[llvm-commits] [llvm] r150978 - /llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

[Duncan Sands]

Hi Benjamin,

>>> InstCombine: Removing the base from the address calculation is only safe when the GEPs are inbounds.
>>
>> since GEP is implicitly a signed computation, isn't this only true for signed
>> comparisons?  I.e. mightn't an inbounds GEP overflow when viewed as an unsigned
>> computation, leading to wrongness for unsigned comparisons?
>
> Not sure if I understand. We're folding this to an index compare so the resulting icmp will always be signed. Also, overflow on inbounds GEPs is undefined behavior.

what does "overflow" mean?  Suppose a pointer has 8 bits, so the values are:

-128, -127, ..., -1, 0, 1, ..., 127

Can I have an object that spans addresses -1, 0, 1?  Can I have an object that
occupies addresses 127 and -128?  If either of these is yes, then we had better
be allowing inbounds GEP that (in the first case) has an unsigned overflow, and
(if the second case is allowed) signed overflow.

Ciao, Duncan.