linux-kernel - Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve

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Message-Id: <20230802110303.1e3ceeba5a96076f723d1d08@linux-foundation.org>
Date:   Wed, 2 Aug 2023 11:03:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev@...glegroups.com, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve_most
 function attribute

On Wed,  2 Aug 2023 17:06:37 +0200 Marco Elver <elver@...gle.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.