[doc, committed] copy-edit x86 transactional memory intrinsics section

[doc, committed] copy-edit x86 transactional memory intrinsics section

[Sandra Loosemore]

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A while ago when I was doing some other work on extend.texi, I noticed 
that the section on x86 transactional memory intrinsics was in need of 
cleanup.  I've checked in this patch to do some content-free 
copy-editing -- fixing typos and markup, rephrasing some awkward 
sentences, etc.

To tell the truth I don't think the documentation in this section is 
useful to anyone who doesn't already know how to use the feature, so  it 
could still use more content-full editing as well.  But, at least this 
is an incremental improvement.

-Sandra


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2015-02-19  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* doc/extend.texi (x86 transactional memory intrinsics):
	Copy-edit.

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Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi	(revision 220818)
+++ gcc/doc/extend.texi	(working copy)
@@ -17257,10 +17257,11 @@ int __builtin_ia32_xtest ()
 @node x86 transactional memory intrinsics
 @subsection x86 Transactional Memory Intrinsics
 
-Hardware transactional memory intrinsics for x86. These allow to use
+These hardware transactional memory intrinsics for x86 allow you to use
 memory transactions with RTM (Restricted Transactional Memory).
-For using HLE (Hardware Lock Elision) see @ref{x86 specific memory model extensions for transactional memory} instead.
 This support is enabled with the @option{-mrtm} option.
+For using HLE (Hardware Lock Elision) see 
+@ref{x86 specific memory model extensions for transactional memory} instead.
 
 A memory transaction commits all changes to memory in an atomic way,
 as visible to other threads. If the transaction fails it is rolled back
@@ -17271,12 +17272,12 @@ and suitable fallback code always needs 
 
 @deftypefn {RTM Function} {unsigned} _xbegin ()
 Start a RTM (Restricted Transactional Memory) transaction. 
-Returns _XBEGIN_STARTED when the transaction
+Returns @code{_XBEGIN_STARTED} when the transaction
 started successfully (note this is not 0, so the constant has to be 
-explicitely tested). When the transaction aborts all side effects
+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
-tested fallback path.
+fallback path.
 @end deftypefn
 
 @smallexample
@@ -17290,38 +17291,38 @@ if ((status = _xbegin ()) == _XBEGIN_STA
 @}
 @end smallexample
 
-Valid abort status bits (when the value is not @code{_XBEGIN_STARTED}) are:
+If the transaction aborts, the return value is one of:
 
 @table @code
 @item _XABORT_EXPLICIT
-Transaction explicitely aborted with @code{_xabort}. The parameter passed
-to @code{_xabort} is available with @code{_XABORT_CODE(status)}
+Transaction was explicitly aborted with @code{_xabort}.  The parameter passed
+to @code{_xabort} is available with @code{_XABORT_CODE(status)}.
 @item _XABORT_RETRY
 Transaction retry is possible.
 @item _XABORT_CONFLICT
-Transaction abort due to a memory conflict with another thread
+Transaction abort due to a memory conflict with another thread.
 @item _XABORT_CAPACITY
-Transaction abort due to the transaction using too much memory
+Transaction abort due to the transaction using too much memory.
 @item _XABORT_DEBUG
-Transaction abort due to a debug trap
+Transaction abort due to a debug trap.
 @item _XABORT_NESTED
-Transaction abort in a inner nested transaction
+Transaction abort in an inner nested transaction.
 @end table
 
 @deftypefn {RTM Function} {void} _xend ()
-Commit the current transaction. When no transaction is active this will
-fault. All memory side effects of the transactions will become visible
-to other threads in an atomic matter.
+Commit the current transaction. When no transaction is active this faults.
+All memory side-effects of the transaction become visible
+to other threads in an atomic manner.
 @end deftypefn
 
 @deftypefn {RTM Function} {int} _xtest ()
-Return a value not zero when a transaction is currently active, otherwise 0.
+Return a nonzero value if a transaction is currently active, otherwise 0.
 @end deftypefn
 
 @deftypefn {RTM Function} {void} _xabort (status)
 Abort the current transaction. When no transaction is active this is a no-op.
-status must be a 8bit constant, that is included in the status code returned
-by @code{_xbegin}
+The @var{status} is an 8-bit constant; its value is encoded in the return 
+value from @code{_xbegin}.
 @end deftypefn
 
 @node Target Format Checks

Re: [doc, committed] copy-edit x86 transactional memory intrinsics section

[Patrick Marlier]

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:

On Thu, Feb 19, 2015 at 5:16 PM, Sandra Loosemore
<sandra@codesourcery.com> wrote:
> A while ago when I was doing some other work on extend.texi, I noticed that
> the section on x86 transactional memory intrinsics was in need of cleanup.
> I've checked in this patch to do some content-free copy-editing -- fixing
> typos and markup, rephrasing some awkward sentences, etc.
>
> To tell the truth I don't think the documentation in this section is useful
> to anyone who doesn't already know how to use the feature, so  it could
> still use more content-full editing as well.  But, at least this is an
> incremental improvement.
>
> -Sandra
>

Re: [doc, committed] copy-edit x86 transactional memory intrinsics section

[Sandra Loosemore]

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.

-Sandra

[doc, rfa] improve x86 transactional memory intrinsics section

[Sandra Loosemore]

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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


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2015-02-21  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 +17317,33 @@ 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_STARTED;
+...
+
+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
+
 @node Target Format Checks
 @section Format Checks Specific to Particular Target Machines
 

Re: [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

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

[Sandra Loosemore]

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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
 

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

[Patrick Marlier]

On 02/24/2015 01:51 AM, Sandra Loosemore wrote:
> 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
>

_XBEGIN_EXPLICIT does not exist right? maybe 0 is better to avoid 
confusion? or ~_XBEGIN_STARTED?
But it sounds good to me. Thanks.
--
Patrick

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

[Sandra Loosemore]

On 02/24/2015 03:01 AM, Patrick Marlier wrote:
> On 02/24/2015 01:51 AM, Sandra Loosemore wrote:
>> [snip]
>> Revised patch attached.  OK to commit this version?
>>
>> -Sandra
>
> _XBEGIN_EXPLICIT does not exist right? maybe 0 is better to avoid
> confusion? or ~_XBEGIN_STARTED?

Argh, I meant to change _XBEGIN_STARTED to _XABORT_EXPLICIT here.  :-P 
I'll fix that before checking it in.

> But it sounds good to me. Thanks.

Thanks for the review; I think we've got the technical corrections 
nailed down now.  Doc maintainers, is this OK?

-Sandra

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

[Gerald Pfeifer]

On Tue, 24 Feb 2015, Sandra Loosemore wrote:
> Thanks for the review; I think we've got the technical corrections 
> nailed down now.  Doc maintainers, is this OK?

You can approve yourself now.  Watch out for my next mail. ;-)

Gerald