Re: [PATCH] xtensa: Optimize stack frame adjustment more

[Max Filippov <jcmvbkbc@gmail.com>]

Hi Suwa-san,

On Thu, Jan 5, 2023 at 3:57 AM Takayuki 'January June' Suwa
<jjsuwa_sys3175@yahoo.co.jp> wrote:
>
> This patch introduces a convenient helper function for integer immediate
> addition with scratch register as needed, that splits and emits either
> up to two ADDI/ADDMI machine instructions or an addition by register
> following an immediate integer load (which may later be transformed by
> constantsynth).
>
> By using the helper function, it makes stack frame adjustment logic
> simplified and instruction count less in some cases.
>
> gcc/ChangeLog:
>
>         * config/xtensa/xtensa.cc
>         (xtensa_split_imm_two_addends, xtensa_emit_add_imm):
>         New helper functions.
>         (xtensa_emit_adjust_stack_ptr, xtensa_set_return_address,
>         xtensa_output_mi_thunk): Change to use the helper function.
> ---
>  gcc/config/xtensa/xtensa.cc | 139 +++++++++++++++++++++++-------------
>  1 file changed, 88 insertions(+), 51 deletions(-)

This change introduces a bunch of failures in the g++ testsuite,
but the culprit is apparently somewhere in the libstdc++.so, I'm
still looking for it.

I see the following pattern change in the generated epilogue code:

-    4aaf:      b0a192          movi    a9, 0x1b0
-    4ab2:      1f9a            add.n   a1, a15, a9
...
-    4abe:      20c112          addi    a1, a1, 32
-    4ac1:      f00d            ret.n
+    4aaf:      02df12          addmi   a1, a15, 0x200
+    4ab2:      b0c112          addi    a1, a1, -80
...
+    4abf:      20c112          addi    a1, a1, 32
+    4ac2:      f00d            ret.n

I.e. a1 is first moved into the parent stack frame, then back to the right
spot. This does not look correct, especially for bare-metal targets.

-- 
Thanks.
-- Max