Re: [patch] Reenable LOOP_HEADER prediction

[Jan Hubicka <hubicka at ucw dot cz>]

> Hello,
> 
> the predictor telling that the copied condition for the first iteration
> of loop is usually true became unused when loop header copying on
> rtl was removed.  This patch makes tree level header copying to use
> it instead.

Perhaps it would make more sense to reorder branch prediction and header
copying in order to allow it to take profile into account (and restrict
duplication in cold regions).  I believe Dale already modified code to
deal with profile updating so perhaps just reordering the two lines in
tree-optimize passmanager would do the trick?

Honza
> 
> Bootstrapped & regtested on i686.
> 
> Zdenek
> 
> 	* tree-ssa-loop-ch.c (predict_entry_edges_taken): New function.
> 	(copy_loop_headers): Call predict_entry_edges_taken.
> 
> Index: tree-ssa-loop-ch.c
> ===================================================================
> RCS file: /cvs/gcc/gcc/gcc/tree-ssa-loop-ch.c,v
> retrieving revision 2.14
> diff -c -3 -p -r2.14 tree-ssa-loop-ch.c
> *** tree-ssa-loop-ch.c	18 Mar 2005 20:15:06 -0000	2.14
> --- tree-ssa-loop-ch.c	29 Mar 2005 08:23:08 -0000
> *************** do_while_loop_p (struct loop *loop)
> *** 116,121 ****
> --- 116,147 ----
>     return true;
>   }
>   
> + /* Predict the edges from COPIED_BBS that lead to entering the LOOP
> +    by PRED_LOOP_HEADER.  N_BBS is number of COPIED_BBS.  */
> + 
> + static void
> + predict_entry_edges_taken (basic_block copied_bbs[], int n_bbs,
> + 			   struct loop *loop)
> + {
> +   int i;
> +   edge e;
> + 
> +   for (i = 0; i < n_bbs - 1; i++)
> +     {
> +       e = EDGE_SUCC (copied_bbs[i], 0);
> +       if (e->dest != copied_bbs[i + 1])
> + 	e = EDGE_SUCC (copied_bbs[i], 1);
> +       gcc_assert (e->dest == copied_bbs[i + 1]);
> +       predict_edge_def (e, PRED_LOOP_HEADER, TAKEN);
> +     }
> + 
> +   e = EDGE_SUCC (copied_bbs[n_bbs - 1], 0);
> +   if (e->dest != loop->header)
> +     e = EDGE_SUCC (copied_bbs[n_bbs - 1], 1);
> +   gcc_assert (e->dest == loop->header);
> +   predict_edge_def (e, PRED_LOOP_HEADER, TAKEN);
> + }
> + 
>   /* For all loops, copy the condition at the end of the loop body in front
>      of the loop.  This is beneficial since it increases efficiency of
>      code motion optimizations.  It also saves one jump on entry to the loop.  */
> *************** copy_loop_headers (void)
> *** 211,216 ****
> --- 237,245 ----
>   	  continue;
>   	}
>   
> +       /* Predict the edges that lead to the loop as taken.  */
> +       predict_entry_edges_taken (copied_bbs, n_bbs, loop);
> + 
>         /* Fix profiling info.  Scaling is done in gcov_type arithmetic to
>   	 avoid losing information; this is slow, but is done at most
>   	 once per loop.  We special case 0 to avoid division by 0;