Re: [PATCH] Fix ODR violations in code using <ext/atomicity.h>

[Jonathan Wakely <jwakely at redhat dot com>]

On 21/06/19 18:13 +0100, Jonathan Wakely wrote:
> On 21/06/19 18:08 +0100, Jonathan Wakely wrote:
> > On 21/06/19 13:01 -0400, Nathan Sidwell wrote:
> > > On 6/21/19 12:01 PM, Jonathan Wakely wrote:
> > > > Nathan noticed that the 'static inline' functions in <ext/atomicity.h>
> > > > cause ODR violations when used from inline functions or templates (see
> > > > [basic.def.odr] p12 bullet (12.2)). His modules branch now diagnoses
> > > > those violations, so we need a fix.
> > > >
> > > > Looking at the history (r114044 by Paolo) I believe the idea was indeed
> > > > to allow different definitions to be used in different TUs. I think
> > > > __attribute__((__always_inline__)) is a better match for that than
> > > > 'static inline', and doesn't violate the ODR (at least, not if all TUs
> > > > have the same values for __GTHREADS and _GLIBCXX_ATOMIC_BUILTINS).
> > > >
> > > > These functions still violate this rule in [dcl.inline]:
> > > >
> > > > C++17: "If a function with external linkage is declared inline in one
> > > > translation unit, it shall be declared inline in all translation units
> > > > in which it appears; no diagnostic is required."
> > > >
> > > > C++2a WP: "If a function or variable with external or module linkage
> > > > is declared inline in one translation unit, there shall be a reachable
> > > > inline declaration in all translation units in which it is declared;
> > > > no diagnostic is required."
> > > >
> > > > But that doesn't seem to be diagnosable by today's implementations.
> > > >
> > > > Does this change seem reasonable?
> > >
> > > yes, thanks!
> >
> > Actually, I think I prefer this version. This produces identical code
> > to the always_inline version, but without the indirection to
> > additional inline functions, i.e. just inline the relevant code into
> > the _dispatch functions.
> >
> > Tests are still running but no failures so far, as expected.
>
> Oops, I spoke too soon:
>
> FAIL: tr1/2_general_utilities/shared_ptr/thread/default_weaktoshared.cc (test for excess errors)
> UNRESOLVED: tr1/2_general_utilities/shared_ptr/thread/default_weaktoshared.cc compilation failed to produce executable
> FAIL: tr1/2_general_utilities/shared_ptr/thread/mutex_weaktoshared.cc (test for excess errors)
> UNRESOLVED: tr1/2_general_utilities/shared_ptr/thread/mutex_weaktoshared.cc compilation failed to produce executable
>
> Those tests explicitly use the __atomic_add function that the patch
> removes. But those would be easy to adjust.

I decided against the simplification in the second patch, and
committed the attached one which is closer to the first patch I sent
(preserving the __atomic_add and __exchange_and_add functions even
when they just call the built-ins).

Tested x86_64-linux, powerpc64-linux, powerpc-aix. Committed to trunk.

commit 0d26b6506f20dcb1c956338baba0c26f623e25f2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jul 5 15:15:46 2019 +0100

    Fix ODR violations in code using <ext/atomicity.h>
    
    Because the inline versions of __exchange_and_add and __atomic_add are
    also marked static, they cannot be used from templates or other inline
    functions without ODR violations. This change gives them external
    linkage, but adds the always_inline attribute.
    
            * include/ext/atomicity.h [_GLIBCXX_ATOMIC_BUILTINS] (__atomic_add)
            (__exchange_and_add): Replace static specifier with always_inline
            attribute.
            (__exchange_and_add_single, __atomic_add_single): Likewise.
            (__exchange_and_add_dispatch, __atomic_add_dispatch): Likewise. Also
            combine !__gthread_active_p() and !__GTHREADS branches.

diff --git a/libstdc++-v3/include/ext/atomicity.h b/libstdc++-v3/include/ext/atomicity.h
index 0783a58e01a..73225b3de20 100644
--- a/libstdc++-v3/include/ext/atomicity.h
+++ b/libstdc++-v3/include/ext/atomicity.h
@@ -44,24 +44,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // __exchange_and_add_dispatch
   // __atomic_add_dispatch
 #ifdef _GLIBCXX_ATOMIC_BUILTINS
-  static inline _Atomic_word 
+  inline _Atomic_word
+  __attribute__((__always_inline__))
   __exchange_and_add(volatile _Atomic_word* __mem, int __val)
   { return __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 
-  static inline void
+  inline void
+  __attribute__((__always_inline__))
   __atomic_add(volatile _Atomic_word* __mem, int __val)
   { __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
 #else
   _Atomic_word
-  __attribute__ ((__unused__))
   __exchange_and_add(volatile _Atomic_word*, int) throw ();
 
   void
-  __attribute__ ((__unused__))
   __atomic_add(volatile _Atomic_word*, int) throw ();
 #endif
 
-  static inline _Atomic_word
+  inline _Atomic_word
+  __attribute__((__always_inline__))
   __exchange_and_add_single(_Atomic_word* __mem, int __val)
   {
     _Atomic_word __result = *__mem;
@@ -69,36 +70,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     return __result;
   }
 
-  static inline void
+  inline void
+  __attribute__((__always_inline__))
   __atomic_add_single(_Atomic_word* __mem, int __val)
   { *__mem += __val; }
 
-  static inline _Atomic_word
-  __attribute__ ((__unused__))
+  inline _Atomic_word
+  __attribute__ ((__always_inline__))
   __exchange_and_add_dispatch(_Atomic_word* __mem, int __val)
   {
 #ifdef __GTHREADS
     if (__gthread_active_p())
       return __exchange_and_add(__mem, __val);
-    else
-      return __exchange_and_add_single(__mem, __val);
-#else
-    return __exchange_and_add_single(__mem, __val);
 #endif
+    return __exchange_and_add_single(__mem, __val);
   }
 
-  static inline void
-  __attribute__ ((__unused__))
+  inline void
+  __attribute__ ((__always_inline__))
   __atomic_add_dispatch(_Atomic_word* __mem, int __val)
   {
 #ifdef __GTHREADS
     if (__gthread_active_p())
       __atomic_add(__mem, __val);
-    else
-      __atomic_add_single(__mem, __val);
-#else
-    __atomic_add_single(__mem, __val);
 #endif
+    __atomic_add_single(__mem, __val);
   }
 
 _GLIBCXX_END_NAMESPACE_VERSION