Re: [PATCH] PR 10011: Introduce gcse_constant_p

[Richard Henderson <rth at redhat dot com>]

On Sun, Mar 16, 2003 at 06:51:37PM -0700, Roger Sayle wrote:
> 	PR middle-end/10011
> 	* gcse.c (gcse_constant_p): New function to identify constants
> 	suitable for constant propagation, including COMPARE with two
> 	integer constant arguments.
> 	(hash_scan_set): Use gcse_constant_p.
> 	(find_avail_set): Likewise.
> 	(cprop_insn): Likewise.
> 	(do_local_cprop): Likewise.
> 	(find_implicit_sets): Likewise.
> 	(find_bypass_set): Likewise.

This is ok, since it is a nice cleanup, but I don't think this is
the whole story.  The real problem, as I see it, is that
purge_builtin_constant_p is being run too late.

In particular, what happens in each case is that we purge the
constant_p_rtx, replace it with a zero, and then loop optimization
moves the assignment out of the loop before the branch is simplfied.
If we run this a bit ealier, then the post-gcse cse pass will take
care of this.

Indeed, this change by itself fixes both prs.


r~


	* toplev.c (rest_of_compilation): Run purge_builtin_constant_p
	before post-gcse cse pass.

Index: toplev.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/toplev.c,v
retrieving revision 1.737
diff -c -p -d -u -r1.737 toplev.c
--- toplev.c	24 Mar 2003 20:44:54 -0000	1.737
+++ toplev.c	30 Mar 2003 23:11:19 -0000
@@ -2929,6 +2929,10 @@ rest_of_compilation (decl)
       save_cfj = flag_cse_follow_jumps;
       flag_cse_skip_blocks = flag_cse_follow_jumps = 0;
 
+      /* Instantiate any remaining CONSTANT_P_RTX nodes.  */
+      if (current_function_calls_constant_p)
+	purge_builtin_constant_p ();
+
       /* If -fexpensive-optimizations, re-run CSE to clean up things done
 	 by gcse.  */
       if (flag_expensive_optimizations)
@@ -2973,10 +2977,6 @@ rest_of_compilation (decl)
       verify_flow_info ();
 #endif
     }
-
-  /* Instantiate any remaining CONSTANT_P_RTX nodes.  */
-  if (optimize > 0 && flag_gcse && current_function_calls_constant_p)
-    purge_builtin_constant_p ();
 
   /* Move constant computations out of loops.  */