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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>,
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        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>,
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Subject: Re: [PATCH v2 1/3] compiler_types: Introduce the Clang
 __preserve_most function attribute
References: <20230804090621.400-1-elver@google.com>
        <87il9rgjvw.fsf@oldenburg.str.redhat.com>
        <CANpmjNN4h2+i3LUG__GHha849PZ3jK=mBoFQWpSz4jffXB4wrw@mail.gmail.com>
        <ZNDlrRrUS2AWTCiw@tucnak>
Date:   Mon, 07 Aug 2023 14:43:39 +0200
In-Reply-To: <ZNDlrRrUS2AWTCiw@tucnak> (Jakub Jelinek's message of "Mon, 7 Aug
        2023 14:38:05 +0200")
Message-ID: <87h6pbf2f8.fsf@oldenburg.str.redhat.com>
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* 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=E2=80=99t app=
ly for
>> > | values returned in callee-saved registers.
>> > |
>> > |  =C2=B7  On X86-64 the callee preserves all general purpose register=
s, 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.
>> > |
>> > |  =C2=B7  On AArch64 the callee preserve all general purpose register=
s, 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>
>>=20
>> Good idea. I had already created
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110899, 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