re PR tree-optimization/85244 (Bad optimisation with flexible array member (may…

re PR tree-optimization/85244 (Bad optimisation with flexible array member (may be related to -ftree-dominator-opts))

2018-04-06  Richard Biener  <rguenther@suse.de>

	PR middle-end/85244
	* tree-dfa.c (get_ref_base_and_extent): Reset seen_variable_array_ref
	after seeing a component reference with an adjacent field.  Treat
	refs to arrays at struct end of external decls similar to
	refs to unconstrained commons.

	* gcc.dg/torture/pr85244-1.c: New testcase.
	* gcc.dg/torture/pr85244-2.c: Likewise.

From-SVN: r259168

gcc/ChangeLog

+2018-04-06  Richard Biener  <rguenther@suse.de>
+
+	PR middle-end/85244
+	* tree-dfa.c (get_ref_base_and_extent): Reset seen_variable_array_ref
+	after seeing a component reference with an adjacent field.  Treat
+	refs to arrays at struct end of external decls similar to
+	refs to unconstrained commons.
+
 2018-04-06  Jakub Jelinek  <jakub@redhat.com>
 
 	PR sanitizer/85213

gcc/testsuite/ChangeLog

+2018-04-06  Richard Biener  <rguenther@suse.de>
+
+	PR middle-end/85244
+	* gcc.dg/torture/pr85244-1.c: New testcase.
+	* gcc.dg/torture/pr85244-2.c: Likewise.
+
 2018-04-06  Jakub Jelinek  <jakub@redhat.com>
 
 	PR sanitizer/85213

gcc/testsuite/gcc.dg/torture/pr85244-1.c

+/* { dg-do run } */
+/* { dg-additional-sources "pr85244-2.c" } */
+
+struct s {
+ long a;
+ int b;
+ int tab[];
+};
+
+extern const struct s val;
+extern int idx;
+extern void abort (void);
+
+int main()
+{
+  if (val.tab[0] != 42 || val.tab[1] != 1337 || val.tab[idx] != 1337)
+    abort ();
+  return 0;
+}

gcc/testsuite/gcc.dg/torture/pr85244-2.c

+struct s {
+    long a;
+    int b;
+    int tab[];
+};
+
+int idx = 1;
+const struct s val = { 0, 0, { 42, 1337 } };

gcc/tree-dfa.c

@@ -438,7 +438,7 @@ get_ref_base_and_extent (tree exp, poly_int64_pod *poffset,
 		   referenced the last field of a struct or a union member
 		   then we have to adjust maxsize by the padding at the end
 		   of our field.  */
-		if (seen_variable_array_ref && known_size_p (maxsize))
+		if (seen_variable_array_ref)
 		  {
 		    tree stype = TREE_TYPE (TREE_OPERAND (exp, 0));
 		    tree next = DECL_CHAIN (field);
@@ -454,7 +454,7 @@ get_ref_base_and_extent (tree exp, poly_int64_pod *poffset,
 			    || ssize == NULL
 			    || !poly_int_tree_p (ssize))
 			  maxsize = -1;
-			else
+			else if (known_size_p (maxsize))
 			  {
 			    poly_offset_int tem
 			      = (wi::to_poly_offset (ssize)
@@ -464,6 +464,11 @@ get_ref_base_and_extent (tree exp, poly_int64_pod *poffset,
 			    maxsize += tem;
 			  }
 		      }
+		    /* An component ref with an adjacent field up in the
+		       structure hierarchy constrains the size of any variable
+		       array ref lower in the access hierarchy.  */
+		    else
+		      seen_variable_array_ref = false;
 		  }
 	      }
 	    else
@@ -622,7 +627,9 @@ get_ref_base_and_extent (tree exp, poly_int64_pod *poffset,
 
   if (DECL_P (exp))
     {
-      if (flag_unconstrained_commons && VAR_P (exp) && DECL_COMMON (exp))
+      if (VAR_P (exp)
+	  && ((flag_unconstrained_commons && DECL_COMMON (exp))
+	      || (DECL_EXTERNAL (exp) && seen_variable_array_ref)))
 	{
 	  tree sz_tree = TYPE_SIZE (TREE_TYPE (exp));
 	  /* If size is unknown, or we have read to the end, assume there