[llvm] r279568 - [InstCombine] remove icmp shr folds that are already handled by InstSimplify

[Sanjay Patel via llvm-commits]

Thanks, Eli. I made a couple of follow-on cleanups before I saw your mail.
Draft posted for review - make sure the comments don't cause more confusion
than they remove. :)
https://reviews.llvm.org/D23819

On Tue, Aug 23, 2016 at 3:36 PM, Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 8/23/2016 2:01 PM, Sanjay Patel via llvm-commits wrote:
>
>> Author: spatel
>> Date: Tue Aug 23 16:01:35 2016
>> New Revision: 279568
>>
>> -  // If we are comparing against bits always shifted out, the
>> -  // comparison cannot succeed.
>> -  APInt Comp = CmpRHSV << ShAmtVal;
>> -  ConstantInt *ShiftedCmpRHS = Builder->getInt(Comp);
>> -  if (Shr->getOpcode() == Instruction::LShr)
>> -    Comp = Comp.lshr(ShAmtVal);
>> -  else
>> -    Comp = Comp.ashr(ShAmtVal);
>> -
>> -  if (Comp != CmpRHSV) { // Comparing against a bit that we know is zero.
>> -    bool IsICMP_NE = ICI.getPredicate() == ICmpInst::ICMP_NE;
>> -    Constant *Cst = Builder->getInt1(IsICMP_NE);
>> -    return replaceInstUsesWith(ICI, Cst);
>> -  }
>> -
>> -  // Otherwise, check to see if the bits shifted out are known to be
>> zero.
>> -  // If so, we can compare against the unshifted value:
>> +  // Check if the bits shifted out are known to be zero. If so, we can
>> compare
>> +  // against the unshifted value:
>>
>
> It's not obvious that it's actually safe to do this... I mean, you don't
> need the same optimization in both InstSimplify and Instcombine, but "Comp
> != CmpRHSV" also acts as a safety check here.  At the very least, there
> needs to be a comment explaining why this is safe.
>
> -Eli
>
>
>     //  (X & 4) >> 1 == 2  --> (X & 4) == 4.
>> +  ConstantInt *ShiftedCmpRHS = Builder->getInt(CmpRHSV << ShAmtVal);
>>     if (Shr->hasOneUse() && Shr->isExact())
>>       return new ICmpInst(ICI.getPredicate(), Shr->getOperand(0),
>> ShiftedCmpRHS);
>>
>>
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>
>
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