Optimized local call conventions

By: anon2 (anon.delete@this.anon.com), July 3, 2021 9:18 pm
Room: Moderated Discussions
Carson (carson.delete@this.example.edu) on July 3, 2021 6:13 pm wrote:
> anon2 (anon.delete@this.anon.com) on July 2, 2021 7:49 pm wrote:
> > Do any compilers actually do this? I don't mean inline or partial inline to create a
> > different function, but compiling a given function with different call convention?
> >
> > Not saying it would be difficult or not worthwhile for a compiler
> > to do. Would have to be careful looking at the assembly though!
>
> Absolutely. PowerPC is the standard example.
>
> The PowerPC calling convention requires the caller set up both the target PC and the "table of contents"
> (TOC) pointer in R2. This latter is used to access global and static variables. A generic
> function pointer actually points to a 2-word descriptor which the caller must load both parts of.
>
> However, within one compilation unit (e.g a static function), all functions
> share the same TOC value. so a simpler calling convention can be used.
>
> I'm sure there's a better source for the information, but the first good description I
> found happens to be https://devblogs.microsoft.com/oldnewthing/20180816-00/?p=99505
>

Yeah, you could be right depending on how you read my question, and yes it is an example of a context specific calling convention. The function is always compiled with the same calling convention(s) though, standard ABI is used, and after the global prologue it looks exactly the same, and it can be called by any caller. It actually wouldn't surprise me if a "fastcall" or "asmlinkage" kind of optimization for static functions was applied by very old x86 compilers either.

What I intended to ask is whether analysis is applied to change the call convention of a particular function, arguments, return value registers, etc based not just on local/global, but a co-optimization between callers and callee.
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