[Bug target/110533] [x86-64] naked with -O0 and register-passed struct/int128 clobbers parameters/callee-saved regs

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110533

--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> >clobbering other parameters and callee-saved registers.
> 
> 
> (insn 2 8 3 2 (set (reg:DI 84)
>         (reg:DI 5 di [ aD.2522 ])) "/app/example.cpp":3:25 -1
>      (nil))
> (insn 3 2 4 2 (set (reg:DI 85)
>         (reg:DI 4 si [ aD.2522+8 ])) "/app/example.cpp":3:25 -1
>      (nil))
> (insn 4 3 5 2 (set (reg:TI 83)
>         (subreg:TI (reg:DI 84) 0)) "/app/example.cpp":3:25 -1
>      (nil))
> (insn 5 4 6 2 (set (subreg:DI (reg:TI 83) 8)
>         (reg:DI 85)) "/app/example.cpp":3:25 -1
>      (nil))
> (insn 6 5 7 2 (set (reg/v:TI 82 [ aD.2522 ])
>         (reg:TI 83)) "/app/example.cpp":3:25 -1
>      (nil))

This is emitted by middle-end to reconstruct a function argument in cases when
argument is passed via multiple registers. Function argument is specified by
the same hook that is used for the caller and the callee, so it is not possible
to simple disable it for naked functions.

When -O2 is used, optimizers figure out that the reconstructed value is unused
and remove the whole reconstruction sequence. This is unfortunately not the
case with -O0.

So, middle-end should provide some sort of mechanism to suppress the generation
of reconstruction sequence. The above sequence is emitted in
function.cc/assign_parm_remove_parallels, but similar functionality can
probably be found elsewhere in the function handling code.

Or only pass simple arguments to naked function. Naked functions are
specialist's tool, not intended for "general public".