[Bug target/113312] Add __attribute__((no_callee_saved_registers)) for Intel FRED

["hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113312

--- Comment #23 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Xin Li from comment #22)
> 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/
> 12c38143a5c33e89f2b3d8906629dd4f23f8d79c.  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
> pressure – intentionally).
> 3) other functions with __attribute__((no_callee_saved_registers)) do get
> rid 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.

Do these mean that __attribute__((no_callee_saved_registers)) works for you?
BTW, my patches also avoid saving callee-saved registers in
__attribute__((noreturn))
functions.