[PATCH] Move transforms from InstCombine to InstSimplify

[Duncan Sands]

Hi David,

On 11/07/13 00:26, David Majnemer wrote:
> InstCombine was pattern matching for (icmp (shl 1, X), Cst) to try to transform
> these icmps to true/false if we could prove that Cst must be in/out of range.
>
> However, there are two problems:
> 1. This sort of thing belongs in InstSimplify
> 2. This can be generalized further
>
> Attached is a patch that moves over these transforms to InstructionSimplify and
> handles arbitrary constants on the left hand side of the shift and constrains
> the possible range further by analyzing the nsw/nuw flags on the shift.

> --- lib/Analysis/InstructionSimplify.cpp	(revision 186043)
> +++ lib/Analysis/InstructionSimplify.cpp	(working copy)
> @@ -1995,6 +1995,27 @@
>          Lower = IntMin.sdiv(Val);
>          Upper = IntMax.sdiv(Val) + 1;
>        }
> +    } else if (match(LHS, m_Shl(m_ConstantInt(CI2), m_Value()))) {
> +      const APInt &CI2Value = CI2->getValue();
> +      if (cast<OverflowingBinaryOperator>(LHS)->hasNoUnsignedWrap()) {
> +        // 'shl nuw CI2, x' produces [CI2, CI2 << CLZ(CI2)]
> +        Lower = CI2Value;
> +        Upper = (CI2Value << CI2Value.countLeadingZeros()) + 1;
> +      } else if (cast<OverflowingBinaryOperator>(LHS)->hasNoSignedWrap()) {
> +        if (CI2->isNegative()) {
> +          // 'shl nsw CI2, x' [CI2 << (CLO(CI2)-1), CI2]
> +          Lower = (CI2Value << (CI2Value.countLeadingOnes() - 1));
> +          Upper = CI2Value + 1;
> +        } else {
> +          // 'shl nsw CI2, x' [CI2, CI2 << (CLZ(CI2)-1)]
> +          Lower = CI2Value;
> +          Upper = (CI2Value << (CI2Value.countLeadingZeros() - 1)) + 1;
> +        }

the cases above look OK, but

> +      } else {
> +        // 'shl CI2, x' produces [CI2, CI2 << (Width-1)]
> +        Lower = CI2Value;
> +        Upper = CI2Value.shl(Width - 1) + 1;

this one seems wrong to me.  Consider for example CI2 (in binary) equal to
   10001000
Then Lower=10001000, Upper=00000001, but a left-shift by 1 of CI2 is
00010000 which is not in the range Lower .. Upper.

Ciao, Duncan.