diff mbox series

[libitm] eh specifications are lax

Message ID a347e239-3fc6-6d34-185a-9162c1d665e0@acm.org
State New
Headers show
Series [libitm] eh specifications are lax | expand

Commit Message

Nathan Sidwell May 5, 2020, 8:08 p.m. UTC 
I discovered that libitm:
(a) declares __cxa_allocate_exception and friends directly,
(b) doesn't mark them as 'throw()'
(c) doesn't mark the replacment fns _ITM_$foo as nothrow either

We happen to get away with it because of code in the compiler that, 
although it checks the parameter types, doesn't check the exception 
specification.  (One reason being they used to not be part of the 
language's type system, but now they are.)  I suspect this can lead us 
to generate pessimal code later, if we've seen one of these decls 
earlier.  Anyway, with modules it becomes trickier[*], so I'm trying to 
clean it up and not be a problem.  I see Jakub fixed part of the problem 
(https://gcc.gnu.org/pipermail/gcc-patches/2018-December/513302.html) 
AFAICT, he did fix libitm's decls, but left the lax parm-type checking 
in the compiler.

libitm.h is not very informative about specification:
   in version 1 of http://www.intel.com/some/path/here.pdf.  */

Anyway, it was too fiddly to have libitm pick up the declarations from 
libsupc++.  Besides it makes them weak declarations, and then provides 
definitions for non-elf systems.  So this patch adds the expected 'throw()'

While I can't be sure, I suspect the _ITM entry points are supposed to 
have the same exception specification as the original entry points.  So 
those are also made 'throw ()'.  libstdc++'s _GLIBCXX_NOTHROW didn't 
seem available, so I make use of a new _ITM_NOTHROW macro, suitably defined.

Because of the lax checking in the compiler, and old compiler with a 
patched libitm.h will be ok...  Until I change the compiler :)

booted & tested on x86_64-linux, ok?

nathan

[*] modules make it harder to have ODR violations, that's why it finds 
ODR violations in existing code.

Comments

Nathan Sidwell May 11, 2020, 1:37 p.m. UTC | #1 
ping?

