On 02/23/2015 03:36 AM, Patrick Marlier wrote:
On 02/22/2015 04:06 AM, Sandra Loosemore wrote:
+Here is an example showing handling for @code{_XABORT_RETRY}
+and a fallback path for other failures:
+
+@smallexample
+#include <immintrin.h>
+
+int n_tries, max_tries;
+unsigned status = _XBEGIN_STARTED;
I would suggest to set it to something different. Indeed if max_tries ==
0, then it will end up to do the transactional code with no transaction
started.
Good catch....
+...
+
+for (n_tries = 0; n_tries < max_tries; n_tries++)
+ @{
+ status = _xbegin ();
+ if (status == _XBEGIN_STARTED || !(status | _XABORT_RETRY))
Should not be || !(status & _XABORT_RETRY) ?
Yup, that was a think-o.
+ break;
+ @}
+if (status == _XBEGIN_STARTED)
+ @{
+ ... transaction code...
+ _xend ();
+ @}
+else
+ @{
+ ... non transactional fallback path...
+ @}
+@end smallexample
Thanks a lot. It gives a good idea on how to use it. I just would like
to mention that the non transactional and transactional code must
synchronize together (in most cases) to ensure consistency.
OK, I added a sentence about that.
Revised patch attached. OK to commit this version?
-Sandra
2015-02-23 Sandra Loosemore <san...@codesourcery.com>
gcc/
* doc/extend.texi (x86 transactional memory intrinsics):
Reorganize discussion of _xbegin. Clarify that the return
value is a bit mask. Expand example and move to end of section.
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 220892)
+++ gcc/doc/extend.texi (working copy)
@@ -17274,24 +17274,11 @@ and suitable fallback code always needs
Start a RTM (Restricted Transactional Memory) transaction.
Returns @code{_XBEGIN_STARTED} when the transaction
started successfully (note this is not 0, so the constant has to be
-explicitly tested). If the transaction aborts, all side-effects
-are undone and an abort code is returned. There is no guarantee
-any transaction ever succeeds, so there always needs to be a valid
-fallback path.
-@end deftypefn
-
-@smallexample
-#include <immintrin.h>
-
-if ((status = _xbegin ()) == _XBEGIN_STARTED) @{
- ... transaction code...
- _xend ();
-@} else @{
- ... non transactional fallback path...
-@}
-@end smallexample
+explicitly tested).
-If the transaction aborts, the return value is one of:
+If the transaction aborts, all side-effects
+are undone and an abort code encoded as a bit mask is returned.
+The following macros are defined:
@table @code
@item _XABORT_EXPLICIT
@@ -17309,6 +17296,11 @@ Transaction abort due to a debug trap.
Transaction abort in an inner nested transaction.
@end table
+There is no guarantee
+any transaction ever succeeds, so there always needs to be a valid
+fallback path.
+@end deftypefn
+
@deftypefn {RTM Function} {void} _xend ()
Commit the current transaction. When no transaction is active this faults.
All memory side-effects of the transaction become visible
@@ -17325,6 +17317,37 @@ The @var{status} is an 8-bit constant; i
value from @code{_xbegin}.
@end deftypefn
+Here is an example showing handling for @code{_XABORT_RETRY}
+and a fallback path for other failures:
+
+@smallexample
+#include <immintrin.h>
+
+int n_tries, max_tries;
+unsigned status = _XBEGIN_EXPLICIT;
+...
+
+for (n_tries = 0; n_tries < max_tries; n_tries++)
+ @{
+ status = _xbegin ();
+ if (status == _XBEGIN_STARTED || !(status & _XABORT_RETRY))
+ break;
+ @}
+if (status == _XBEGIN_STARTED)
+ @{
+ ... transaction code...
+ _xend ();
+ @}
+else
+ @{
+ ... non-transactional fallback path...
+ @}
+@end smallexample
+
+@noindent
+Note that, in most cases, the transactional and non-transactional code
+must synchronize together to ensure consistency.
+
@node Target Format Checks
@section Format Checks Specific to Particular Target Machines
