[LLVMdev] What does "noalias sret" mean?

[Qiuping Yi]

Got it, thank you for your perfect answer.



--------------------------------------------
Qiuping Yi
Institute Of Software
Chinese Academy of Sciences

On Thu, Jul 16, 2015 at 12:08 PM, Tim Northover <t.p.northover at gmail.com>
wrote:

> > Could you tell me why function "func" with a return value
> > is changed to be one with a void return value and another
> > more parameter %o. Does "noalias sret" play a special role?
>
> Have you found the reference page for LLVM IR yet? It describes sret:
> http://llvm.org/docs/LangRef.html
>
> > What is the exact meaning of "noalias sret"?
>
> The "sret" (struct-return) is the important one. When a struct gets
> too big (as decided by the ABI writers) to be returned directly, what
> often happens is that the caller has to allocate storage for the
> object and pass this pointer to the function being called.
>
> Because this demotion is often handled differently from a normal
> argument (AArch64 requires register "X8" to be used; as I recall x86
> requires the pointer to be re-returned) we need a special argument
> attribute to mark this so that the correct code can be generated.
>
> "noalias" is just an optimisation hint: because of how this argument
> came to exist (allocated by the caller directly before the call), no
> other pointer accessible to the function can alias it. This allows
> some useful transformations in the mid-end.
>
> Cheers.
>
> Tim.
>
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