On 5/5/20 4:08 PM, Nathan Sidwell wrote:
> I discovered that libitm:
> (a) declares __cxa_allocate_exception and friends directly,
> (b) doesn't mark them as 'throw()'
> (c) doesn't mark the replacment fns _ITM_$foo as nothrow either
> 
> We happen to get away with it because of code in the compiler that, 
> although it checks the parameter types, doesn't check the exception 
> specification.  (One reason being they used to not be part of the 
> language's type system, but now they are.)  I suspect this can lead us 
> to generate pessimal code later, if we've seen one of these decls 
> earlier.  Anyway, with modules it becomes trickier[*], so I'm trying to 
> clean it up and not be a problem.  I see Jakub fixed part of the problem 
> (https://gcc.gnu.org/pipermail/gcc-patches/2018-December/513302.html) 
> AFAICT, he did fix libitm's decls, but left the lax parm-type checking 
> in the compiler.
> 
> libitm.h is not very informative about specification:
>    in version 1 of http://www.intel.com/some/path/here.pdf.  */
> 
> Anyway, it was too fiddly to have libitm pick up the declarations from 
> libsupc++.  Besides it makes them weak declarations, and then provides 
> definitions for non-elf systems.  So this patch adds the expected 'throw()'
> 
> While I can't be sure, I suspect the _ITM entry points are supposed to 
> have the same exception specification as the original entry points.  So 
> those are also made 'throw ()'.  libstdc++'s _GLIBCXX_NOTHROW didn't 
> seem available, so I make use of a new _ITM_NOTHROW macro, suitably 
> defined.
> 
> Because of the lax checking in the compiler, and old compiler with a 
> patched libitm.h will be ok...  Until I change the compiler :)
> 
> booted & tested on x86_64-linux, ok?
> 
> nathan
> 
> [*] modules make it harder to have ODR violations, that's why it finds 
> ODR violations in existing code.
>
Li, Pan2 via Gcc-patches May 12, 2020, 4:19 p.m. UTC | #2 
On Tue, 2020-05-05 at 16:08 -0400, Nathan Sidwell wrote:
> I discovered that libitm:
> (a) declares __cxa_allocate_exception and friends directly,
> (b) doesn't mark them as 'throw()'
> (c) doesn't mark the replacment fns _ITM_$foo as nothrow either
> 
> We happen to get away with it because of code in the compiler that, 
> although it checks the parameter types, doesn't check the exception 
> specification.  (One reason being they used to not be part of the 
> language's type system, but now they are.)  I suspect this can lead us 
> to generate pessimal code later, if we've seen one of these decls 
> earlier.  Anyway, with modules it becomes trickier[*], so I'm trying to 
> clean it up and not be a problem.  I see Jakub fixed part of the problem 
> (https://gcc.gnu.org/pipermail/gcc-patches/2018-December/513302.html) 
> AFAICT, he did fix libitm's decls, but left the lax parm-type checking 
> in the compiler.
> 
> libitm.h is not very informative about specification:
>    in version 1 of http://www.intel.com/some/path/here.pdf.  */
> 
> Anyway, it was too fiddly to have libitm pick up the declarations from 
> libsupc++.  Besides it makes them weak declarations, and then provides 
> definitions for non-elf systems.  So this patch adds the expected 'throw()'
> 
> While I can't be sure, I suspect the _ITM entry points are supposed to 
> have the same exception specification as the original entry points.  So 
> those are also made 'throw ()'.  libstdc++'s _GLIBCXX_NOTHROW didn't 
> seem available, so I make use of a new _ITM_NOTHROW macro, suitably defined.
> 
> Because of the lax checking in the compiler, and old compiler with a 
> patched libitm.h will be ok...  Until I change the compiler :)
> 
> booted & tested on x86_64-linux, ok?
> 
> nathan
> 
> [*] modules make it harder to have ODR violations, that's why it finds 
> ODR violations in existing code.
> 
> > 2020-05-05  Nathan Sidwell  <nathan@acm.org>
> > 
> > 	Fix throw specifiers on interface.
> > 	* libitm/libitm.h (_ITM_NOTHROW): Define.
> > 	(_ITM_cxa_allocate_exception, _ITM_cxa_free_exception)
> > 	(_ITM_cxa_begin_catch): Use it.
> > 	* eh_cpp.cc: Add throw() to __cxa_allocate_exception,
> > 	__cxa_free_exception, __cxa_begin_catch, __cxa_tm_cleanup,
> > 	__cxa_get_globals.
> > 	(_ITM_cxa_allocate_exception, _ITM_cxa_free_exception)
> > 	(_ITM_cxa_begin_catch): Likewise.
> > 
OK
jeff
diff mbox series

Patch

2020-05-05  Nathan Sidwell  <nathan@acm.org>

	Fix throw specifiers on interface.
	* libitm/libitm.h (_ITM_NOTHROW): Define.
	(_ITM_cxa_allocate_exception, _ITM_cxa_free_exception)
	(_ITM_cxa_begin_catch): Use it.
	* eh_cpp.cc: Add throw() to __cxa_allocate_exception,
	__cxa_free_exception, __cxa_begin_catch, __cxa_tm_cleanup,
	__cxa_get_globals.
	(_ITM_cxa_allocate_exception, _ITM_cxa_free_exception)
	(_ITM_cxa_begin_catch): Likewise.

diff --git a/libitm/eh_cpp.cc b/libitm/eh_cpp.cc
index 7d570e74fc2..6c9da2d06e4 100644
--- a/libitm/eh_cpp.cc
+++ b/libitm/eh_cpp.cc
@@ -87,23 +87,23 @@  struct __cxa_eh_globals
   unsigned int	uncaughtExceptions;
 };
 
-extern void *__cxa_allocate_exception (size_t) WEAK;
-extern void __cxa_free_exception (void *) WEAK;
+extern void *__cxa_allocate_exception (size_t) _ITM_NOTHROW WEAK;
+extern void __cxa_free_exception (void *) _ITM_NOTHROW WEAK;
 extern void __cxa_throw (void *, void *, void (*) (void *)) WEAK;
-extern void *__cxa_begin_catch (void *) WEAK;
+extern void *__cxa_begin_catch (void *) _ITM_NOTHROW WEAK;
 extern void __cxa_end_catch (void) WEAK;
-extern void __cxa_tm_cleanup (void *, void *, unsigned int) WEAK;
-extern __cxa_eh_globals *__cxa_get_globals (void) WEAK;
+extern void __cxa_tm_cleanup (void *, void *, unsigned int) throw () WEAK;
+extern __cxa_eh_globals *__cxa_get_globals (void) _ITM_NOTHROW WEAK;
 
