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[patch] tree-cfg.c: Speed up tree_forwarder_block_p.


Attached is a patch to speed up tree_forwarder_block_p by scanning
through the statements backwards (as suggested by Jeff).

One of the requirements of a forwarder block is that a basic block
have statements except local labels.  Without this patch,
tree_forwarder_block_p scans the statements forward, so the first
statement that we encounter is almost always a label, which does not
quite tell if a given basic block is a forwarder block or not.  With
this patch, it scans the statements backward, so if there is any
non-label statement, we can find that out in the first iteration.

With this patch, cleanup_forwarder_blocks fairly consistently takes
21ms instead of 24ms without the patch while compiling fold-const.c.
So this is a micro optimization, but its effect is certainly

Tested on i686-pc-linux-gnu.  OK to apply?

Kazu Hirata

2005-01-10  Kazu Hirata  <>

	* tree-cfg.c (tree_forwarder_block_p): Speed up by walking
	through the statements backward.

Index: tree-cfg.c
RCS file: /cvs/gcc/gcc/gcc/tree-cfg.c,v
retrieving revision 2.137
diff -u -d -p -r2.137 tree-cfg.c
--- tree-cfg.c	1 Jan 2005 16:15:10 -0000	2.137
+++ tree-cfg.c	10 Jan 2005 19:45:18 -0000
@@ -3911,9 +3911,9 @@ tree_forwarder_block_p (basic_block bb)
   gcc_assert (bb != ENTRY_BLOCK_PTR);
-  /* Now walk through the statements.  We can ignore labels, anything else
-     means this is not a forwarder block.  */
-  for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
+  /* Now walk through the statements backward.  We can ignore labels,
+     anything else means this is not a forwarder block.  */
+  for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_next (&bsi))
       tree stmt = bsi_stmt (bsi);

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