re PR tree-optimization/78965 (Invalid -fprintf-return-value optimization)

	PR tree-optimization/78965
	* gimple-ssa-sprintf.c (pass_sprintf_length::compute_format_length):
	Change first argument from const call_info & to call_info &.  For %n
	set info.nowrite to false.

	* gcc.dg/pr78965.c: New test.

From-SVN: r244014

gcc/ChangeLog

 2017-01-03  Jakub Jelinek  <jakub@redhat.com>
 
+	PR tree-optimization/78965
+	* gimple-ssa-sprintf.c (pass_sprintf_length::compute_format_length):
+	Change first argument from const call_info & to call_info &.  For %n
+	set info.nowrite to false.
+
 	PR middle-end/78901
 	* gimple-ssa-sprintf.c (try_substitute_return_value): Don't change
 	possibly throwing calls.

gcc/gimple-ssa-sprintf.c

@@ -131,7 +131,7 @@ public:
   void handle_gimple_call (gimple_stmt_iterator*);
 
   struct call_info;
-  bool compute_format_length (const call_info &, format_result *);
+  bool compute_format_length (call_info &, format_result *);
 };
 
 bool
@@ -710,7 +710,8 @@ struct pass_sprintf_length::call_info
 
   /* True for bounded functions like snprintf that specify a zero-size
      buffer as a request to compute the size of output without actually
-     writing any.  */
+     writing any.  NOWRITE is cleared in response to the %n directive
+     which has side-effects similar to writing output.  */
   bool nowrite;
 };
 
@@ -2357,7 +2358,7 @@ add_bytes (const pass_sprintf_length::call_info &info,
    that caused the processing to be terminated early).  */
 
 bool
-pass_sprintf_length::compute_format_length (const call_info &info,
+pass_sprintf_length::compute_format_length (call_info &info,
 					    format_result *res)
 {
   /* The variadic argument counter.  */
@@ -2624,6 +2625,9 @@ pass_sprintf_length::compute_format_length (const call_info &info,
 	  return false;
 
 	case 'n':
+	  /* %n has side-effects even when nothing is actually printed to
+	     any buffer.  */
+	  info.nowrite = false;
 	  break;
 
 	case 'c':

gcc/testsuite/ChangeLog

 2017-01-03  Jakub Jelinek  <jakub@redhat.com>
 
+	PR tree-optimization/78965
+	* gcc.dg/pr78965.c: New test.
+
 	PR middle-end/78901
 	* g++.dg/opt/pr78901.C: New test.
 

gcc/testsuite/gcc.dg/pr78965.c

+/* PR tree-optimization/78965 */
+/* { dg-do run { target c99_runtime } } */
+/* { dg-options "-O2" } */
+/* { dg-add-options c99_runtime } */
+
+int
+main ()
+{
+  int a = 5, b = 6;
+  int c = __builtin_snprintf (0, 0, "a%nb%nc", &a, &b);
+  if (a + b + c != 6)
+    __builtin_abort ();
+  return 0;
+}