Re: PATCH RFC: PR 33094: Test DECL_INITIAL in make_rtl_for_nonlocal_decl

[Mark Mitchell <mark at codesourcery dot com>]

Ian Lance Taylor wrote:

> namespace
> {
> 
> template <typename T>
> class A
> {
>   virtual T f1() { return c; }
>   static const T c = 0;
> };

Thanks for the analysis!

> All those steps seem reasonable.  If the anonymous namespace is
> removed from the test case, all is well because DECL_EXTERNAL is set.
> When using the anonymous namespace, DECL_EXTERNAL is not set.

That seems wrong.  This declaration is not a definition.  For example, a
reference to &A<int>::c from within this file should be unresolved at
link-time.  (The standard says that this is an invalid thing to do; I
don't know if it says that a diagnostic is required.)

> 	  /* If this static data member is needed, provide it to the
> 	     back end.  */
> 	  if (DECL_NOT_REALLY_EXTERN (decl) && decl_needed_p (decl))
> 	    DECL_EXTERNAL (decl) = 0;

I think DECL_NOT_REALLY_EXTERN is the wrong test here, now that things
with anonymous namespaces get ELF local linkage.  The confusion here is
that the language says they have external linkage; it's an optimization
that, in generating object files, we now give them ELF local linkage.

There's no such thing as a "static" forward declaration (e.g., "static
int i;" is a definition, not a declaration that "i" exists) -- but now
we've essentially created such a thing.

I think DECL_EXTERNAL has historically only applied to TREE_PUBLIC
things -- but probably that's the best way to represent that.  In other
words, interpret DECL_EXTERNAL as "there is no definition of this entity
in this translation unit (yet)".

> The DECL_NOT_REALLY_EXTERN flag is only set when using an anonymous
> namespace.  It happens in constrain_visibility:

I think that's wrong.  I think we should leave DECL_NOT_REALLY_EXTERN
clear until/unless we actually encounter a definition.  I don't think
constrain_visibility should have any effect on DECL_NOT_REALLY_EXTERN,
given the above.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713