gcc/

	PR tree-optimization/71691
	* bitmap.h (class auto_bitmap): New.
	* tree-ssa-loop-unswitch.c (tree_may_unswitch_on): Call
	is_maybe_undefined instead of ssa_undefined_value_p.

gcc/testsuite/

	PR tree-optimization/71691
	* gcc.dg/loop-unswitch-5.c: Test that we actually unswitch a loop.

diff --git a/gcc/bitmap.h b/gcc/bitmap.h
index 196738b..f158b44 100644
--- a/gcc/bitmap.h
+++ b/gcc/bitmap.h
@@ -802,4 +802,25 @@ bmp_iter_and_compl (bitmap_iterator *bi, unsigned *bit_no)
        bmp_iter_and_compl (&(ITER), &(BITNUM));				\
        bmp_iter_next (&(ITER), &(BITNUM)))
 
+/* A class that ties the lifetime of a bitmap to its scope.  */
+class auto_bitmap
+{
+ public:
+  auto_bitmap () { bits = BITMAP_ALLOC (NULL); }
+  ~auto_bitmap () { BITMAP_FREE (bits); }
+  // Allow calling bitmap functions on our bitmap.
+  operator bitmap () { return bits; }
+
+ private:
+  // Prevent making a copy that references our bitmap.
+  auto_bitmap (const auto_bitmap &);
+  auto_bitmap &operator = (const auto_bitmap &);
+#if __cplusplus >= 201103L
+  auto_bitmap (auto_bitmap &&);
+  auto_bitmap &operator = (auto_bitmap &&);
+#endif
+
+  bitmap bits;
+};
+
 #endif /* GCC_BITMAP_H */
diff --git a/gcc/testsuite/gcc.dg/loop-unswitch-1.c b/gcc/testsuite/gcc.dg/loop-unswitch-1.c
index 930364c..f6fc41d 100644
--- a/gcc/testsuite/gcc.dg/loop-unswitch-1.c
+++ b/gcc/testsuite/gcc.dg/loop-unswitch-1.c
@@ -1,6 +1,6 @@
 /* For PR rtl-optimization/27735  */
 /* { dg-do compile } */
-/* { dg-options "-O2 -funswitch-loops" } */
+/* { dg-options "-O2 -funswitch-loops -fdump-tree-unswitch-details" } */
 
 void set_color(void);
 void xml_colorize_line(unsigned int *p, int state)
@@ -32,3 +32,5 @@ parse_tag: ;
     }
 }
 
+/* Test that we actually unswitched something.  */
+/* { dg-final { scan-tree-dump ";; Unswitching loop" "unswitch" } } */
diff --git a/gcc/testsuite/gcc.dg/loop-unswitch-5.c b/gcc/testsuite/gcc.dg/loop-unswitch-5.c
new file mode 100644
index 0000000..b41e853
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/loop-unswitch-5.c
@@ -0,0 +1,51 @@
+/* PR middle-end/71691 */
+/* { dg-do run } */
+/* { dg-options "-fno-tree-vrp -O2 -funswitch-loops -fdump-tree-unswitch-details" } */
+
+/* Note: The -fno-tree-vrp above is only there to avoid VRP papering
+   over the problem.  */
+
+char b;
+short f;
+unsigned e;
+int g = 20;
+
+void
+foo ()
+{
+  int l, h;
+  for (l = 0; l <= 7; l++)
+    {
+      int j = 38;
+      if (g)
+	h = 0;
+      for (; h <= 7; h++)
+	{
+	  int i, k = b % (j % 4);
+	  g = f;
+	  for (;;)
+	    {
+	      j = 6 || b;
+	      if (e)
+		{
+		  for (; j; --j)
+		    if (k)
+		      __builtin_printf ("%d", 9);
+		  if (i)
+		    __builtin_printf ("%d", j);
+		}
+	      if (l)
+		continue;
+	      break;
+	    }
+	  i = f || b;
+	}
+    }
+}
+
+int
+main ()
+{
+  foo ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-loop-unswitch.c b/gcc/tree-ssa-loop-unswitch.c
index 92599fb..28ae7dc 100644
--- a/gcc/tree-ssa-loop-unswitch.c
+++ b/gcc/tree-ssa-loop-unswitch.c
@@ -109,6 +109,84 @@ tree_ssa_unswitch_loops (void)
   return 0;
 }
 
