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

[Richard Osborne]

Thanks for your reply, I've filed this as PR14045:

http://llvm.org/bugs/show_bug.cgi?id=14045

On 08/10/12 14:54, Duncan Sands wrote:
> 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?
>>
>
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