Re: [PATCH] xtensa: Optimize stack pointer updates in function pro/epilogue under certain conditions

[Max Filippov <jcmvbkbc@gmail.com>]

On Wed, Aug 17, 2022 at 12:32 PM Takayuki 'January June' Suwa
<jjsuwa_sys3175@yahoo.co.jp> wrote:
>
> This patch enforces the use of "addmi" machine instruction instead of
> addition/subtraction with two source registers for adjusting the stack
> pointer, if the adjustment fits into a signed 16-bit and is also a multiple
> of 256.
>
>     /* example */
>     void test(void) {
>       char buffer[4096];
>       __asm__(""::"m"(buffer));
>     }
>
>     ;; before
>     test:
>         movi.n  a9, 1
>         slli    a9, a9, 12
>         sub     sp, sp, a9
>         movi.n  a9, 1
>         slli    a9, a9, 12
>         add.n   sp, sp, a9
>         addi    sp, sp, 0
>         ret.n
>
>     ;; after
>     test:
>         addmi   sp, sp, -0x1000
>         addmi   sp, sp, 0x1000
>         ret.n
>
> gcc/ChangeLog:
>
>         * config/xtensa/xtensa.cc (xtensa_expand_prologue):
>         Use an "addmi" machine instruction for updating the stack pointer
>         rather than addition/subtraction via hard register A9, if the amount
>         of change satisfies the literal value conditions of that instruction
>         when the CALL0 ABI is used.
>         (xtensa_expand_epilogue): Ditto.
>         And also inhibit the stack pointer addition of constant zero.
> ---
>  gcc/config/xtensa/xtensa.cc | 79 +++++++++++++++++++++++++------------
>  1 file changed, 54 insertions(+), 25 deletions(-)

Regtested for target=xtensa-linux-uclibc, no new regressions.
Committed to master.

-- 
Thanks.
-- Max