[llvm-commits] [llvm] r120049 - in /llvm/trunk: lib/Target/README.txt lib/Transforms/InstCombine/InstCombineShifts.cpp test/Transforms/InstCombine/shift.ll

[Eli Friedman]

On Tue, Nov 23, 2010 at 12:33 PM, Benjamin Kramer
<benny.kra at googlemail.com> wrote:
> Author: d0k
> Date: Tue Nov 23 14:33:57 2010
> New Revision: 120049
>
> URL: http://llvm.org/viewvc/llvm-project?rev=120049&view=rev
> Log:
> The srem -> urem transform is not safe for any divisor that's not a power of two.
> E.g. -5 % 5 is 0 with srem and 1 with urem.
>
> Also addresses Frits van Bommel's comments.
>
> Modified:
>    llvm/trunk/lib/Target/README.txt
>    llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
>    llvm/trunk/test/Transforms/InstCombine/shift.ll
>
> Modified: llvm/trunk/lib/Target/README.txt
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=120049&r1=120048&r2=120049&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/README.txt (original)
> +++ llvm/trunk/lib/Target/README.txt Tue Nov 23 14:33:57 2010
> @@ -1704,8 +1704,8 @@
>   %384 = shl i64 %381, %383                       ; [#uses=1]
>   %385 = icmp slt i32 %tmp14.i, 64                ; [#uses=1]
>
> -The srem can be transformed to an and because if x is negative, the shift is
> -undefined.  Testcase derived from 403.gcc.
> +The srem can be transformed to an and because if %tmp14.i is negative, the
> +shift is undefined.  Testcase derived from 403.gcc.
>
>  //===---------------------------------------------------------------------===//
>
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp?rev=120049&r1=120048&r2=120049&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp Tue Nov 23 14:33:57 2010
> @@ -54,19 +54,17 @@
>     if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
>       return Res;
>
> -  // X shift (A srem B) -> X shift (A urem B) iff B is positive.
> +  // X shift (A srem B) -> X shift (A and B-1) iff B is a power of 2.
>   // Because shifts by negative values are undefined.
>   if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
> -    if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
> -      // Make sure the divisor's sign bit is zero.
> -      APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
> -      if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
> -        Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
> -                                          BO->getName());
> -        I.setOperand(1, URem);
> -        return &I;
> -      }
> -    }
> +    if (BO->hasOneUse() && BO->getOpcode() == Instruction::SRem)
> +      if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
> +        if (CI->getValue().isPowerOf2()) {
> +          Constant *C = ConstantInt::get(BO->getType(), CI->getValue()-1);
> +          Value *Rem = Builder->CreateAnd(BO->getOperand(0), C, BO->getName());
> +          I.setOperand(1, Rem);
> +          return &I;
> +        }

Is this check correct for "X << (A % INT_MIN)"?

-Eli