[llvm] r224097 - Fix another infinite loop in InstCombine

[Bob Wilson]

One minor stylistic comment below.

> On Dec 11, 2014, at 8:34 PM, Steven Wu <stevenwu at apple.com> wrote:
> 
> Author: steven_wu
> Date: Thu Dec 11 22:34:07 2014
> New Revision: 224097
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=224097&view=rev
> Log:
> Fix another infinite loop in InstCombine
> 
> Summary:
> InstCombine infinite-loops for the testcase added
> It is because InstCombine is generating instructions that can be
> optimized by itself. Fix by not optimizing frem if the optimized
> type is the same as original type.
> rdar://problem/19150820
> 
> Reviewers: majnemer
> 
> Differential Revision: http://reviews.llvm.org/D6634
> 
> Modified:
>    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
>    llvm/trunk/test/Transforms/InstCombine/fpcast.ll
> 
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=224097&r1=224096&r2=224097&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Thu Dec 11 22:34:07 2014
> @@ -1269,16 +1269,19 @@ Instruction *InstCombiner::visitFPTrunc(
>         // type of OpI doesn't enter into things at all.  We simply evaluate
>         // in whichever source type is larger, then convert to the
>         // destination type.
> -        if (LHSWidth < SrcWidth)
> -          LHSOrig = Builder->CreateFPExt(LHSOrig, RHSOrig->getType());
> -        else if (RHSWidth <= SrcWidth)
> -          RHSOrig = Builder->CreateFPExt(RHSOrig, LHSOrig->getType());
> -        if (LHSOrig != OpI->getOperand(0) || RHSOrig != OpI->getOperand(1)) {
> -          Value *ExactResult = Builder->CreateFRem(LHSOrig, RHSOrig);
> -          if (Instruction *RI = dyn_cast<Instruction>(ExactResult))
> -            RI->copyFastMathFlags(OpI);
> -          return CastInst::CreateFPCast(ExactResult, CI.getType());
> +        if (SrcWidth != OpWidth) {

Can you flip this comparison and break out of the case when the widths are equal? Then you don’t need to increase the indentation of the other code.

> +          if (LHSWidth < SrcWidth)
> +            LHSOrig = Builder->CreateFPExt(LHSOrig, RHSOrig->getType());
> +          else if (RHSWidth <= SrcWidth)
> +            RHSOrig = Builder->CreateFPExt(RHSOrig, LHSOrig->getType());
> +          if (LHSOrig != OpI->getOperand(0) || RHSOrig != OpI->getOperand(1)) {
> +            Value *ExactResult = Builder->CreateFRem(LHSOrig, RHSOrig);
> +            if (Instruction *RI = dyn_cast<Instruction>(ExactResult))
> +              RI->copyFastMathFlags(OpI);
> +            return CastInst::CreateFPCast(ExactResult, CI.getType());
> +          }
>         }
> +        break;
>     }
> 
>     // (fptrunc (fneg x)) -> (fneg (fptrunc x))
> 
> Modified: llvm/trunk/test/Transforms/InstCombine/fpcast.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/fpcast.ll?rev=224097&r1=224096&r2=224097&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/fpcast.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/fpcast.ll Thu Dec 11 22:34:07 2014
> @@ -73,3 +73,15 @@ define float @test7(double %V) {
> ; CHECK-NEXT: %[[trunc:.*]] = fptrunc double %frem to float
> ; CHECK-NEXT: ret float %trunc
> }
> +
> +define float @test8(float %V) {
> +  %fext = fpext float %V to double
> +  %frem = frem double %fext, 1.000000e-01
> +  %trunc = fptrunc double %frem to float
> +  ret float %trunc
> +; CHECK-LABEL: @test8
> +; CHECK-NEXT: %[[fext:.*]]  = fpext float %V to double
> +; CHECK-NEXT: %[[frem:.*]]  = frem double %fext, 1.000000e-01
> +; CHECK-NEXT: %[[trunc:.*]] = fptrunc double %frem to float
> +; CHECK-NEXT: ret float %trunc
> +}
> 
> 
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