[C PATCH] Warn for optimize attribute on decl after definition (PR c/70255)

[Marek Polacek <polacek at redhat dot com>]

In this PR, Richi pointed out that we don't warn for the case when a
declaration with attribute optimize follows the definition which is lacking
that attribute.  This patch adds such a warning.  Though the question is
whether this shouldn't apply to more attributes than just "optimize".  And,
as can be seen in the testcase, we'll warn for even for the case when the
definition has
  optimize ("no-associative-math,O2")
and the declaration
  optimize ("O2,no-associative-math")
Not sure if we have something better than attribute_value_equal, though.

(The C++ FE lacks these kind of warnings; I opened PR71024 for that.)

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-05-09  Marek Polacek  <polacek@redhat.com>

	PR c/70255
	* c-decl.c (diagnose_mismatched_decls): Warn for optimize attribute
	on a declaration following the definition.

	* gcc.dg/attr-opt-1.c: New test.

diff --git gcc/c/c-decl.c gcc/c/c-decl.c
index 7094efc..6f97ed9 100644
--- gcc/c/c-decl.c
+++ gcc/c/c-decl.c
@@ -2228,6 +2228,18 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 
   if (TREE_CODE (newdecl) == FUNCTION_DECL)
     {
+      tree a1 = lookup_attribute ("optimize", DECL_ATTRIBUTES (olddecl));
+      tree a2 = lookup_attribute ("optimize", DECL_ATTRIBUTES (newdecl));
+      /* An optimization attribute applied on a declaration after the
+	 definition is likely not what the user wanted.  */
+      if (a2 != NULL_TREE
+	  && DECL_SAVED_TREE (olddecl) != NULL_TREE
+	  && (a1 == NULL_TREE || !attribute_value_equal (a1, a2)))
+	warned |= warning (OPT_Wattributes,
+			   "optimization attribute on %qD follows "
+			   "definition but the attribute doesn%'t match",
+			   newdecl);
+
       /* Diagnose inline __attribute__ ((noinline)) which is silly.  */
       if (DECL_DECLARED_INLINE_P (newdecl)
 	  && lookup_attribute ("noinline", DECL_ATTRIBUTES (olddecl)))
diff --git gcc/testsuite/gcc.dg/attr-opt-1.c gcc/testsuite/gcc.dg/attr-opt-1.c
index e69de29..07fa4db 100644
--- gcc/testsuite/gcc.dg/attr-opt-1.c
+++ gcc/testsuite/gcc.dg/attr-opt-1.c
@@ -0,0 +1,30 @@
+/* PR c/70255 */
+/* { dg-do compile } */
+
+double
+fn1 (double h, double l) /* { dg-message "previous definition" } */
+{
+  return h + l;
+}
+double fn1 (double, double) __attribute__ ((optimize ("no-associative-math"))); /* { dg-warning "optimization attribute on .fn1. follows definition" } */
+
+__attribute__ ((optimize ("no-associative-math"))) double
+fn2 (double h, double l)
+{
+  return h + l;
+}
+double fn2 (double, double) __attribute__ ((optimize ("no-associative-math")));
+
+__attribute__ ((optimize ("no-associative-math"))) double
+fn3 (double h, double l) /* { dg-message "previous definition" } */
+{
+  return h + l;
+}
+double fn3 (double, double) __attribute__ ((optimize ("O2,no-associative-math"))); /* { dg-warning "optimization attribute on .fn3. follows definition" } */
+
+__attribute__ ((optimize ("no-associative-math,O2"))) double
+fn4 (double h, double l) /* { dg-message "previous definition" } */
+{
+  return h + l;
+}
+double fn4 (double, double) __attribute__ ((optimize ("O2,no-associative-math"))); /* { dg-warning "optimization attribute on .fn4. follows definition" } */

	Marek