[llvm-commits] [llvm] r139450 - /llvm/trunk/lib/Analysis/ScalarEvolution.cpp

[Dan Gohman]

Hi Andy,

The reason this wasn't done is because of potential trouble with
code like this:

  if (some condition which is true when a+b doesn't overflow) {
    t0 = add nsw a, b
    use(t0);
  } else {
    t1 = add a, b)
    use(t1);
  }

Because ScalarEvolution doesn't currently include the nsw flag when
hashing expressions for uniqueness, the nsw flag could get set for
a+b when examining the first add instruction, and remain when that
expression is reused for the second add instruction.

There actually was a comment about this way back in r83649, but
it appears to have gotten lost somewhere along the way.

Dan

On Sep 9, 2011, at 6:09 PM, Andrew Trick wrote:

> Author: atrick
> Date: Fri Sep  9 20:09:50 2011
> New Revision: 139450
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=139450&view=rev
> Log:
> Set NSW/NUW flags on SCEVAddExpr when the operation is flagged as
> such.
> 
> I'm doing this now for completeness because I can't think of/remember
> any reason that it was left out. I'm not sure it will help anything,
> but if we don't do it we need to explain why in comments.
> 
> 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=139450&r1=139449&r2=139450&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
> +++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Fri Sep  9 20:09:50 2011
> @@ -3546,7 +3546,13 @@
>         AddOps.push_back(Op1);
>     }
>     AddOps.push_back(getSCEV(U->getOperand(0)));
> -    return getAddExpr(AddOps);
> +    SCEV::NoWrapFlags Flags = SCEV::FlagAnyWrap;
> +    OverflowingBinaryOperator *OBO = cast<OverflowingBinaryOperator>(V);
> +    if (OBO->hasNoSignedWrap())
> +      setFlags(Flags, SCEV::FlagNSW);
> +    if (OBO->hasNoUnsignedWrap())
> +      setFlags(Flags, SCEV::FlagNUW);
> +    return getAddExpr(AddOps, Flags);
>   }
>   case Instruction::Mul: {
>     // See the Add code above.
> 
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits