2009-01-16 Kenneth Zadeck <zadeck@naturalbridge.com>

	* dce.c (delete_unmarked_insns): Reversed the order that insns are
	examined before deleting them.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@143433 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index c40950f..3a7a72c 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2009-01-16  Kenneth Zadeck <zadeck@naturalbridge.com>
+
+       * dce.c (delete_unmarked_insns): Reversed the order that insns are
+       examined before deleting them.
+       
 2009-01-16  Richard Earnshaw  <rearnsha@arm.com>
 
        * function.c (aggregate_value_p): Correctly extract the function

diff --git a/gcc/dce.c b/gcc/dce.c
index 5a64fb2..f6a10d6 100644 (file)
--- a/gcc/dce.c
+++ b/gcc/dce.c
@@ -510,8 +510,8 @@ delete_unmarked_insns (void)
   rtx insn, next;
   bool must_clean = false;
 
-  FOR_EACH_BB (bb)
-    FOR_BB_INSNS_SAFE (bb, insn, next)
+  FOR_EACH_BB_REVERSE (bb)
+    FOR_BB_INSNS_REVERSE_SAFE (bb, insn, next)
       if (INSN_P (insn))
        {
          /* Always delete no-op moves.  */
@@ -522,13 +522,24 @@ delete_unmarked_insns (void)
          else if (marked_insn_p (insn))
            continue;
 
-         /* Beware that reaching a dbg counter limit here can rarely
-            result in miscompiled file.  This occurs when a group of
-            insns must be deleted together.  Currently this only
-            can happen on non-looping pure and constant calls
-            on machines where ACCUMULATE_OUTGOING_ARGS is true.  By
-            using the dbg_cnt, it is possible to remove the call, but
-            leave the argument pushes to the stack.  */
+         /* Beware that reaching a dbg counter limit here can result
+            in miscompiled file.  This occurs when a group of insns
+            must be deleted together, typically because the kept insn
+            depends on the output from the deleted insn.  Deleting
+            this insns in reverse order (both at the bb level and
+            when looking at the blocks) minimizes this, but does not
+            eliminate it, since it is possible for the using insn to
+            be top of a block and the producer to be at the bottom of
+            the block.  However, in most cases this will only result
+            in an uninitialized use of an insn that is dead anyway.
+
+            However, there is one rare case that will cause a
+            miscompile: deletion of non-looping pure and constant
+            calls on a machine where ACCUMULATE_OUTGOING_ARGS is true.
+            In this case it is possible to remove the call, but leave
+            the argument pushes to the stack.  Because of the changes
+            to the stack pointer, this will almost always lead to a
+            miscompile.  */
          if (!dbg_cnt (dce))
            continue;