commit 9008d4a610dccb5ec47f9c1e506492b8615a36fd
Author: Torvald Riegel <triegel@redhat.com>
Date:   Sun Jan 17 19:21:13 2016 +0100

    libstdc++: Fix usage of weak symbols in TM TS support.

diff --git a/libstdc++-v3/src/c++11/cow-stdexcept.cc b/libstdc++-v3/src/c++11/cow-stdexcept.cc
index e2d7e76..8ccc7f5 100644
--- a/libstdc++-v3/src/c++11/cow-stdexcept.cc
+++ b/libstdc++-v3/src/c++11/cow-stdexcept.cc
@@ -179,6 +179,13 @@ _GLIBCXX_END_NAMESPACE_VERSION
 // Furthermore, _Rep will always have been allocated or deallocated via
 // global new or delete, so nontransactional writes we do to _Rep cannot
 // interfere with transactional accesses.
+
+// We require support for weak symbols because we need to call libitm but must
+// not depend on it.  The exception classes will not be declared
+// transaction-safe if there is no such support, so do not create
+// transactional clones in that case at all.
+#ifdef __GXX_WEAK__
+
 extern "C" {
 
 #ifndef _GLIBCXX_MANGLE_SIZE_T
@@ -195,7 +202,6 @@ extern "C" {
 # define ITM_REGPARM
 #endif
 
-#if __GXX_WEAK__
 // Declare all libitm symbols we rely on, but make them weak so that we do
 // not depend on libitm.
 extern void* _ZGTtnaX (size_t sz) __attribute__((weak));
@@ -213,9 +219,10 @@ extern void _ITM_memcpyRnWt(void *, const void *, size_t)
 extern void _ITM_addUserCommitAction(void (*)(void *), uint64_t, void *)
   ITM_REGPARM __attribute__((weak));
 
-#else
-// If there is no support for weak symbols, create dummies.  The exceptions
-// will not be declared transaction_safe in this case.
+// Also create stubs because some targets (e.g., AIX) do not support weak
+// symbols that do not have a definition.  These stubs will not be used
+// (unless users call the transactional clones directly or run transactions
+// without using libitm as well)
 void* _ZGTtnaX (size_t) { return NULL; }
 void _ZGTtdlPv (void*) { }
 uint8_t _ITM_RU1(const uint8_t *) { return 0; }
@@ -224,7 +231,6 @@ uint64_t _ITM_RU8(const uint64_t *) { return 0; }
 void _ITM_memcpyRtWn(void *, const void *, size_t) { }
 void _ITM_memcpyRnWt(void *, const void *, size_t) { }
 void _ITM_addUserCommitAction(void (*)(void *), uint64_t, void *) { };
-#endif
 
 }
 
@@ -440,3 +446,5 @@ CTORDTOR(14overflow_error, std::overflow_error, runtime_error)
 CTORDTOR(15underflow_error, std::underflow_error, runtime_error)
 
 }
+
+#endif  // __GXX_WEAK__