[llvm] r300746 - [SCEV] Make SCEV or modeling more aggressive.
Mikhail Zolotukhin via llvm-commits
llvm-commits at lists.llvm.org
Thu Apr 20 13:02:07 PDT 2017
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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