[PATCH] Clarify wording in the LangRef around !invariant.load

[Nick Lewycky]

Andrew Trick wrote:
Comparing pointers of different objects should be considered illegal in 
LLVM, although I'm not sure where that's spelled out.

"If the operands are pointer typed, the pointer values are compared as 
if they were integers." - http://llvm.org/docs/LangRef.html#id251

I disagree, and think llvm's current model is right. There used to be 
(still are?) some interesting rules on the inbounds gep instruction, but 
nothing that applies to pointers in general.

We do end up with weird cases such as %ptr1 noalias %ptr2 while %ptr1 == 
%ptr2, but that only comes up because we recognize C functions and 
graciously handle C's aliasing rules for things like freed pointers and 
such. It's not actually part of llvm's execution semantics, nor does it 
need to be. It might hurt support for other languages if we did, and it 
would make %p eq %q different from ptrtoint %p eq ptrtoint %q, which in 
turn means you can't do transforms which that a given i64 in memory is 
always used as a pointer and replace the type loads and stores referring 
to it.

Nick