re PR tree-optimization/45970 (tree DSE misses many obvious dead stores)

2010-10-13  Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/45970
	* tree-ssa-alias.h (stmt_kills_ref_p): Declare.
	* tree-ssa-alias.c (stmt_kills_ref_p_1): New function.
	(stmt_kills_ref_p): Likewise.
	* tree-ssa-dse.c (dse_optimize_stmt): Use it.

	* gcc.dg/tree-ssa/ssa-dse-13.c: New testcase.

From-SVN: r165422

gcc/ChangeLog

@@ -1,3 +1,11 @@
+2010-10-13  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/45970
+	* tree-ssa-alias.h (stmt_kills_ref_p): Declare.
+	* tree-ssa-alias.c (stmt_kills_ref_p_1): New function.
+	(stmt_kills_ref_p): Likewise.
+	* tree-ssa-dse.c (dse_optimize_stmt): Use it.
+
 2010-10-13  Richard Guenther  <rguenther@suse.de>
 
 	PR tree-optimization/45982

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2010-10-13  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/45970
+	* gcc.dg/tree-ssa/ssa-dse-13.c: New testcase.
+
 2010-10-13  Richard Guenther  <rguenther@suse.de>
 
 	PR tree-optimization/45982

gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-13.c

@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1-details" } */
+
+struct A { char c[4]; } a, b;
+
+void
+f1 (void)
+{
+  __builtin_memcpy (&a.c[0], "a", 1);
+  a = b;
+}
+
+void
+f2 (void)
+{
+  __builtin_memcpy (&a.c[0], "a", 1);
+  __builtin_memcpy (&a.c[0], "cdef", 4);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 2 "dse1" } } */
+/* { dg-final { cleanup-tree-dump "dse1" } } */

gcc/tree-ssa-alias.c

@@ -1522,6 +1522,45 @@ stmt_may_clobber_ref_p (gimple stmt, tree ref)
   return stmt_may_clobber_ref_p_1 (stmt, &r);
 }
 
+/* If STMT kills the memory reference REF return true, otherwise
+   return false.  */
+
+static bool
+stmt_kills_ref_p_1 (gimple stmt, ao_ref *ref)
+{
+  if (gimple_has_lhs (stmt)
+      && TREE_CODE (gimple_get_lhs (stmt)) != SSA_NAME)
+    {
+      tree base, lhs = gimple_get_lhs (stmt);
+      HOST_WIDE_INT size, offset, max_size;
+      ao_ref_base (ref);
+      base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
+      /* We can get MEM[symbol: sZ, index: D.8862_1] here,
+	 so base == ref->base does not always hold.  */
+      if (base == ref->base)
+	{
+	  /* For a must-alias check we need to be able to constrain
+	     the accesses properly.  */
+	  if (size != -1 && size == max_size
+	      && ref->max_size != -1)
+	    {
+	      if (offset <= ref->offset
+		  && offset + size >= ref->offset + ref->max_size)
+		return true;
+	    }
+	}
+    }
+  return false;
+}
+
+bool
+stmt_kills_ref_p (gimple stmt, tree ref)
+{
+  ao_ref r;
+  ao_ref_init (&r, ref);
+  return stmt_kills_ref_p_1 (stmt, &r);
+}
+
 
 /* Walk the virtual use-def chain of VUSE until hitting the virtual operand
    TARGET or a statement clobbering the memory reference REF in which

gcc/tree-ssa-alias.h

@@ -105,6 +105,7 @@ extern bool ref_maybe_used_by_stmt_p (gimple, tree);
 extern bool stmt_may_clobber_ref_p (gimple, tree);
 extern bool stmt_may_clobber_ref_p_1 (gimple, ao_ref *);
 extern bool call_may_clobber_ref_p (gimple, tree);
+extern bool stmt_kills_ref_p (gimple, tree);
 extern tree get_continuation_for_phi (gimple, ao_ref *, bitmap *);
 extern void *walk_non_aliased_vuses (ao_ref *, tree,
 				     void *(*)(ao_ref *, tree, void *),

gcc/tree-ssa-dse.c

@@ -301,8 +301,9 @@ dse_optimize_stmt (struct dse_global_data *dse_gd,
 	 virtual uses from stmt and the stmt which stores to that same
 	 memory location, then we may have found redundant store.  */
       if (bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
-	  && operand_equal_p (gimple_assign_lhs (stmt),
+	  && (operand_equal_p (gimple_assign_lhs (stmt),
-			      gimple_assign_lhs (use_stmt), 0))
+			       gimple_assign_lhs (use_stmt), 0)
+	      || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt))))
 	{
 	  /* If use_stmt is or might be a nop assignment, e.g. for
 	     struct { ... } S a, b, *p; ...