[llvm] r300746 - [SCEV] Make SCEV or modeling more aggressive.

[Mikhail Zolotukhin via llvm-commits]

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>
> +
> 
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits