PR c/77336 - -Wsuggest-attribute=format warning overly simplistic

gcc/c-family/ChangeLog:

	PR c/77336
	* c-format.c (check_function_format): Avoid issuing warnings for
	functions unless they call format functions with non-constant
	format strings.

gcc/testsuite/ChangeLog:

	PR c/77336
	* gcc.dg/format/miss-7.c: New test.

From-SVN: r240013

gcc/c-family/ChangeLog

+2016-09-06 Martin Sebor  <msebor@redhat.com>
+
+	PR c/77336
+	* c-format.c (check_function_format): Avoid issuing warnings for
+	functions unless they call format functions with non-constant
+	format strings.
+
 2016-09-06  Richard Biener  <rguenther@suse.de>
 
 	PR c/77450

gcc/c-family/c-format.c

@@ -31,6 +31,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "c-format.h"
 #include "diagnostic.h"
 #include "selftest.h"
+#include "builtins.h"
 
 /* Handle attributes associated with format checking.  */
 
@@ -1212,9 +1213,17 @@ check_function_format (tree attrs, int nargs, tree *argarray)
 		params = tree_cons (NULL_TREE, argarray[i], params);
 	      check_format_info (&info, params);
 	    }
+
+	  /* Attempt to detect whether the current function might benefit
+	     from the format attribute if the called function is decorated
+	     with it.  Avoid using calls with string literal formats for
+	     guidance since those are unlikely to be viable candidates.  */
 	  if (warn_suggest_attribute_format && info.first_arg_num == 0
 	      && (format_types[info.format_type].flags
-		  & (int) FMT_FLAG_ARG_CONVERT))
+		  & (int) FMT_FLAG_ARG_CONVERT)
+	      /* c_strlen will fail for a function parameter but succeed
+		 for a literal or constant array.  */
+	      && !c_strlen (argarray[info.format_num - 1], 1))
 	    {
 	      tree c;
 	      for (c = TYPE_ATTRIBUTES (TREE_TYPE (current_function_decl));
@@ -1242,8 +1251,9 @@ check_function_format (tree attrs, int nargs, tree *argarray)
 			break;
 		    }
 		  if (args != 0)
-		    warning (OPT_Wsuggest_attribute_format, "function might "
-			     "be possible candidate for %qs format attribute",
+		    warning (OPT_Wsuggest_attribute_format, "function %qD "
+			     "might be a candidate for %qs format attribute",
+			     current_function_decl,
 			     format_types[info.format_type].name);
 		}
 	    }

gcc/testsuite/ChangeLog

+2016-09-06 Martin Sebor  <msebor@redhat.com>
+
+	PR c/77336
+	* gcc.dg/format/miss-7.c: New test.
+
 2016-09-06  Uros Bizjak  <ubizjak@gmail.com>
 
 	* gfortran.dg/c_by_val_1.f: Correct the call to c_to_c and c_to_c8.

gcc/testsuite/gcc.dg/format/miss-7.c

+/* PR77336 - -Wsuggest-attribute=format warning overly simplistic */
+/* { dg-do compile } */
+/* { dg-options "-Wsuggest-attribute=format" } */
+
+#include "format.h"
+
+const char format[] = "%i";
+
+void foo (char *d, unsigned n, va_list va)
+{
+  (void)&n;
+
+  /* The following calls don't imply that the enclosing function is
+     a candiate for the format attribute because it uses a string
+     constant as the format.  */
+  vsnprintf (d, n, "%i", va);
+
+  vsnprintf (d, n, format, va);
+
+  /* In theory this should not trigger the warning either but GCC
+     doesn't treat the local static constant the same way as the
+     global and issues a false positive.
+  const char fmt[] = "%i";
+  vsnprintf (d, n, fmt, va);
+  */
+}
+
+void bar (char *d, unsigned n, const char *f, va_list va)
+{
+  (void)&n;
+
+  /* The following call suggests that the enclosing function might
+     be a candiate for the format attribute because it doesn't use
+     a string literal as the format.  */
+  vsnprintf (d, n, f, va);   /* { dg-warning "function .bar. might be a candidate for .gnu_printf. format attribute" } */
+}