Re: [PATCH v2] xtensa: Eliminate the use of callee-saved register that saves and restores only once

[Max Filippov <jcmvbkbc@gmail.com>]

Hi Suwa-san,

On Mon, Jan 16, 2023 at 8:12 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 (the frame pointer is needed to recover the stack pointer
> and must be excluded).
>
> e.g. in the following example, if there are no other occurrences of
> register A14:
>
> ;; before
>         ; prologue {
>   ...
>         s32i.n  a14, sp, 16
>   ...
>         ; } prologue
>   ...
>         mov.n   a14, a6
>   ...
>         call0   foo
>   ...
>         mov.n   a8, a14
>   ...
>         ; epilogue {
>   ...
>         l32i.n  a14, sp, 16
>   ...
>         ; } epilogue
>
> It can be possible like this:
>
> ;; after
>         ; prologue {
>   ...
>         (deleted)
>   ...
>         ; } prologue
>   ...
>         s32i.n  a6, sp, 16
>   ...
>         call0   foo
>   ...
>         l32i.n  a8, sp, 16
>   ...
>         ; epilogue {
>   ...
>         (deleted)
>   ...
>         ; } epilogue
>
> This patch introduces a new peephole2 pattern that implements the above.
>
> gcc/ChangeLog:
>
>         * config/xtensa/xtensa.md: New peephole2 pattern that eliminates
>         the use of callee-saved register that saves and restores only once
>         for other register, by using its stack slot directly.
> ---
>  gcc/config/xtensa/xtensa.md | 60 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)

There's still a few regressions in tests with -fcompare-debug because
code generated with -g and without it is different:

+FAIL: gcc.dg/pr41241.c (test for excess errors)
+FAIL: gcc.dg/pr48159-1.c (test for excess errors)
+FAIL: gcc.dg/pr65521.c (test for excess errors)
+FAIL: gcc.dg/torture/pr42878-1.c   -O2  (test for excess errors)
+FAIL: gcc.dg/torture/pr42878-1.c   -O3 -fomit-frame-pointer
-funroll-loops -fpeel-loops -ftracer -finline-functions  (test for
excess errors)
+FAIL: gcc.dg/torture/pr42878-1.c   -O3 -g  (test for excess errors)
+FAIL: gcc.dg/torture/pr42878-1.c   -Os  (test for excess errors)
+FAIL: gcc.dg/torture/pr42878-1.c   -O2 -flto -fno-use-linker-plugin
-flto-partition=none  (test for excess errors)

E.g. check the following test with -g0 and -g:

gcc/cc1 gcc/testsuite/gcc.dg/torture/pr42878-1.c -mlongcalls
-mtext-section-literals -fdiagnostics-plain-output -O3
-fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer
-finline-functions

-- 
Thanks.
-- Max