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Date:   Wed, 2 Aug 2023 11:03:03 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Marco Elver <elver@google.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Miguel Ojeda <ojeda@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com, linux-toolchains@vger.kernel.org
Subject: Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve_most
 function attribute
Message-Id: <20230802110303.1e3ceeba5a96076f723d1d08@linux-foundation.org>
In-Reply-To: <20230802150712.3583252-1-elver@google.com>
References: <20230802150712.3583252-1-elver@google.com>
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On Wed,  2 Aug 2023 17:06:37 +0200 Marco Elver <elver@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.