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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: Thu, 18 Jan 2024 06:42:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113312

Xin Li <xin at zytor dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xin at zytor dot com

--- Comment #22 from Xin Li <xin at zytor dot com> ---
Per Peter's suggestion, I added __attribute__((no_callee_saved_registers)) =
to a
linux source tree containing FRED patches:
https://github.com/xinli-intel/linux-fred-public/commit/12c38143a5c33e89f2b=
3d8906629dd4f23f8d79c.
 And I compiled the linux code with a gcc built from
https://gitlab.com/x86-gcc/gcc/-/tree/users/hjl/pr113312/gcc-13.

Following are my observations:
1) the generated kernel boots fine on both FRED Simics model and bare metal.
2) the asm code generated for fred_entry_from_{user,kernel}() are the same,
i.e., __attribute__((no_callee_saved_registers)) makes no difference (Peter
said the FRED dispatch points simply do not have significant register press=
ure
=E2=80=93 intentionally).
3) other functions with __attribute__((no_callee_saved_registers)) do get r=
id
of pushing/popping clobbered registers, and cause no issues because they are
top-level functions, only invoked by tail call, meaning the following case
won't happen:

<func>:
...
mov (%rbx), %r13
call foo
mov %rax, (%r13)
...

otherwise foo() is NOT a top-level function.=