Re: [PATCH v2 1/2] xtensa: Eliminate unused stack frame allocation/freeing

[Max Filippov <jcmvbkbc@gmail.com>]

Hi Suwa-san,

On Fri, Sep 2, 2022 at 3:57 AM Takayuki 'January June' Suwa
<jjsuwa_sys3175@yahoo.co.jp> wrote:
>
> Changes from v1:
>   (xtensa_expand_epilogue): Fixed forgetting to consider hard_frame_pointer_rtx when sharing codes.
>
> ---
> 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 member to track
>         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 record the insns for stack pointer adjustment.
>         (xtensa_expand_prologue): Change to use the function mentioned
>         above when using the CALL0 ABI.
>         (xtensa_expand_epilogue): Ditto.
>         And also change to cancel emitting the insns for the stack pointer
>         adjustment if only used for its own.
> ---
>  gcc/config/xtensa/xtensa.cc | 230 ++++++++++++++++++------------------
>  1 file changed, 118 insertions(+), 112 deletions(-)

With this change xtensa-linux-uclibc configured for call0 ABI
produces ICE while building libgcc:

gcc/libgcc/libgcc2.c: In function ‘__divdi3’:
gcc/libgcc/libgcc2.c:1230:1: internal compiler error: Segmentation fault
1230 | }
     | ^
0xea801f crash_signal
       gcc/gcc/toplev.cc:314
0x7f1fb4be5d5f ???
       ./signal/../sysdeps/unix/sysv/linux/x86_64/sigaction.c:0
0x9602e3 lookup_page_table_entry
       gcc/gcc/ggc-page.cc:630
0x9602e3 ggc_set_mark(void const*)
       gcc/gcc/ggc-page.cc:1550
0x121d7bd gt_ggc_mx_hash_set_rtx_insn__(void*)
       ./gt-xtensa.h:38
0xbe6187 gt_ggc_mx_function(void*)
       gcc-13-2434-g2f4d626e13dc-call0-le/gcc/gtype-desc.cc:1694
0xbe6187 gt_ggc_mx_function(void*)
       gcc-13-2434-g2f4d626e13dc-call0-le/gcc/gtype-desc.cc:1678
0x85634a gt_ggc_mx_lang_tree_node(void*)
       ./gt-c-c-decl.h:289
0xbe07d2 gt_ggc_mx_rtx_def(void*)
       gcc-13-2434-g2f4d626e13dc-call0-le/gcc/gtype-desc.cc:725
0xa47074 void gt_ggc_mx<dw_attr_struct>(vec<dw_attr_struct, va_gc, vl_embed>*)
       gcc/gcc/vec.h:1363
0xa47074 gt_ggc_mx_vec_dw_attr_node_va_gc_(void*)
       ./gt-dwarf2out.h:261
0xa47074 gt_ggc_mx_vec_dw_attr_node_va_gc_(void*)
       ./gt-dwarf2out.h:256
0xa47074 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:45
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:27
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:47
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:27
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:47
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:27
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:47
0xa470d7 gt_ggc_mx_die_struct(void*)
       ./gt-dwarf2out.h:27

-- 
Thanks.
-- Max