[doc, rfa] improve x86 transactional memory intrinsics section

[Patrick Marlier]

On 02/22/2015 04:06 AM, Sandra Loosemore wrote:
> On 02/19/2015 12:36 PM, Sandra Loosemore wrote:
>> On 02/19/2015 09:38 AM, Patrick Marlier wrote:
>>> Thanks Sandra. Just a minor comment.
>>>
>>> -Valid abort status bits (when the value is not
>>> @code{_XBEGIN_STARTED}) are:
>>> +If the transaction aborts, the return value is one of:
>>>
>>> Here it is really bits. So maybe something like that:
>>> If the transaction aborts, the return value is a combination of the
>>> following bits:
>>
>> So "combination" == "bit mask"?  Can there be more than one abort
>> condition at a time?  (If there's more than one _xabort call, it seems
>> there can be only one status....)
>>
>> I'll draft a patch to fix this, and expand the example to show how a
>> user should test for these conditions, once I understand how it's
>> supposed to work.
>
> I found some additional documentation online that gave me a few clues. 
> Is the attached patch OK to commit?
>
> -Sandra
>


+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.

+...
+
+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) ?

+      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.
--
Patrick