[llvm-dev] Why is method call treated as a safe point for GC

[Kavindu Gimhan Zoysa via llvm-dev]

Hi David,

Thank you for your detailed response.

Kavindu Gimhan Zoysa,
BSc(Hons) | ENTC | UoM,
SSE | WSO2

GitHub <https://github.com/KavinduZoysa> LinkedIn
<https://www.linkedin.com/in/kavindu-gimhan-zoysa-85939a122/> Medium
<https://medium.com/@kavindugimhanzoysa>


On Wed, 5 May 2021 at 14:20, David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I believe this is to permit local reasoning.  A method call is not
> necessarily a real safe point, but if the callee reaches a safe point
> then the entire stack must be treated as a safe point.  Without this
> assumption, you could reach a safe point in the current leaf function
> but the caller would not be in a safe point and so your stack would be
> in an inconsistent state.
>
> In theory, you could make may-be-a-safepoint an attribute on methods and
> avoid making the call site a safe point for cases where you can
> statically prove that nothing reachable from the callee contains a safe
> point.
>
> In practice, if you have sufficient analysis to be able to guarantee
> that the callee doesn't contain safe points (which also implies that it
> has a sufficiently short execution time that the lack of safepoints will
> not impact overall performance) then you probably want to inline it.  At
> this point, the method call no longer exists and so you don't have to
> worry about it as a potential safe point.
>
> David
>
> On 05/05/2021 05:26, Kavindu Gimhan Zoysa via llvm-dev wrote:
> > Hi all,
> >
> > In the LLVM GC framework, we are talking about the safe points which are
> > used to run the GC. When selecting the safe points, methods call are
> > considered as safe points. I cannot understand the reason to treat
> > method calls as safe points. Can you explain the reason?
> >
> > Thank you,
> > Kavindu
> >
> > Kavindu Gimhan Zoysa,
> > BSc(Hons) | ENTC | UoM,
> > SSE | WSO2
> >
> > GitHub <https://github.com/KavinduZoysa> LinkedIn
> > <https://www.linkedin.com/in/kavindu-gimhan-zoysa-85939a122/> Medium
> > <https://medium.com/@kavindugimhanzoysa>
> >
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
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