On Tue, 28 Mar 2023, Jakub Jelinek wrote:
> 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?
Yes. I have reverted the change and will revisit in GCC 14 (I have
tested an adjusted patch that avoids the above regression).
Richard.
> 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
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
