Hi Suwa-san,
On Sun, Jan 15, 2023 at 6:53 PM Takayuki 'January June' Suwa
<jjsuwa_sys3...@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.
>
> 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 | 58 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 58 insertions(+)
This change introduces a bunch of test failures in cases where alloca
or similar mechanisms are used in a function and a15 is used as a
stack frame pointer. E.g., gcc.c-torture/execute/pr82210.c has the
following diff:
@@ -20,9 +20,8 @@
srli a10, a10, 4
slli a10, a10, 4
s32i.n a12, sp, 8
- s32i.n a15, sp, 0
s32i.n a0, sp, 12
- mov.n a15, sp
+ s32i.n sp, sp, 0
sub sp, sp, a10
mov.n a6, sp
mov.n a12, a6
@@ -59,11 +58,10 @@
addi.n a2, a2, 4
bne a12, a13, .L6
.L1:
- mov.n sp, a15
+ l32i.n sp, sp, 0
l32i.n a0, sp, 12
l32i.n a12, sp, 8
l32i.n a13, sp, 4
- l32i.n a15, sp, 0
addi sp, sp, 16
ret.n
.size foo, .-foo
--
Thanks.
-- Max
