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Subject: Re: [PATCH] riscv: eliminate unreliable __builtin_frame_address(1)
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <87y23b5t9g.fsf@igel.home>
Date:   Wed, 19 Jan 2022 21:27:46 +0000
Cc:     Changbin Du <changbin.du@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        linux-kernel@vger.kernel.org
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To:     Andreas Schwab <schwab@linux-m68k.org>
Precedence: bulk

On 19 Jan 2022, at 21:07, Andreas Schwab <schwab@linux-m68k.org> wrote:
>=20
> On Jan 19 2022, Jessica Clarke wrote:
>=20
>> My point is that the only thing that can possibly read the incoming
>> frame pointer of a leaf function is the leaf function itself, and =
since
>> it knows where it=E2=80=99s putting it then there is no ABI issue, it =
just
>> remembers where it put it and loads it from there.
>=20
> llvm sidesteps that issue by always saving ra when creating a frame,
> even in a leaf function, so it can use a constant offset.

What=E2=80=99s your point? That=E2=80=99s a correct implementation, just =
a simple one.
If it wanted to RISCVFrameLowering::spillCalleeSavedRegisters could
easily save that information, or recompute it when trying to load s0,
it just doesn=E2=80=99t because there=E2=80=99s no need. Also saving s0 =
alongside ra is
deliberate, it aids debugging when calling noreturn functions (e.g.
panic in an OS kernel). So yes, we avoid complexity in LLVM by not
doing things we don=E2=80=99t need to; there=E2=80=99s nothing wrong =
with that and it
doesn=E2=80=99t mean other toolchains that do need that to be correct =
should
just do something wrong.

Jess