Regarding the nocapture attribute the language ref says: "the callee does not make any copies of the pointer that outlive the callee itself".
From I inferred that it is OK for the callee to make a copy of the
pointer that doesn't outlive the call. However if I write some code that does this the optimizers don't do what I'd expect. Consider the following the example:
declare void @g(i32** %p, i32* %q) nounwind
define i32 @f(i32* noalias nocapture %p) nounwind {
entry:
%q = alloca i32*
call void @g(i32** %q, i32* %p) nounwind
store i32 0, i32* %p
%0 = load i32** %q
store i32 1, i32* %0
%1 = load i32* %p
ret i32 %1
}
I would expect it to be valid for g() to store the value of its second argument to the object pointed to by its first argument. Because of this I would expect a possible memory dependency between the last load (%1 = load i32* %p) and the last store (store i32 1, i32* %0). However if I run the example through opt -basicaa -gvn then the return instruction is optimized to ret i32 0 suggesting basicaa doesn't think there is any such dependency.
Is this a bug in the basic alias analysis pass or am I misunderstanding the semantics of nocapture?