Hi Suwa-san,
On Wed, Sep 7, 2022 at 2:08 AM Takayuki 'January June' Suwa
<jjsuwa_sys3...@yahoo.co.jp> wrote:
>
> Changes from v2:
> (xtensa_expand_prologue): Changed to check conditions for suppressing emit
> insns in advance, instead of tracking emitted and later replacing them with
> NOPs if they are found to be unnecessary.
>
> ---
>
> In the example below, 'x' is once placed on the stack frame and then read
> into registers as the argument value of bar():
>
> /* example */
> struct foo {
> int a, b;
> };
> extern struct foo bar(struct foo);
> struct foo test(void) {
> struct foo x = { 0, 1 };
> return bar(x);
> }
>
> Thanks to the dead store elimination, the initialization of 'x' turns into
> merely loading the immediates to registers, but corresponding stack frame
> growth is not rolled back. As a result:
>
> ;; prereq: the CALL0 ABI
> ;; before
> test:
> addi sp, sp, -16 // unused stack frame allocation/freeing
> movi.n a2, 0
> movi.n a3, 1
> addi sp, sp, 16 // because no instructions that refer to
> j.l bar, a9 // the stack pointer between the two
>
> This patch eliminates such unused stack frame allocation/freeing:
>
> ;; after
> test:
> movi.n a2, 0
> movi.n a3, 1
> j.l bar, a9
>
> gcc/ChangeLog:
>
> * config/xtensa/xtensa.cc (machine_function): New boolean member as
> a flag that controls whether to emit the insns for stack pointer
> adjustment inside of the pro/epilogue.
> (xtensa_emit_adjust_stack_ptr): New function to share the common
> codes and to emit insns if not inhibited.
> (xtensa_expand_epilogue): Change to use the function mentioned
> above when using the CALL0 ABI.
> (xtensa_expand_prologue): Ditto.
> And also change to set the inhibit flag used by
> xtensa_emit_adjust_stack_ptr() to true if the stack pointer is only
> used for its own adjustment.
> ---
> gcc/config/xtensa/xtensa.cc | 162 +++++++++++++++++-------------------
> 1 file changed, 78 insertions(+), 84 deletions(-)
There's still a minor issue here: this change introduces a new regression
in the following test:
g++.dg/opt/pr100469.C
AFAICS it generates different code with and without the '-g' option: no
stack modification without -g, but adjustment in prologue and epilogue
with -g.
--
Thanks.
-- Max
