Re: [PATCH v7] xtensa: Eliminate the use of callee-saved register that saves and restores only once

[Max Filippov <jcmvbkbc@gmail.com>]

On Thu, Feb 16, 2023 at 11:54 PM Takayuki 'January June' Suwa
<jjsuwa_sys3175@yahoo.co.jp> wrote:
>
> In the case of the CALL0 ABI, values that must be retained before and
> after function calls are placed in the callee-saved registers (A12
> through A15) and referenced later.  However, it is often the case that
> the save and the reference are each only once and a simple register-
> register move (with two exceptions; i. the register saved to/restored
> from is the stack pointer, ii. the function needs an additional stack
> pointer adjustment to grow the stack).
>
> e.g. in the following example, if there are no other occurrences of
> register A14:
>
> ;; before
>         ; prologue {
>   ...
>         s32i.n  a14, sp, 16
>   ...                           ;; no frame pointer needed
>                                 ;; no additional stack growth
>         ; } prologue
>   ...
>         mov.n   a14, a6         ;; A6 is not SP
>   ...
>         call0   foo
>   ...
>         mov.n   a8, a14         ;; A8 is not SP
>   ...
>         ; epilogue {
>   ...
>         l32i.n  a14, sp, 16
>   ...
>         ; } epilogue
>
> It can be possible like this:
>
> ;; after
>         ; prologue {
>   ...
>         (no save needed)
>   ...
>         ; } prologue
>   ...
>         s32i.n  a6, sp, 16      ;; replaced with A14's slot
>   ...
>         call0   foo
>   ...
>         l32i.n  a8, sp, 16      ;; through SP
>   ...
>         ; epilogue {
>   ...
>         (no restoration needed)
>   ...
>         ; } epilogue
>
> This patch adds the abovementioned logic to the function prologue/epilogue
> RTL expander code.
>
> gcc/ChangeLog:
>
>         * config/xtensa/xtensa.cc (machine_function): Add new member
>         'eliminated_callee_saved_regs'.
>         (xtensa_can_eliminate_callee_saved_reg_p): New function to
>         determine whether the register can be eliminated or not.
>         (xtensa_expand_prologue): Add invoking the above function and
>         elimination the use of callee-saved register by using its stack
>         slot through the stack pointer (or the frame pointer if needed)
>         directly.
>         (xtensa_expand_prologue): Modify to not emit register restoration
>         insn from its stack slot if the register is already eliminated.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/xtensa/elim_callee_saved.c: New.
> ---
>  gcc/config/xtensa/xtensa.cc                   | 134 ++++++++++++++----
>  .../gcc.target/xtensa/elim_callee_saved.c     |  37 +++++
>  2 files changed, 146 insertions(+), 25 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/xtensa/elim_callee_saved.c

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

-- 
Thanks.
-- Max