[PATCH] PR c/46921 Lost side effect when struct initializer expression uses comma operator

[dave.pagan@oracle.com]

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This patch fixes improper handling of comma operator expression in a 
struct field initializer as described in:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46921

Currently, function output_init_element () does not evaluate the left 
hand expression in a comma operator that's used for a struct initializer 
field if the right hand side is zero-sized. However, the left hand 
expression must be evaluated if it's found to have side effects (for 
example, a function call).

Patch was successfully bootstrapped and tested on x86_64-linux.

--Dave



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/c
2018-02-26  David Pagan  <dave.pagan@oracle.com>

	PR c/46921
	* c-typeck.c (output_init_element): Ensure field initializer 
	expression is always evaluated if there are side effects.

/testsuite
2018-02-26  David Pagan  <dave.pagan@oracle.com>

	PR c/46921
	* gcc.dg/pr46921.c: New test.

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Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c	(revision 257975)
+++ gcc/c/c-typeck.c	(working copy)
@@ -9208,12 +9208,14 @@ output_init_element (location_t loc, tree value, t
 		      "enum conversion in initialization is invalid in C++");
     }
 
-  /* If this field is empty (and not at the end of structure),
-     don't do anything other than checking the initializer.  */
+  /* If this field is empty and does not have side effects (and is not at 
+     the end of structure), don't do anything other than checking the 
+     initializer.  */
   if (field
       && (TREE_TYPE (field) == error_mark_node
 	  || (COMPLETE_TYPE_P (TREE_TYPE (field))
 	      && integer_zerop (TYPE_SIZE (TREE_TYPE (field)))
+	      && !TREE_SIDE_EFFECTS (new_value)
 	      && (TREE_CODE (constructor_type) == ARRAY_TYPE
 		  || DECL_CHAIN (field)))))
     return;
Index: gcc/testsuite/gcc.dg/pr46921.c
===================================================================
--- gcc/testsuite/gcc.dg/pr46921.c	(revision 0)
+++ gcc/testsuite/gcc.dg/pr46921.c	(working copy)
@@ -0,0 +1,40 @@
+/* PR c/46921  lost side effect when struct initializer expr uses comma
+      operator  */
+
+/* { dg-do run } */
+/* { dg-options "" } */
+
+extern int printf(const char *, ...);
+extern void abort (void);
+
+typedef struct __uws_0 { } uw_unit;
+uw_unit uw_unit_v = {};
+
+struct __uws_1 
+{
+  struct __uws_0 __uwf_1;
+  struct __uws_1* __uwf_2;
+};
+
+static int left_hand_eval = 0;
+
+static void
+foo (const char *s)
+{
+  ++left_hand_eval;
+  printf("%s", s);
+}
+
+int
+main ()
+{
+  struct __uws_1 tmp = {(foo("Inner\n"), uw_unit_v)};
+  
+  printf("Outer\n");
+  /* left hand expression in comma operator initializer must always be
+     evaluated if there are side effects.  */
+  if (!left_hand_eval)
+    abort ();
+  
+  return 0;
+}