assigned to @majnemer

Line 650 in FoldGEPICmp [*]:

  // If all indices are the same, just compare the base pointers.
  if (IndicesTheSame)
    return new ICmpInst(ICmpInst::getSignedPredicate(Cond),
                        GEPLHS->getOperand(0), GEPRHS->getOperand(0));

We always use the signed version of the condition for some reason.

[*] http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?revision=185111&view=markup

While the code has danced around over the years, it looks like this was introduced when LLVM switched from having setcc to icmp [*].

http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp?r1=32690&r2=32751

A consequence of this is that the following two functions aren't the same!
struct blah {
char thing[1];
};
bool f(struct blah *a, struct blah *b) {
char *c = a->thing;
char *d = b->thing;
return c < d;
}

bool f2(struct blah *a, struct blah *b) {
return a < b;
}

gives us:
define zeroext i1 @​Z1fP4blahS0(%struct.blah* %a, %struct.blah* %b) #​0 {
entry:
%cmp = icmp slt %struct.blah* %a, %b
ret i1 %cmp
}

; Function Attrs: nounwind readnone uwtable
define zeroext i1 @​Z2f2P4blahS0(%struct.blah* %a, %struct.blah* %b) #​0 {
entry:
%cmp = icmp ult %struct.blah* %a, %b
ret i1 %cmp
}

Fixed in r185259.