Re: optimization/9319: slower code generated for simple loop on linux

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

> I don't think it's worth much time.  The only way to Really Fix
> this is to move to CFG-based loop analysis.

Yes, and the pessimization is a direct effect of the patch, not a mere side 
effect. Before, loops such as

	top:
 	   /* empty */
	body:
	   if (!cond) goto end;
	   statement;
	   goto top;
	end:

were rotated, identically to canonical loops such as

	top:
 	   if (!cond) goto end;
	body:
	   statement;
	   goto top;
	end:

into

	   goto first;
	top:
	   statement;
	first:
	   if (cond) goto top;


Now the patch explicitly forbids that, in order to preserve the unrolling 
pass:

!      We rely on the presence of NOTE_INSN_LOOP_END_TOP_COND to mark
!      the end of the entry condtional.  Without this, our lexical scan
!      can't tell the difference between an entry conditional and a
!      body conditional that exits the loop.  Mistaking the two means
!      that we can misplace the NOTE_INSN_LOOP_CONT note, which can 
!      screw up loop unrolling.


Would you support relaxing the code when unrolling is disabled, in order to 
recover the optimization loss in that case ?

-- 
Eric Botcazou