Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

From: Florian Weimer <fweimer@redhat.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Marco Elver <elver@google.com>,
	 Andrew Morton <akpm@linux-foundation.org>,
	 Kees Cook <keescook@chromium.org>,
	 Guenter Roeck <linux@roeck-us.net>,
	 Peter Zijlstra <peterz@infradead.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	 Steven Rostedt <rostedt@goodmis.org>,
	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>,
	 Nathan Chancellor <nathan@kernel.org>,
	 Nick Desaulniers <ndesaulniers@google.com>,
	 Tom Rix <trix@redhat.com>,  Miguel Ojeda <ojeda@kernel.org>,
	 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 v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute
Date: Mon, 07 Aug 2023 14:43:39 +0200	[thread overview]
Message-ID: <87h6pbf2f8.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <ZNDlrRrUS2AWTCiw@tucnak> (Jakub Jelinek's message of "Mon, 7 Aug 2023 14:38:05 +0200")

* Jakub Jelinek:

> On Mon, Aug 07, 2023 at 02:24:26PM +0200, Marco Elver wrote:
>> > | If the arguments are passed in callee-saved registers, then they will
>> > | be preserved by the callee across the call. This doesn’t apply for
>> > | values returned in callee-saved registers.
>> > |
>> > |  ·  On X86-64 the callee preserves all general purpose registers, except
>> > |     for R11. R11 can be used as a scratch register. Floating-point
>> > |     registers (XMMs/YMMs) are not preserved and need to be saved by the
>> > |     caller.
>> > |
>> > |  ·  On AArch64 the callee preserve all general purpose registers, except
>> > |     X0-X8 and X16-X18.
>> >
>> > Ideally, this would be documented in the respective psABI supplement.
>> > I filled in some gaps and filed:
>> >
>> >   Document the ABI for __preserve_most__ function calls
>> >   <https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/45>
>> 
>> Good idea. I had already created
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899, and we need
>> better spec to proceed for GCC anyway.
>
> "Registers used for passing arguments
> are preserved by the called function, but registers used for
> returning results are not."
>
> You mean just GPRs or also vector SSE or MMX registers?

I think this is pretty clear for x86-64:

| Floating-point registers (XMMs/YMMs) are not preserved and need to be
| saved by the caller.

The issue is more with future GPR extensions like APX.

Thanks,
Florian