[PATCH] InstCombineCompare with constant return false if we know it is never going to be equal

[Duncan Sands]

On 04/06/14 15:28, Duncan Sands wrote:
> Hi Suyog,
>> +  // If the KnownBits of LHS and RHS at same bit position differ,
>> +  // LHS and RHS are not equal.
>
> I think this comment is misleading.  If KnownBitsLHS != KnownBitsRHS it doesn't
> mean that LHS != RHS, but the comment seems to be saying that.
>
>> +  if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
>> +      uint32_t BitWidth = CI->getBitWidth();
>> +      APInt LHSKnownZero(BitWidth, 0, 1);
>
> Why not just: APInt LHSKnownZero(BitWidth);

PS: I have to admit that all callers of computeKnownBits initialize their APInt 
to zero, eg:
   APInt LHSKnownZero(BitWidth, 0);
In my opinion this should either be documented as a requirement of 
computeKnownBits, or documented as not being a requirement and computeKnownBits 
should zero out the APInts as the first step.  Setting the APInts to be signed 
is definitely pointless.