[LLVMdev] Meaning of the nocapture attribute (possible bug?)

[Duncan Sands]

Hi Richard, I think it is a bug.

Ciao, Duncan.

On 08/10/12 14:34, Richard Osborne wrote:
> 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?
>