[llvm] r300746 - [SCEV] Make SCEV or modeling more aggressive.
Mikhail Zolotukhin via llvm-commits
llvm-commits at lists.llvm.org
Thu Apr 20 15:10:17 PDT 2017
> On Apr 20, 2017, at 2:39 PM, Friedman, Eli <efriedma at codeaurora.org> wrote:
>
> The testcase is derived from a loop we were looking at while doing some polly-related work; not sure if it has any visible performance benefit elsewhere.
>
> That said, I wasn't expecting a compile-time regression, and the codesize regression is definitely weird. Can you get a preprocessed source file for me to look at?
Here it is:
It's one of source files of ClamAV, which is a part of LLVM test-suite. Compile time on this file increased almost 3x in my preliminary measurements (and looking at the source, function "body" has a lot of ORs, so probably it's the culprit).
Michael
>
> -Eli
>
> On 4/20/2017 1:02 PM, Mikhail Zolotukhin wrote:
>> Hi Eli,
>>
>> It looks like this change caused a big compile time and codesize regression on clamscan [1]. Is it expected, and if so, are there any gains from this change that overweigh the losses? I realize that the change itself looks right useful, but if we get nothing from it then probably we should find a way to mitigate negative effects first.
>>
>> Thanks,
>> Michael
>>
>> [1] http://104.154.54.203/db_default/v4/nts/44526
>>
>>
>>> On Apr 19, 2017, at 1:19 PM, Eli Friedman via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>>>
>>> Author: efriedma
>>> Date: Wed Apr 19 15:19:58 2017
>>> New Revision: 300746
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=300746&view=rev
>>> Log:
>>> [SCEV] Make SCEV or modeling more aggressive.
>>>
>>> Use haveNoCommonBitsSet to figure out whether an "or" instruction
>>> is equivalent to addition. This handles more cases than just
>>> checking for a constant on the RHS.
>>>
>>> Differential Revision: https://reviews.llvm.org/D32239
>>>
>>>
>>> Added:
>>> llvm/trunk/test/Analysis/ScalarEvolution/or-as-add.ll
>>> Modified:
>>> llvm/trunk/lib/Analysis/ScalarEvolution.cpp
>>>
>>> Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=300746&r1=300745&r2=300746&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
>>> +++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Wed Apr 19 15:19:58 2017
>>> @@ -5328,28 +5328,12 @@ const SCEV *ScalarEvolution::createSCEV(
>>> break;
>>>
>>> case Instruction::Or:
>>> - // If the RHS of the Or is a constant, we may have something like:
>>> - // X*4+1 which got turned into X*4|1. Handle this as an Add so loop
>>> - // optimizations will transparently handle this case.
>>> - //
>>> - // In order for this transformation to be safe, the LHS must be of the
>>> - // form X*(2^n) and the Or constant must be less than 2^n.
>>> - if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->RHS)) {
>>> - const SCEV *LHS = getSCEV(BO->LHS);
>>> - const APInt &CIVal = CI->getValue();
>>> - if (GetMinTrailingZeros(LHS) >=
>>> - (CIVal.getBitWidth() - CIVal.countLeadingZeros())) {
>>> - // Build a plain add SCEV.
>>> - const SCEV *S = getAddExpr(LHS, getSCEV(CI));
>>> - // If the LHS of the add was an addrec and it has no-wrap flags,
>>> - // transfer the no-wrap flags, since an or won't introduce a wrap.
>>> - if (const SCEVAddRecExpr *NewAR = dyn_cast<SCEVAddRecExpr>(S)) {
>>> - const SCEVAddRecExpr *OldAR = cast<SCEVAddRecExpr>(LHS);
>>> - const_cast<SCEVAddRecExpr *>(NewAR)->setNoWrapFlags(
>>> - OldAR->getNoWrapFlags());
>>> - }
>>> - return S;
>>> - }
>>> + // Use ValueTracking to check whether this is actually an add.
>>> + if (haveNoCommonBitsSet(BO->LHS, BO->RHS, getDataLayout(), &AC,
>>> + nullptr, &DT)) {
>>> + // There aren't any common bits set, so the add can't wrap.
>>> + auto Flags = SCEV::NoWrapFlags(SCEV::FlagNUW | SCEV::FlagNSW);
>>> + return getAddExpr(getSCEV(BO->LHS), getSCEV(BO->RHS), Flags);
>>> }
>>> break;
>>>
>>>
>>> Added: llvm/trunk/test/Analysis/ScalarEvolution/or-as-add.ll
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/or-as-add.ll?rev=300746&view=auto
>>> ==============================================================================
>>> --- llvm/trunk/test/Analysis/ScalarEvolution/or-as-add.ll (added)
>>> +++ llvm/trunk/test/Analysis/ScalarEvolution/or-as-add.ll Wed Apr 19 15:19:58 2017
>>> @@ -0,0 +1,38 @@
>>> +; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s
>>> +
>>> +declare void @z(i32)
>>> +declare void @z2(i64)
>>> +
>>> +define void @fun(i1 %bool, i32 %x) {
>>> +entry:
>>> + br label %body
>>> +body:
>>> + %i = phi i32 [ 0, %entry ], [ %i.next, %body ]
>>> + %bottom_zero = mul i32 %i, 2
>>> + %a = or i32 %bottom_zero, 1
>>> + call void @z(i32 %a)
>>> + %bool_ext = zext i1 %bool to i32
>>> + %b = or i32 %bool_ext, %bottom_zero
>>> + call void @z(i32 %b)
>>> + %shifted = lshr i32 %x, 31
>>> + %c = or i32 %shifted, %bottom_zero
>>> + call void @z(i32 %c)
>>> + %i_ext = zext i32 %i to i64
>>> + %d = or i64 %i_ext, 4294967296
>>> + call void @z2(i64 %d)
>>> + %i.next = add i32 %i, 1
>>> + %cond = icmp eq i32 %i.next, 10
>>> + br i1 %cond, label %exit, label %body
>>> +exit:
>>> + ret void
>>> +}
>>> +
>>> +; CHECK: %a = or i32 %bottom_zero, 1
>>> +; CHECK-NEXT: --> {1,+,2}<%body>
>>> +; CHECK: %b = or i32 %bool_ext, %bottom_zero
>>> +; CHECK-NEXT: --> {(zext i1 %bool to i32),+,2}
>>> +; CHECK: %c = or i32 %shifted, %bottom_zero
>>> +; CHECK-NEXT: --> {(%x /u -2147483648),+,2}<%body>
>>> +; CHECK: %d = or i64 %i_ext, 4294967296
>>> +; CHECK-NEXT: --> {4294967296,+,1}<nuw><nsw><%body>
>>> +
>>>
>>>
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>
> --
> Employee of Qualcomm Innovation Center, Inc.
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