[PATCH] Fix for PR52734 (-ftree-tail-merge)

[Tom de Vries <Tom_deVries at mentor dot com>]

Richard,

this patch fixes PR52743.

The problem is as follows: blocks 3 and 5, with successor 6 are considered equal
and merged.
...
  # BLOCK 3 freq:6102
  # PRED: 2 [61.0%]  (true,exec)
  # VUSE <.MEMD.1734_10>
  dddD.1710_3 = bbbD.1703;
  goto <bb 6>;
  # SUCC: 6 [100.0%]  (fallthru,exec)

  # BLOCK 5 freq:2378
  # PRED: 4 [61.0%]  (false,exec)
  # SUCC: 6 [100.0%]  (fallthru,exec)

  # BLOCK 6 freq:10000
  # PRED: 3 [100.0%]  (fallthru,exec) 7 [100.0%]  (fallthru) 5 [100.0%]
(fallthru,exec)
  # dddD.1710_1 = PHI <dddD.1710_3(3), 0(7), dddD.1710_4(5)>
  # .MEMD.1734_8 = PHI <.MEMD.1734_10(3), .MEMD.1734_11(7), .MEMD.1734_11(5)>
  # VUSE <.MEMD.1734_8>
  return dddD.1710_1;
  # SUCC: EXIT [100.0%]
...

Tail merge considers 2 blocks equal if the effect at the tail is equal,
meaning:
- the sequence of side effects produced by each block is equal
- the value phis are equal

There are no side effects in block 3 and 5, and the phi alternatives of
dddD.1710_1 for 3 (dddD.1710_3)  and 5 (dddD.1710_4)  are proven equal by gvn.

The problem is that changing the (4->5) edge into a (4->3) edge changes the
value of dddD.1710_3, because block 4 contains a store that affects the load in
block 3.
...
  # BLOCK 4 freq:3898
  # PRED: 2 [39.0%]  (false,exec)
  # VUSE <.MEMD.1734_10>
  dddD.1710_4 = bbbD.1703;
  # .MEMD.1734_11 = VDEF <.MEMD.1734_10>
  # USE = nonlocal null
  # CLB = nonlocal null
  D.1724_5 = aaaD.1705 ();
  if (D.1724_5 != 0)
    goto <bb 7>;
  else
    goto <bb 5>;
  # SUCC: 7 [39.0%]  (true,exec) 5 [61.0%]  (false,exec)
...

Or, put differently, the incoming vuse of block 3 affects a value phi
alternative for that block (dddD.1710_3), so the 2 blocks are equal only under
the condition that the incoming vuses are equal.

We could build an analysis that addresses that precisely, but for now I
implemented a more coarse-grained fix: if the incoming vuses are not equal, and
at least one of the vuses influenced a non-virtual result, we don't consider the
blocks equal.

Bootstrapped and reg-tested on x86_64.

ok for trunk, 4.7.1?

Thanks,
- Tom

2012-04-13  Tom de Vries  <tom@codesourcery.com>

	* tree-ssa-tail-merge.c (gsi_advance_bw_nondebug_nonlocal): Add
	parameters vuse and vuse_escaped.
	(find_duplicate): Init vuse1, vuse2 and vuse_escaped.  Pass to
	gsi_advance_bw_nondebug_nonlocal.  Return if vuse_escaped and
	vuse1 != vuse2.

	* gcc.dg/pr52734.c: New test.

