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Re: [doc, rfa] (v2) improve x86 transactional memory intrinsics section


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  <sandra@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
 

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