From: Sandra Loosemore <sandra@codesourcery.com>
To: Patrick Marlier <patrick.marlier@gmail.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [doc, rfa] (v2) improve x86 transactional memory intrinsics section
Date: Tue, 24 Feb 2015 03:38:00 -0000 [thread overview]
Message-ID: <54EBCB24.6080903@codesourcery.com> (raw)
In-Reply-To: <54EB0295.5080105@gmail.com>
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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
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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.
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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
next prev parent reply other threads:[~2015-02-24 0:52 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-19 16:20 [doc, committed] copy-edit " Sandra Loosemore
2015-02-19 16:45 ` Patrick Marlier
2015-02-19 20:04 ` Sandra Loosemore
2015-02-22 8:54 ` [doc, rfa] improve " Sandra Loosemore
2015-02-23 11:59 ` Patrick Marlier
2015-02-24 3:38 ` Sandra Loosemore [this message]
2015-02-24 12:21 ` [doc, rfa] (v2) " Patrick Marlier
2015-02-25 5:18 ` Sandra Loosemore
2015-02-27 0:26 ` Gerald Pfeifer
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