>>> 
>>> 
>>> On Mon, Jan 23, 2017 at 11:13 AM, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com <mailto:Evgeny.Astigeevich at arm.com>> wrote:
>>> Confirm there is no change in IR if the hack is disabled in the sources.
>>> 
>>> David wrote that these instructions are created by SCEV.
>>> 
>>> Are other targets affected by the changes, e.g. X86?
>>> 
>>>  
>>> 
>>> Kind regards,
>>> Evgeny Astigeevich
>>> Senior Compiler Engineer
>>> Compilation Tools
>>> ARM
>>> 
>>>  
>>> 
>>> From: Sanjay Patel [mailto:spatel at rotateright.com <mailto:spatel at rotateright.com>] 
>>> Sent: Sunday, January 22, 2017 10:45 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
>>> 
>>>  
>>> 
>>> I tried an experiment to remove the integer min/max bailouts from InstCombine, and it doesn't appear to change the IR in the attachment, so I doubt there's going to be any improvement.
>>> 
>>> If I haven't messed up this example, this is amazing:
>>> https://godbolt.org/g/yzoxeY <https://godbolt.org/g/yzoxeY>
>>>  
>>> 
>>> On Sun, Jan 22, 2017 at 1:06 PM, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com <mailto:Evgeny.Astigeevich at arm.com>> wrote:
>>> 
>>> 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 <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/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/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/InstructionCombining.cpp#L803>
>>> https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L409 <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/InstCombineCompares.cpp#L4780>
>>> https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCasts.cpp#L1376 <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 <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 <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 <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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