Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve_most function attribute

[Marco Elver]

On Wed, 2 Aug 2023 at 20:03, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed,  2 Aug 2023 17:06:37 +0200 Marco Elver <elver@xxxxxxxxxx> wrote:
>
> > [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> > convention of a function. The preserve_most calling convention attempts
> > to make the code in the caller as unintrusive as possible. This
> > convention behaves identically to the C calling convention on how
> > arguments and return values are passed, but it uses a different set of
> > caller/callee-saved registers. This alleviates the burden of saving and
> > recovering a large register set before and after the call in the
> > caller."
> >
> > [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> >
> > Use of this attribute results in better code generation for calls to
> > very rarely called functions, such as error-reporting functions, or
> > rarely executed slow paths.
> >
> > Introduce the attribute to compiler_attributes.h.
>
> That sounds fairly radical.  And no changes are needed for assembly
> code or asm statements?

The callee in this case is supposed to save the registers and restore
them. If the caller (such as in asm) redundantly saves the registers
as well, this would be safe although redundant. That being said, there
are no plans to call functions marked with the attribute from asm.
Only issue would be if someone implements such a function in asm with
a C decl with the attribute - but also, there are no plans to do this.

I'll need to spin a v2 to always add notrace: one way this can go
wrong if something inserts itself between the callers and callee, such
as tracing would.