Index: gcc/tree-ssa-tail-merge.c
===================================================================
--- gcc/tree-ssa-tail-merge.c (revision 185028)
+++ gcc/tree-ssa-tail-merge.c (working copy)
@@ -1123,18 +1123,31 @@ gimple_equal_p (same_succ same_succ, gim
     }
 }
 
-/* Let GSI skip backwards over local defs.  */
+/* Let GSI skip backwards over local defs.  Return the earliest vuse in VUSE.
+   Return true in VUSE_ESCAPED if the vuse influenced a SSA_OP_DEF of one of the
+   processed statements.  */
 
 static void
-gsi_advance_bw_nondebug_nonlocal (gimple_stmt_iterator *gsi)
+gsi_advance_bw_nondebug_nonlocal (gimple_stmt_iterator *gsi, tree *vuse,
+				  bool *vuse_escaped)
 {
   gimple stmt;
+  tree lvuse;
 
   while (true)
     {
       if (gsi_end_p (*gsi))
 	return;
       stmt = gsi_stmt (*gsi);
+
+      lvuse = gimple_vuse (stmt);
+      if (lvuse != NULL_TREE)
+	{
+	  *vuse = lvuse;
+	  if (!ZERO_SSA_OPERANDS (stmt, SSA_OP_DEF))
+	    *vuse_escaped = true;
+	}
+
       if (!(is_gimple_assign (stmt) && local_def (gimple_get_lhs (stmt))
 	    && !gimple_has_side_effects (stmt)))
 	return;
@@ -1150,9 +1163,11 @@ find_duplicate (same_succ same_succ, bas
 {
   gimple_stmt_iterator gsi1 = gsi_last_nondebug_bb (bb1);
   gimple_stmt_iterator gsi2 = gsi_last_nondebug_bb (bb2);
+  tree vuse1 = NULL_TREE, vuse2 = NULL_TREE;
+  bool vuse_escaped = false;
 
-  gsi_advance_bw_nondebug_nonlocal (&gsi1);
-  gsi_advance_bw_nondebug_nonlocal (&gsi2);
+  gsi_advance_bw_nondebug_nonlocal (&gsi1, &vuse1, &vuse_escaped);
+  gsi_advance_bw_nondebug_nonlocal (&gsi2, &vuse2, &vuse_escaped);
 
   while (!gsi_end_p (gsi1) && !gsi_end_p (gsi2))
     {
@@ -1161,13 +1176,20 @@ find_duplicate (same_succ same_succ, bas
 
       gsi_prev_nondebug (&gsi1);
       gsi_prev_nondebug (&gsi2);
-      gsi_advance_bw_nondebug_nonlocal (&gsi1);
-      gsi_advance_bw_nondebug_nonlocal (&gsi2);
+      gsi_advance_bw_nondebug_nonlocal (&gsi1, &vuse1, &vuse_escaped);
+      gsi_advance_bw_nondebug_nonlocal (&gsi2, &vuse2, &vuse_escaped);
     }
 
   if (!(gsi_end_p (gsi1) && gsi_end_p (gsi2)))
     return;
 
+  /* If the incoming vuses are not the same, and the vuse escaped into an
+     SSA_OP_DEF, then merging the 2 blocks will change the value of the def,
+     which potentially means the semantics of one of the blocks will be changed.
+     TODO: make this check more precise.  */
+  if (vuse_escaped && vuse1 != vuse2)
+    return;
+
   if (dump_file)
     fprintf (dump_file, "find_duplicates: <bb %d> duplicate of <bb %d>\n",
 	     bb1->index, bb2->index);
Index: gcc/testsuite/gcc.dg/pr52734.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.dg/pr52734.c (revision 0)
@@ -0,0 +1,35 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+int bbb = 0;
+
+int __attribute__((noinline,noclone)) aaa(void)
+{
+    ++bbb;
+    return 0;
+}
+
+int __attribute__((noinline,noclone)) ccc(void)
+{
+  int ddd;
+  /* bbb == 0 */
+  if (aaa())
+    return bbb;
+
+  /* bbb == 1 */
+  ddd = bbb;
+  /* bbb == ddd == 1 */
+  if (aaa ())
+    return 0;
+  /* bbb == 2, ddd == 1 */
+
+  return ddd;
+}
+
+int main(void)
+{
+    if (ccc() != 1)
+	__builtin_abort();
+    return 0;
+}
+