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Subject: Re: [PATCH] riscv: eliminate unreliable __builtin_frame_address(1)
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <8735lj78wu.fsf@igel.home>
Date:   Wed, 19 Jan 2022 20:48:48 +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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References: <20220117154433.3124-1-changbin.du@gmail.com>
 <C2470F2D-9E45-49D7-A03B-E6A7BB4B9738@jrtc27.com> <87v8yg6lhf.fsf@igel.home>
 <AAAA7852-EBCA-47A3-B74E-A425023468C6@jrtc27.com> <8735lj78wu.fsf@igel.home>
To:     Andreas Schwab <schwab@linux-m68k.org>
Precedence: bulk

On 19 Jan 2022, at 20:44, Andreas Schwab <schwab@linux-m68k.org> wrote:
>=20
> On Jan 19 2022, Jessica Clarke wrote:
>=20
>> Leaf functions by definition don=E2=80=99t have callees that are =
trying to read
>> their frame pointer so aren=E2=80=99t relevant here.
>=20
> ??? __builtin_frame_address(1) is about the caller, not the callee.

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. The issue of whether
it=E2=80=99s part of the ABI or not only arises when you=E2=80=99re =
trying to read the
incoming frame pointer from a caller, which by definition is not a leaf
function.

Jess