[Bug c/92086] Provide way to avoid saving callee-saved registers in functions without callers

[Bug c/92086] Provide way to avoid saving callee-saved registers in functions without callers

[gdisirio at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92086

Giovanni Di Sirio <gdisirio at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |gdisirio at gmail dot com

--- Comment #8 from Giovanni Di Sirio <gdisirio at gmail dot com> ---
I would like to support the introduction of a "root" attribute, I can bring
several examples in embedded development where this would be beneficial:

1) Thread functions in an RTOS, callee-saved registers are just stealing
precious RAM for each thread.
2) Multiple main()-like functions in multi-core MCUs, each core has its own
entry function, saving registers is again wasting space.
3) Functions called by ASM veneers, for example ISR code called by a common ASM
entry-point.

Note that "naked" and "noreturn" attributes do not help in the above scenarios
for various reasons. I experienced all the above points while developing my
ChibiOS project.

I would suggest the following behavior:
- Root functions cannot be called by C code (warning).
- The "used" attribute is implicit for "root" functions.
- Parameters and return values would work as usual, just the callee-saved
registers stacking/unstacking is skipped.
- main() would have implicitly the "root" attribute.

[Bug c/92086] Provide way to avoid saving callee-saved registers in functions without callers

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92086

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #7)
> This is related to the compiler saving the return address for noreturn
> sibcalls, like in
> 
>   void g(void) __attribute__((noreturn));
>   void f(void) { g(); }
> 
> Maybe we should have an option like "-fimprove-debugging-experience" that
> defaults to true only for -O0 and -Og (and maybe -O1)?

This was discussed in PR 10837.

[Bug c/92086] Provide way to avoid saving callee-saved registers in functions without callers

[mcarbone at vmware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92086

Martim Carbone <mcarbone at vmware dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |mcarbone at vmware dot com

--- Comment #10 from Martim Carbone <mcarbone at vmware dot com> ---
Another example where the proposed optimization would make sense is a
hypervisor's VM-exit handler. This is the entry point for guest-to-hypervisor
mode transitions, and is often implemented by having an asm preamble call a
noreturn C function that handles the exit and then directly resumes the guest.
Saving callee-saved registers in this function just adds unnecessary overhead
to a very hot path.