 #if !defined (HAVE_ELF_STYLE_WEAKREF) 
-void *__cxa_allocate_exception (size_t) { return NULL; }
-void __cxa_free_exception (void *) { return; }
+void *__cxa_allocate_exception (size_t) _ITM_NOTHROW { return NULL; }
+void __cxa_free_exception (void *) _ITM_NOTHROW { return; }
 void __cxa_throw (void *, void *, void (*) (void *)) { return; }
-void *__cxa_begin_catch (void *) { return NULL; }
+void *__cxa_begin_catch (void *) _ITM_NOTHROW { return NULL; }
 void __cxa_end_catch (void) { return; }
-void __cxa_tm_cleanup (void *, void *, unsigned int) { return; }
+void __cxa_tm_cleanup (void *, void *, unsigned int) throw () { return; }
 void _Unwind_DeleteException (_Unwind_Exception *) { return; }
-__cxa_eh_globals *__cxa_get_globals (void) { return NULL; }
+__cxa_eh_globals *__cxa_get_globals (void) _ITM_NOTHROW { return NULL; }
 #endif /* HAVE_ELF_STYLE_WEAKREF */
 
 }
@@ -117,7 +117,7 @@  free_any_exception (void *exc_ptr)
 }
 
 void *
-_ITM_cxa_allocate_exception (size_t size)
+_ITM_cxa_allocate_exception (size_t size) _ITM_NOTHROW
 {
   void *r = __cxa_allocate_exception (size);
   gtm_thr()->record_allocation (r, free_any_exception);
@@ -125,7 +125,7 @@  _ITM_cxa_allocate_exception (size_t size)
 }
 
 void
-_ITM_cxa_free_exception (void *exc_ptr)
+_ITM_cxa_free_exception (void *exc_ptr) _ITM_NOTHROW
 {
   // __cxa_free_exception can be called from user code directly if
   // construction of an exception object throws another exception, in which
@@ -143,7 +143,7 @@  _ITM_cxa_throw (void *obj, void *tinfo, void (*dest) (void *))
 }
 
 void *
-_ITM_cxa_begin_catch (void *exc_ptr)
+_ITM_cxa_begin_catch (void *exc_ptr) _ITM_NOTHROW
 {
   // If this exception object has been allocated by this transaction, we
   // discard the undo log entry for the allocation; we are entering phase (3)
diff --git a/libitm/libitm.h b/libitm/libitm.h
index 69f551e2675..4f8051bdfb7 100644
--- a/libitm/libitm.h
+++ b/libitm/libitm.h
@@ -45,6 +45,15 @@  extern "C" {
 
 #define ITM_NORETURN	__attribute__((noreturn))
 #define ITM_PURE __attribute__((transaction_pure))
+#ifdef _GLIBCXX_NOTHROW
+# define _ITM_NOTHROW _GLIBCXX_NOTHROW
+#elif !defined (__cplusplus)
+# define _ITM_NOTHROW __attribute__((__nothrow__))
+#elif __cplusplus < 201103L
+# define _ITM_NOTHROW throw ()
+#else
+# define _ITM_NOTHROW noexcept
+#endif  
 
 /* The following are externally visible definitions and functions, though
    only very few of these should be called by user code.  */
@@ -282,11 +291,11 @@  extern void *_ITM_getTMCloneSafe (void *) ITM_REGPARM;
 extern void _ITM_registerTMCloneTable (void *, size_t);
 extern void _ITM_deregisterTMCloneTable (void *);
 
-extern void *_ITM_cxa_allocate_exception (size_t);
-extern void _ITM_cxa_free_exception (void *exc_ptr);
+extern void *_ITM_cxa_allocate_exception (size_t) _ITM_NOTHROW;
+extern void _ITM_cxa_free_exception (void *exc_ptr) _ITM_NOTHROW;
 extern void _ITM_cxa_throw (void *obj, void *tinfo, void (*dest) (void *));
-extern void *_ITM_cxa_begin_catch (void *exc_ptr);
-extern void _ITM_cxa_end_catch (void);
+extern void *_ITM_cxa_begin_catch (void *exc_ptr) _ITM_NOTHROW;
+extern void _ITM_cxa_end_catch (void); /* This can throw.  */
 extern void _ITM_commitTransactionEH(void *exc_ptr) ITM_REGPARM;
 
 #ifdef __cplusplus