[llvm-dev] Confusions around nocapture and sret

[Xun Li via llvm-dev]

If sret implies nocapture, should we modify
CallBase::doesNotCapture(unsigned OpNo) to include this case?
Currently it only checks whether it has nocapture attribute.

On Fri, Feb 19, 2021 at 11:35 AM Johannes Doerfert
<johannesdoerfert at gmail.com> wrote:
>
> 1) I don't think so. sret is an attribute for *arguments* that declares
> they were created from source language return values, it's not a return
> value attribute.
>
> 2) I believe sret shuould imply nocapture when the frontend generates
> it. So yes.
>
> 3) noalias has no direct implications towards nocapture.
>
> ~ Johannes
>
>
> On 2/19/21 1:18 PM, Xun Li via llvm-dev wrote:
> > In the language reference (https://llvm.org/docs/LangRef.html), in the
> > definition of nocapture, it says "This is not a valid attribute for
> > return values". However I have seen it often that nocapture comes
> > together with sret attribute:
> > https://github.com/llvm/llvm-project/search?q=noalias+nocapture+sret
> >
> > A few questions:
> > 1. Is the LangRef out-of-date for sret+nocapture?
> > 2. Is an sret argument to a function guaranteed to be nocapture? (i.e.
> > a pointer cannot escape through a sret argument?)
> > 3. What about sret+noalias? Is that guaranteed to be nocapture?



-- 
Xun