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From: "xin at zytor dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113312] Add __attribute__((no_callee_saved_registers))
 for Intel FRED
Date: Fri, 19 Jan 2024 06:23:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113312

--- Comment #24 from Xin Li <xin at zytor dot com> ---
(In reply to H.J. Lu from comment #23)
> (In reply to Xin Li from comment #22)
> > Per Peter's suggestion, I added __attribute__((no_callee_saved_register=
s))
> > to a linux source tree containing FRED patches:
> > https://github.com/xinli-intel/linux-fred-public/commit/
> > 12c38143a5c33e89f2b3d8906629dd4f23f8d79c.  And I compiled the linux code
> > with a gcc built from
> > https://gitlab.com/x86-gcc/gcc/-/tree/users/hjl/pr113312/gcc-13.
> >=20
> > Following are my observations:
> > 1) the generated kernel boots fine on both FRED Simics model and bare m=
etal.
> > 2) the asm code generated for fred_entry_from_{user,kernel}() are the s=
ame,
> > i.e., __attribute__((no_callee_saved_registers)) makes no difference (P=
eter
> > said the FRED dispatch points simply do not have significant register
> > pressure =E2=80=93 intentionally).
> > 3) other functions with __attribute__((no_callee_saved_registers)) do g=
et
> > rid of pushing/popping clobbered registers, and cause no issues because=
 they
> > are top-level functions, only invoked by tail call, meaning the followi=
ng
> > case won't happen:
> >=20
> > <func>:
> > ...
> > mov (%rbx), %r13
> > call foo
> > mov %rax, (%r13)
> > ...
> >=20
> > otherwise foo() is NOT a top-level function.
>=20
> Do these mean that __attribute__((no_callee_saved_registers)) works for y=
ou?

Yes, it works as expected.

> BTW, my patches also avoid saving callee-saved registers in
> __attribute__((noreturn))
> functions.

I also checked do_exit, which is the same as what you posted.=