[llvm-commits] [llvm] r147391 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineShifts.cpp test/Transforms/InstCombine/shift.ll

[Duncan Sands]

Hi Nick,

> @@ -603,9 +602,16 @@
>         // (X>>? C1)<<  C2 -->  X>>? (C1-C2)&  (-1<<  C2)
>         if (I.getOpcode() == Instruction::Shl&&
>             ShiftOp->getOpcode() != Instruction::Shl) {
> -        Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(), X,
> -                                            ConstantInt::get(Ty, ShiftDiff));
> -
> +        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
> +        if (ShiftOp->isExact()) {
> +          // (X>>?exact C1)<<  C2 -->  X>>?exact (C1-C2)

what happens if C2 is bigger than C1?

Ciao, Duncan.

> +          BinaryOperator *NewShr = BinaryOperator::Create(ShiftOp->getOpcode(),
> +                                                          X, ShiftDiffCst);
> +          NewShr->setIsExact(true);
> +          return NewShr;
> +        }
> +        Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(),
> +                                            X, ShiftDiffCst);
>           APInt Mask(APInt::getHighBitsSet(TypeBits, TypeBits - ShiftAmt2));
>           return BinaryOperator::CreateAnd(Shift,
>                                            ConstantInt::get(I.getContext(),Mask));
>
> Modified: llvm/trunk/test/Transforms/InstCombine/shift.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/shift.ll?rev=147391&r1=147390&r2=147391&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/shift.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/shift.ll Sat Dec 31 15:30:22 2011
> @@ -542,3 +542,21 @@
>   ; CHECK-NEXT: %y = lshr i32 %a, 5
>   ; CHECK-NEXT: ret i32 %y
>   }
> +
> +define i32 @test46(i32 %a) {
> +  %y = ashr exact i32 %a, 3
> +  %z = shl i32 %y, 1
> +  ret i32 %z
> +; CHECK: @test46
> +; CHECK-NEXT: %z = ashr exact i32 %a, 2
> +; CHECK-NEXT: ret i32 %z
> +}
> +
> +define i32 @test47(i32 %a) {
> +  %y = lshr exact i32 %a, 3
> +  %z = shl i32 %y, 1
> +  ret i32 %z
> +; CHECK: @test47
> +; CHECK-NEXT: %z = lshr exact i32 %a, 2
> +; CHECK-NEXT: ret i32 %z
> +}
>
>
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