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

[Andrew Morton]

On Wed,  2 Aug 2023 17:06:37 +0200 Marco Elver <elver at google.com> 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?

I'll add "LLVM" to the patch title to make it clear that gcc isn't
affected.