[llvm] r216045 - InstCombine: Annotate sub with nuw when we prove it's safe

[Pete Cooper]

Hi David

Can this be extended to be NumLeadingOnes(LHS) > NumLeadingOnes(RHS)?  (Note i made up NumLeadingOnes, its just for descriptive purposes here).

Thanks,
Pete
> On Aug 20, 2014, at 12:17 AM, David Majnemer <david.majnemer at gmail.com> wrote:
> 
> Author: majnemer
> Date: Wed Aug 20 02:17:31 2014
> New Revision: 216045
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=216045&view=rev
> Log:
> InstCombine: Annotate sub with nuw when we prove it's safe
> 
> We can prove that a 'sub' can be a 'sub nuw' if the left-hand side is
> negative and the right-hand side is non-negative.
> 
> Modified:
>    llvm/trunk/lib/Transforms/InstCombine/InstCombine.h
>    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>    llvm/trunk/test/Transforms/InstCombine/sub.ll
> 
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombine.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombine.h?rev=216045&r1=216044&r2=216045&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombine.h (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombine.h Wed Aug 20 02:17:31 2014
> @@ -251,6 +251,7 @@ private:
>   bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS);
>   bool WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS);
>   bool WillNotOverflowSignedSub(Value *LHS, Value *RHS);
> +  bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS);
>   Value *EmitGEPOffset(User *GEP);
>   Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
>   Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
> 
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=216045&r1=216044&r2=216045&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Wed Aug 20 02:17:31 2014
> @@ -988,6 +988,20 @@ bool InstCombiner::WillNotOverflowSigned
>   return false;
> }
> 
> +/// \brief Return true if we can prove that:
> +///    (sub LHS, RHS)  === (sub nuw LHS, RHS)
> +bool InstCombiner::WillNotOverflowUnsignedSub(Value *LHS, Value *RHS) {
> +  // If the LHS is negative and the RHS is non-negative, no unsigned wrap.
> +  bool LHSKnownNonNegative, LHSKnownNegative;
> +  bool RHSKnownNonNegative, RHSKnownNegative;
> +  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);
> +  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);
> +  if (LHSKnownNegative && RHSKnownNonNegative)
> +    return true;
> +
> +  return false;
> +}
> +
> // Checks if any operand is negative and we can convert add to sub.
> // This function checks for following negative patterns
> //   ADD(XOR(OR(Z, NOT(C)), C)), 1) == NEG(AND(Z, C))
> @@ -1660,6 +1674,10 @@ Instruction *InstCombiner::visitSub(Bina
>     Changed = true;
>     I.setHasNoSignedWrap(true);
>   }
> +  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {
> +    Changed = true;
> +    I.setHasNoUnsignedWrap(true);
> +  }
> 
>   return Changed ? &I : nullptr;
> }
> 
> Modified: llvm/trunk/test/Transforms/InstCombine/sub.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub.ll?rev=216045&r1=216044&r2=216045&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/sub.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/sub.ll Wed Aug 20 02:17:31 2014
> @@ -506,6 +506,18 @@ define i4 @test42(i4 %x, i4 %y) {
> ; CHECK-LABEL: @test42(
> ; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
> ; CHECK-NEXT: [[AND1:%.*]] = and i4 %x, 7
> -; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 %a, %b
> +; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 [[AND]], [[AND1]]
> +; CHECK: ret i4 [[RET]]
> +}
> +
> +define i4 @test43(i4 %x, i4 %y) {
> +  %a = or i4 %x, -8
> +  %b = and i4 %y, 7
> +  %c = sub i4 %a, %b
> +  ret i4 %c
> +; CHECK-LABEL: @test43(
> +; CHECK-NEXT: [[OR:%.*]] = or i4 %x, -8
> +; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
> +; CHECK-NEXT: [[RET:%.*]] = sub nuw i4 [[OR]], [[AND]]
> ; CHECK: ret i4 [[RET]]
> }
> 
> 
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