Re: [v3 PATCH] PR libstdc++/77288 and the newest proposed resolution for LWG 2756

[Jonathan Wakely <jwakely@redhat.com>]

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On 22/09/16 12:15 +0100, Jonathan Wakely wrote:
>On 22/09/16 11:16 +0100, Jonathan Wakely wrote:
>>(Somebody should fix PR58938 so exception_ptr is portable).
>
>Christophe, would you be able to test this patch?
>
>It uses a single global mutex for exception_ptr objects, which doesn't
>scale well but that probably isn't a problem for processors without
>lock-free atomics for single words.
>
>This also solves the problem of mismatched -march options, where the
>header is compiled for a CPU that supports the atomics but
>libstdc++.so was built for an older CPU that doesn't support them, and
>linking fails (as in https://gcc.gnu.org/PR58938#c13).

We'd also need something like this extra piece, to ensure we don't
leak exceptions. Currently __gxx_exception_cleanup assumes that if
ATOMIC_INT_LOCK_FREE < 2 the referenceCount can never be greater than
1, because there are not exception_ptr objects that could increase it.

If we enable exception_ptr unconditionally then that assumption
doesn't hold. This patch uses the exception_ptr code to do the
cleanup, under the same mutex as any other increments and decrements
of the reference count.

It's a bit of a hack though.


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diff --git a/libstdc++-v3/libsupc++/Makefile.am b/libstdc++-v3/libsupc++/Makefile.am
index 2df31ff..dbfd00f 100644
--- a/libstdc++-v3/libsupc++/Makefile.am
+++ b/libstdc++-v3/libsupc++/Makefile.am
@@ -155,9 +155,9 @@ eh_terminate.o: eh_terminate.cc
 	$(CXXCOMPILE) -std=gnu++11 -c $<
 
 eh_throw.lo: eh_throw.cc
-	$(LTCXXCOMPILE) -std=gnu++11 -c $<
+	$(LTCXXCOMPILE) -std=gnu++11 -fno-access-control -c $<
 eh_throw.o: eh_throw.cc
-	$(CXXCOMPILE) -std=gnu++11 -c $<
+	$(CXXCOMPILE) -std=gnu++11 -fno-access-control -c $<
 
 guard.lo: guard.cc
 	$(LTCXXCOMPILE) -std=gnu++11 -c $<
diff --git a/libstdc++-v3/libsupc++/Makefile.in b/libstdc++-v3/libsupc++/Makefile.in
index e828ed9..f3e2193 100644
--- a/libstdc++-v3/libsupc++/Makefile.in
+++ b/libstdc++-v3/libsupc++/Makefile.in
@@ -884,9 +884,9 @@ eh_terminate.o: eh_terminate.cc
 	$(CXXCOMPILE) -std=gnu++11 -c $<
 
 eh_throw.lo: eh_throw.cc
-	$(LTCXXCOMPILE) -std=gnu++11 -c $<
+	$(LTCXXCOMPILE) -std=gnu++11 -fno-access-control -c $<
 eh_throw.o: eh_throw.cc
-	$(CXXCOMPILE) -std=gnu++11 -c $<
+	$(CXXCOMPILE) -std=gnu++11 -fno-access-control -c $<
 
 guard.lo: guard.cc
 	$(LTCXXCOMPILE) -std=gnu++11 -c $<
diff --git a/libstdc++-v3/libsupc++/eh_throw.cc b/libstdc++-v3/libsupc++/eh_throw.cc
index a05f4eb..0f61fbf 100644
--- a/libstdc++-v3/libsupc++/eh_throw.cc
+++ b/libstdc++-v3/libsupc++/eh_throw.cc
@@ -24,6 +24,7 @@
 
 #include <bits/c++config.h>
 #include "unwind-cxx.h"
+#include "exception_ptr.h"
 
 using namespace __cxxabiv1;
 
@@ -42,17 +43,8 @@ __gxx_exception_cleanup (_Unwind_Reason_Code code, _Unwind_Exception *exc)
   if (code != _URC_FOREIGN_EXCEPTION_CAUGHT && code != _URC_NO_REASON)
     __terminate (header->exc.terminateHandler);
 
-#if ATOMIC_INT_LOCK_FREE > 1
-  if (__atomic_sub_fetch (&header->referenceCount, 1, __ATOMIC_ACQ_REL) == 0)
-    {
-#endif
-      if (header->exc.exceptionDestructor)
-	header->exc.exceptionDestructor (header + 1);
-
-      __cxa_free_exception (header + 1);
-#if ATOMIC_INT_LOCK_FREE > 1
-    }
-#endif
+  std::__exception_ptr::exception_ptr ptr;
+  ptr._M_exception_object = header + 1;
 }
 
 extern "C" __cxa_refcounted_exception*