+/* Return TRUE if an SSA_NAME maybe undefined and is therefore
+   unsuitable for unswitching.  STMT is the statement we are
+   considering for unswitching and LOOP is the loop it appears in.  */
+
+static bool
+is_maybe_undefined (const tree name, gimple *stmt, struct loop *loop)
+{
+  /* The loop header is the only block we can trivially determine that
+     will always be executed.  If the comparison is in the loop
+     header, we know it's OK to unswitch on it.  */
+  if (gimple_bb (stmt) == loop->header)
+    return false;
+
+  auto_bitmap visited_ssa;
+  auto_vec<tree> worklist;
+  worklist.safe_push (name);
+  bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
+  while (!worklist.is_empty ())
+    {
+      tree t = worklist.pop ();
+
+      /* If it's obviously undefined, avoid further computations.  */
+      if (ssa_undefined_value_p (t, true))
+	return true;
+
+      /* A PARM_DECL will not have an SSA_NAME_DEF_STMT.  Parameters
+	 get their initial value from function entry.  */
+      if (SSA_NAME_VAR (t) && TREE_CODE (SSA_NAME_VAR (t)) == PARM_DECL)
+	continue;
+
+      gimple *def = SSA_NAME_DEF_STMT (t);
+
+#if 0
+      /* Uses in stmts always executed when the region header executes
+	 are fine.  */
+      if (dominated_by_p (CDI_DOMINATORS, loop->header, gimple_bb (def)))
+	continue;
+#endif
+
+      /* Check that all the PHI args are fully defined.  */
+      if (gphi *phi = dyn_cast <gphi *> (def))
+	{
+	  for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
+	    {
+	      tree t = gimple_phi_arg_def (phi, i);
+	      /* If an SSA has already been seen, it may be a loop,
+		 but we can continue and ignore this use.  Otherwise,
+		 add the SSA_NAME to the queue and visit it later.  */
+	      if (TREE_CODE (t) == SSA_NAME
+		  && bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (t)))
+		worklist.safe_push (t);
+	    }
+	  continue;
+	}
+
+      /* Handle calls and memory loads conservatively.  */
+      if (!is_gimple_assign (def)
+	  || (gimple_assign_single_p (def)
+	      && gimple_vuse (def)))
+	return true;
+
+      /* Check that any SSA names used to define NAME are also fully
+	 defined.  */
+      use_operand_p use_p;
+      ssa_op_iter iter;
+      FOR_EACH_SSA_USE_OPERAND (use_p, def, iter, SSA_OP_USE)
+	{
+	  tree t = USE_FROM_PTR (use_p);
+	  /* If an SSA has already been seen, it may be a loop,
+	     but we can continue and ignore this use.  Otherwise,
+	     add the SSA_NAME to the queue and visit it later.  */
+	  if (bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (t)))
+	    worklist.safe_push (t);
+	}
+    }
+  return false;
+}
+
 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
    basic blocks (for what it means see comments below).  */
 
@@ -136,15 +214,15 @@ tree_may_unswitch_on (basic_block bb, struct loop *loop)
   /* Condition must be invariant.  */
   FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
     {
-      /* Unswitching on undefined values would introduce undefined
-	 behavior that the original program might never exercise.  */
-      if (ssa_undefined_value_p (use, true))
-	return NULL_TREE;
       def = SSA_NAME_DEF_STMT (use);
       def_bb = gimple_bb (def);
       if (def_bb
 	  && flow_bb_inside_loop_p (loop, def_bb))
 	return NULL_TREE;
+      /* Unswitching on undefined values would introduce undefined
+	 behavior that the original program might never exercise.  */
+      if (is_maybe_undefined (use, stmt, loop))
+	return NULL_TREE;
     }
 
   cond = build2 (gimple_cond_code (stmt), boolean_type_node,