[llvm-commits] [llvm] r153419 - in /llvm/trunk: lib/Analysis/InstructionSimplify.cpp test/Transforms/InstSimplify/compare.ll

[Jay Foad]

Hi Chandler,

On 26 March 2012 11:26, Duncan Sands <baldrick at free.fr> wrote:
>> +  case CmpInst::ICMP_EQ:
>> +  case CmpInst::ICMP_NE:
>> +    break;
>> +
>> +    // We can only handle unsigned relational comparisons because 'inbounds' on
>> +    // a GEP only protects against unsigned wrapping.
>> +  case CmpInst::ICMP_UGT:
>> +  case CmpInst::ICMP_UGE:
>> +  case CmpInst::ICMP_ULT:
>> +  case CmpInst::ICMP_ULE:
>> +    // However, we have to switch them to their signed variants to handle
>> +    // negative indices from the base pointer.
>> +    Pred = ICmpInst::getSignedPredicate(Pred);
>
> Is this correct?  For example, suppose the base pointer is null (otherwise
> known as 0), LHSOffset is 2^31-1, RHSOffset is 2^31, and you are doing a
> unsigned < comparison.  The result should be true, but you will return false.

Duncan's analysis seems right to me. Is there some guarantee that
objects are smaller than half the address space? If not, this
optimisation seems broken.

Thanks,
Jay.