Re: RFA: PATCH to gimple-fold.c for c++/80916, bogus "static but not defined" warning

[Jason Merrill]

On Thu, Feb 28, 2019 at 12:18 PM Jason Merrill <ja...@redhat.com> wrote:
> On Thu, Feb 28, 2019 at 11:58 AM Jan Hubicka <hubi...@ucw.cz> wrote:
> > sorry for late reply - I did not identify it as a patch to symbol table.
> > Indeed we want can_refer_decl_in_current_unit_p is a good place to test
> > this.  Is there a reason to resrict this to functions with no body?
>
> If the function has a definition, then of course we can refer to it in
> its own unit.  Am I missing something?

Ah, yes, I was.  You mean, why do we care about DECL_INITIAL if
DECL_EXTERNAL is set?  I think I added that check out of caution.

This would be a more straightforward change:

commit 6af927c40585a4ff75a83b7cdabe8f9074a8d391
Author: Jason Merrill <ja...@redhat.com>
Date:   Fri Jan 25 09:09:17 2019 -0500

            PR c++/80916 - spurious "static but not defined" warning.
    
    Nothing can refer to an internal decl with no definition, so we shouldn't
    treat such a decl as a possible devirtualization target.
    
            * gimple-fold.c (can_refer_decl_in_current_unit_p): Return false
            for an internal function with no definition.

diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
index 7ef5004f5f9..62d2e0abc26 100644
--- a/gcc/gimple-fold.c
+++ b/gcc/gimple-fold.c
@@ -121,9 +121,12 @@ can_refer_decl_in_current_unit_p (tree decl, tree from_decl)
       || !VAR_OR_FUNCTION_DECL_P (decl))
     return true;
 
-  /* Static objects can be referred only if they was not optimized out yet.  */
-  if (!TREE_PUBLIC (decl) && !DECL_EXTERNAL (decl))
+  /* Static objects can be referred only if they are defined and not optimized
+     out yet.  */
+  if (!TREE_PUBLIC (decl))
     {
+      if (DECL_EXTERNAL (decl))
+	return false;
       /* Before we start optimizing unreachable code we can be sure all
 	 static objects are defined.  */
       if (symtab->function_flags_ready)
diff --git a/gcc/testsuite/g++.dg/warn/unused-fn1.C b/gcc/testsuite/g++.dg/warn/unused-fn1.C
new file mode 100644
index 00000000000..aabc01b3f44
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/unused-fn1.C
@@ -0,0 +1,16 @@
+// PR c++/80916
+// { dg-options "-Os -Wunused" }
+
+struct j {
+  virtual void dispatch(void *) {}
+};
+template <typename>
+struct i : j {
+  void dispatch(void *) {} // warning: 'void i< <template-parameter-1-1> >::dispatch(void*) [with <template-parameter-1-1> = {anonymous}::l]' declared 'static' but never defined [-Wunused-function]
+};
+namespace {
+  struct l : i<l> {};
+}
+void f(j *k) {
+  k->dispatch(0);
+}