llvm dev - Apr 2017 - Return on nocapture pointer

Hi,
I have a question about semantics of nocapture attribute:
"This indicates that the callee does not make any copies of the pointer
that outlive the callee itself. "
Is returing a pointer considered outliving callee? For example is this code
valid:

define i8* @foo(i8* nocapture %p)
  ret i8* %p
}

The documentation also mention that " This is not a valid attribute for
return values.", but I interpret that it is is about this case:

declare i8* nocapture @bar(i8* %p)

Piotr
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I believe it is invalid. It's making a copy that outlives the call. I
don't
think functionattrs will infer this, for example.

On Fri, Apr 28, 2017 at 8:22 AM, Piotr Padlewski via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
> I have a question about semantics of nocapture attribute:
> "This indicates that the callee does not make any copies of the
pointer
> that outlive the callee itself. "
> Is returing a pointer considered outliving callee? For example is this
> code valid:
>
> define i8* @foo(i8* nocapture %p)
>   ret i8* %p
> }
>
> The documentation also mention that " This is not a valid attribute
for
> return values.", but I interpret that it is is about this case:
>
> declare i8* nocapture @bar(i8* %p)
>
> Piotr
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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On 04/28/2017 10:22 AM, Piotr Padlewski via llvm-dev
wrote:
> Hi,
> I have a question about semantics of nocapture attribute:
> "This indicates that the callee does not make any copies of the 
> pointer that outlive the callee itself. "
> Is returing a pointer considered outliving callee? For example is this 
> code valid:
Yes, it includes returning the pointer. The code below is invalid. The 
return value outlives the callee itself.
>
> define i8* @foo(i8* nocapture %p)
>   ret i8* %p
> }
>
> The documentation also mention that "This is not a valid attribute for
> return values.", but I interpret that it is is about this case:
>
> declare i8* nocapture @bar(i8* %p)
Correct.

  -Hal
>
> Piotr
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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Thanks guys.
Do you it make sense to extend the definition in LangRef? If so I will be
happy to upload a patch.

Piotr

2017-04-28 17:58 GMT+02:00 Hal Finkel <hfinkel at anl.gov>:
>
>
> On 04/28/2017 10:22 AM, Piotr Padlewski via llvm-dev wrote:
>
> Hi,
> I have a question about semantics of nocapture attribute:
> "This indicates that the callee does not make any copies of the
pointer
> that outlive the callee itself. "
> Is returing a pointer considered outliving callee? For example is this
> code valid:
>
>
> Yes, it includes returning the pointer. The code below is invalid. The
> return value outlives the callee itself.
>
>
> define i8* @foo(i8* nocapture %p)
>   ret i8* %p
> }
>
> The documentation also mention that " This is not a valid attribute
for
> return values.", but I interpret that it is is about this case:
>
> declare i8* nocapture @bar(i8* %p)
>
>
> Correct.
>
>  -Hal
>
>
> Piotr
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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