PR tree-optimization/31602

	* tree-ssa-loop-ch.c (copy_loop_headers): Set TREE_NO_WARNING for
	conditionals in the copied loop header.
	* tree-cfg.c (fold_cond_expr_cond): Don't issue undefined overflow
	warnings if TREE_NO_WARNING is set.
	* doc/invoke.texi (Warning Options): Clarify that
	-Wstrict-overflow does not warn about loops.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-4_2-branch@124127 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 22 insertions, 1 deletions

diff --git a/gcc/tree-ssa-loop-ch.c b/gcc/tree-ssa-loop-ch.c
index 1663807127a..7ad1695f66f 100644
--- a/gcc/tree-ssa-loop-ch.c
+++ b/gcc/tree-ssa-loop-ch.c
@@ -1,5 +1,5 @@
 /* Loop header copying on trees.
-   Copyright (C) 2004, 2005 Free Software Foundation, Inc.
+   Copyright (C) 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
    
 This file is part of GCC.
    
@@ -204,6 +204,27 @@ copy_loop_headers (void)
 	  continue;
 	}
 
+      /* If the loop has the form "for (i = j; i < j + 10; i++)" then
+	 this copying can introduce a case where we rely on undefined
+	 signed overflow to eliminate the preheader condition, because
+	 we assume that "j < j + 10" is true.  We don't want to warn
+	 about that case for -Wstrict-overflow, because in general we
+	 don't warn about overflow involving loops.  Prevent the
+	 warning by setting TREE_NO_WARNING.  */
+      if (warn_strict_overflow > 0)
+	{
+	  unsigned int i;
+
+	  for (i = 0; i < n_bbs; ++i)
+	    {
+	      tree last;
+
+	      last = last_stmt (copied_bbs[i]);
+	      if (TREE_CODE (last) == COND_EXPR)
+		TREE_NO_WARNING (last) = 1;
+	    }
+	}
+
       /* Ensure that the latch and the preheader is simple (we know that they
 	 are not now, since there was the loop exit condition.  */
       loop_split_edge_with (loop_preheader_edge (loop), NULL);