linux-kernel - Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve

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Date:   Mon, 07 Aug 2023 14:43:39 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Jakub Jelinek <jakub@...hat.com>
Cc:     Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
        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 v2 1/3] compiler_types: Introduce the Clang
 __preserve_most function attribute

* 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