| Message ID | 1453062105.26597.419.camel@localhost.localdomain |
|---|---|
| State | New |
| Headers | show |
On Sun, Jan 17, 2016 at 09:21:45PM +0100, Torvald Riegel wrote: > The attached patch works around this by always definining stubs for the > libitm functions, yet declaring them weak at the same time. If This doesn't look like a good idea. The dynamic linker doesn't make difference between weak and non-weak definitions, only ld(1) does, so if for whatever reason libstdc++ appears earlier in the search scope than libitm, TM programs would stop working because of this. Jakub
On Sun, Jan 17, 2016 at 3:21 PM, Torvald Riegel <triegel@redhat.com> wrote: > On Sat, 2016-01-16 at 15:38 -0500, David Edelsohn wrote: >> On Sat, Jan 16, 2016 at 8:35 AM, Jakub Jelinek <jakub@redhat.com> wrote: >> > On Sat, Jan 16, 2016 at 07:47:33AM -0500, David Edelsohn wrote: >> >> stage1 libstdc++ builds just fine. the problem is stage2 configure >> >> fails due to missing ITM_xxx symbols when configure tries to compile >> >> and run conftest programs. >> > >> > On x86_64-linux, the _ITM_xxx symbols are undef weak ones and thus it is >> > fine to load libstdc++ without libitm and libstdc++ doesn't depend on >> > libitm. >> > >> > So, is AIX defining __GXX_WEAK__ or not? Perhaps some other macro or >> > configure check needs to be used to determine if undefined weak symbols >> > work the way libstdc++ needs them to. >> >> __GXX_WEAK__ appears to be defined by gcc/c-family/c-cppbuiltin.c >> based on SUPPORTS_ONE_ONLY. gcc/defaults.h defines SUPPORTS_ONE_ONLY >> if the target supports MAKE_DECL_ONE_ONLY and link-once semantics. >> AIX weak correctly supports link-once semantics. AIX also supports >> the definition of __GXX_WEAK__ in gcc/doc/cpp.texi, namely collapsing >> symbols with vague linkage in multiple translation units. >> >> libstdc++/src/c++11/cow-stdexcept.cc appears to be using __GXX_WEAK__ >> and __attribute__ ((weak)) for references to symbols that may not be >> defined at link time or run time. AIX does not allow undefined symbol >> errors by default. And the libstdc++ inference about the semantics of >> __GXX_WEAK__ are different than the documentation. >> >> AIX supports MAKE_DECL_ONE_ONLY and the documented meaning of >> __GXX_WEAK__. AIX does not support extension of the meaning to >> additional SVR4 semantics not specified in the documentation. > > I see, so we might be assuming that __GXX_WEAK__ means more than it > actually does (I'm saying "might" because personally, I don't know; your > information supports this is the case, but the initial info I got was > that __GXX_WEAK__ would mean we could have weak decls without > definitions). I believe that libstdc++ must continue with the weak undefined references to the symbols as designed, but protect them with a different macro. For example, __GXX_WEAK_REF__ or __GXX_WEAK_UNDEF__ defined in defaults.h based on configure test or simply overridden in config/rs6000/aix.h. Or the macro could be local to libstdc++ and overridden in config/os/aix/os_defines.h. Thanks, David
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__