Re: [PATCH] Fix cut-and-pasto in ifcvt.c/find_if_case_2

[tm <tm at mail dot kloo dot net>]

On Wed, 7 Aug 2002, tm wrote:

Patch survived a three-stage bootstrap.

Please review and approve if appropriate.

Toshi

> 
> 
> I think I've found a bug in ifcvt.c/find_if_case_2().
> 
> I noticed this problem because map_fog.i fails to be optimized
> as well as before. I tracked this down to an optimization which
> is not being performed; specifically case 2 of the IF-THEN-ELSE
> conversion:
> 
> /* Look for IF-THEN-ELSE cases in which one of THEN or ELSE is
>    transformable, but not necessarily the other.  There need be no
>    JOIN block.
> 
>    Return TRUE if we were successful at converting the the block.
> 
>    Cases we'd like to look at:
> ...
>    (2)
>         if (test) goto E; // x not live
>         x = big();
>         goto L;
>         E:
>         x = b;
>         goto M;
> 
>    becomes
> 
>         x = b;
>         if (test) goto M;
>         x = big();
>         goto L;
> 
> This optimization is performed by find_if_case_2(). One of the
> conditions for applying this optimization is failing:
> 
>   /* ELSE is small.  */
>   if (count_bb_insns (then_bb) > BRANCH_COST)
>     return FALSE;
> 
> This code looks suspicious because the comments do not match the code.
> The comment says "check if ELSE is small" but it's actually testing
> the size of the THEN basic block.
> 
> This code appears in gcc-3.0.4 as well, but the RTX generated by 3.0.4
> just happens to match this condition, e.g.  (count_bb_insns(then_bb) < BRANCH_COST)
> so the optimization was applied successfully.
> 
> If I patch this code to:
> 
>   /* ELSE is small.  */
>   if (count_bb_insns (else_bb) > BRANCH_COST) 
>     return FALSE;
> 
> then the optimization works as expected and the number of cache alignments
> generated due to BARRIER decreases significantly.
> 
> Before patch:
> 
> [tm@localhost test]$ grep "align 5" map_fog_partial.s | wc -l
>     113
> 
> After patch:
> 
> [tm@localhost test]$ grep "align 5" map_fog_partial.s | wc -l
>      17
> 
> I'm currently bootstrapping on x86-linux, but as this machine is slow
> (P2/300) it will take a while...can anybody with a faster machine help?
> 
> Here is the patch:
> 
> 2002-08-07  Toshiyasu Morita  <toshiyasu.morita@hsa.hitachi.com>
> 
> 	* ifcvt.c (find_if_case_2): Test correct basic block for size.
> 
> *** ifcvt.c.bak	Wed Aug  7 10:18:02 2002
> --- ifcvt.c	Wed Aug  7 16:13:12 2002
> *************** find_if_case_2 (test_bb, then_edge, else
> *** 2769,2775 ****
>   	     test_bb->index, else_bb->index);
>   
>     /* ELSE is small.  */
> !   if (count_bb_insns (then_bb) > BRANCH_COST)
>       return FALSE;
>   
>     /* Registers set are dead, or are predicable.  */
> --- 2769,2775 ----
>   	     test_bb->index, else_bb->index);
>   
>     /* ELSE is small.  */
> !   if (count_bb_insns (else_bb) > BRANCH_COST)
>       return FALSE;
>   
>     /* Registers set are dead, or are predicable.  */
> 
>