Re: [PATCH] rtl-optimization/109237 - speedup bb_is_just_return

[Jakub Jelinek <jakub@redhat.com>]

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