diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 5fd5c7a2476..cf7505c41c1 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2016-10-06  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/77855
+	* tree-ssa-pre.c (prune_clobbered_mems): Queue exprs to remove
+	instead of removing the current item while iterating over the set
+	which is not safe.
+
 2016-10-06  James Clarke  <jrtc27@jrtc27.com>
             Eric Botcazou  <ebotcazou@adacore.com>
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 085884660fa..e180375cd7c 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-10-06  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/77855
+	* gcc.dg/torture/pr77855.c: New testcase.
+
 2016-10-06  James Clarke  <jrtc27@jrtc27.com>
             Eric Botcazou  <ebotcazou@adacore.com>
 
diff --git a/gcc/testsuite/gcc.dg/torture/pr77855.c b/gcc/testsuite/gcc.dg/torture/pr77855.c
new file mode 100644
index 00000000000..bec5af32013
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr77855.c
@@ -0,0 +1,48 @@
+/* { dg-do run } */
+
+int a, b = 1, c, e, f, g, k, m, n, o;
+char d, h, i, j, l; 
+char res[2];
+
+void __attribute__ ((noinline,noclone)) fn2 ()
+{
+  d = 2;
+}
+
+void fn3 ()
+{
+  for (;;)
+    {
+      for (; b; b--)
+	{
+	  fn2 ();  
+	  if (e)
+	    j = 1;
+	  if (f)
+	    L1:
+		k = j | (a & l);
+	  for (;;)
+	    {
+	      __builtin_snprintf (res, 2, "%d\n", d);
+	      if (d)
+		break;
+	      for (; o; o--)
+		for (; n;)
+		  for (; m; m++)
+		    ;
+	      goto L1;
+	    }
+	}
+      g = h;
+      c = i;
+      break;
+    }
+}
+
+int main ()
+{
+  fn3 ();
+  if (res[0] != '2')
+    __builtin_abort ();
+  return 0; 
+}
diff --git a/gcc/tree-ssa-pre.c b/gcc/tree-ssa-pre.c
index 39bc2548c10..4f172005f26 100644
--- a/gcc/tree-ssa-pre.c
+++ b/gcc/tree-ssa-pre.c
@@ -2025,9 +2025,17 @@ prune_clobbered_mems (bitmap_set_t set, basic_block block)
 {
   bitmap_iterator bi;
   unsigned i;
+  pre_expr to_remove = NULL;
 
   FOR_EACH_EXPR_ID_IN_SET (set, i, bi)
     {
+      /* Remove queued expr.  */
+      if (to_remove)
+	{
+	  bitmap_remove_from_set (set, to_remove);
+	  to_remove = NULL;
+	}
+
       pre_expr expr = expression_for_id (i);
       if (expr->kind == REFERENCE)
 	{
@@ -2041,7 +2049,7 @@ prune_clobbered_mems (bitmap_set_t set, basic_block block)
 					   block, gimple_bb (def_stmt)))
 		      || (gimple_bb (def_stmt) == block
 			  && value_dies_in_block_x (expr, block))))
-		bitmap_remove_from_set (set, expr);
+		to_remove = expr;
 	    }
 	}
       else if (expr->kind == NARY)
@@ -2053,9 +2061,13 @@ prune_clobbered_mems (bitmap_set_t set, basic_block block)
 	     as the available expression might be after the exit point.  */
 	  if (BB_MAY_NOTRETURN (block)
 	      && vn_nary_may_trap (nary))
-	    bitmap_remove_from_set (set, expr);
+	    to_remove = expr;
 	}
     }
+
+  /* Remove queued expr.  */
+  if (to_remove)
+    bitmap_remove_from_set (set, to_remove);
 }
 
 static sbitmap has_abnormal_preds;