On Wed, Mar 22, 2023 at 10:03:42AM +0000, Richard Biener via Gcc-patches wrote:
> For the testcase bb_is_just_return is on top of the profile, changing
> it to walk BB insns backwards puts it off the profile. That's because
> in the forward walk you have to process possibly many debug insns
> but in a backward walk you very likely run into control insns first.
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> OK?
>
> For the record, the profile was (after the delete_trivially_dead_insns
> fix)
>
> Samples: 289K of event 'cycles:u', Event count (approx.): 384226334976
>
> Overhead Samples Command Shared Object Symbol
>
> 3.52% 9747 cc1 cc1 [.] bb_is_just_return
>
> #
>
> and after the fix bb_is_just_return has no recorded samples anymore.
>
> Thanks,
> Richard.
>
> PR rtl-optimization/109237
> * cfgcleanup.cc (bb_is_just_return): Walk insns backwards.
This seems to regress
+FAIL: gcc.dg/guality/pr54200.c -Os -DPREVENT_OPTIMIZATION line 20 z == 3
on x86_64.
This was meant just as a performance improvement, right?
Just with -Os -g0 the assembly difference is:
--- pr54200.s1 2023-03-28 09:45:57.120647323 +0200
+++ pr54200.s2 2023-03-28 09:46:00.423599004 +0200
@@ -19,10 +19,11 @@ foo:
cmpl $1, %esi
jne .L3
movl $2, o(%rip)
- ret
+ jmp .L4
.L3:
addl %esi, %eax
addl %edx, %eax
+.L4:
ret
.cfi_endproc
.LFE1:
so 1 byte longer (ret is 1 byte, jmp 2 bytes), but might be also slower.
The difference starts during the pro_and_epilogue pass.
> --- a/gcc/cfgcleanup.cc
> +++ b/gcc/cfgcleanup.cc
> @@ -2608,7 +2608,7 @@ bb_is_just_return (basic_block bb, rtx_insn **ret,
> rtx_insn **use)
> if (bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
> return false;
>
> - FOR_BB_INSNS (bb, insn)
> + FOR_BB_INSNS_REVERSE (bb, insn)
> if (NONDEBUG_INSN_P (insn))
> {
> rtx pat = PATTERN (insn);
> --
> 2.35.3
Jakub
