From: Jonathan Wakely <jwakely@redhat.com>
To: gcc-patches@gcc.gnu.org
Cc: Sandra Loosemore <sandra@codesourcery.com>
Subject: [PATCH] Clarify __atomic_compare_exchange_n docs
Date: Tue, 29 Sep 2015 12:42:00 -0000 [thread overview]
Message-ID: <20150929120038.GW12094@redhat.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 1123 bytes --]
Someone on IRC incorrectly parsed the docs at
https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/_005f_005fatomic-Builtins.html#index-g_t_005f_005fatomic_005fcompare_005fexchange_005fn-3536
as:
IF
(
desired is written into *ptr
AND
the execution is considered to conform to the memory model
specified by success_memmodel.
)
{
true is returned
}
otherwise ...
rather than the intended:
IF ( desired is written into *ptr )
{
true is returned
AND
the execution is considered to conform to the memory model
specified by success_memmodel.
}
otherwise ...
So they asked:
> What is otherwise, here? Can I make the function return false even
> when 'desired' has been written into 'ptr'? How do I do it? I could
> not write an example, so far.
This patch rewords it to avoid the ambiguity.
I've also replaced the rather clunky "the operation is considered to
conform to" phrasing. (It's only _considered_ to? So does it or doesn't
it use that memory order?) Instead I've used the terminology from the
C and C++ standards, which say "memory is affected according to".
OK for trunk?
[-- Attachment #2: patch.txt --]
[-- Type: text/x-patch, Size: 1698 bytes --]
commit 370a92b7f4d318957a70d0d3f1185f1c6f282ff3
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Tue Sep 29 12:45:21 2015 +0100
* doc/extend.texi (__atomic Builtins): Clarify compare_exchange
effects.
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8406945..0de94f2 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -9353,17 +9353,17 @@ This compares the contents of @code{*@var{ptr}} with the contents of
@code{*@var{expected}}. If equal, the operation is a @emph{read-modify-write}
operation that writes @var{desired} into @code{*@var{ptr}}. If they are not
equal, the operation is a @emph{read} and the current contents of
-@code{*@var{ptr}} is written into @code{*@var{expected}}. @var{weak} is true
+@code{*@var{ptr}} are written into @code{*@var{expected}}. @var{weak} is true
for weak compare_exchange, and false for the strong variation. Many targets
only offer the strong variation and ignore the parameter. When in doubt, use
the strong variation.
-True is returned if @var{desired} is written into
-@code{*@var{ptr}} and the operation is considered to conform to the
+If @var{desired} is written into @code{*@var{ptr}} then true is returned
+and memory is affected according to the
memory order specified by @var{success_memorder}. There are no
restrictions on what memory order can be used here.
-False is returned otherwise, and the operation is considered to conform
+Otherwise, false is returned and memory is affected according
to @var{failure_memorder}. This memory order cannot be
@code{__ATOMIC_RELEASE} nor @code{__ATOMIC_ACQ_REL}. It also cannot be a
stronger order than that specified by @var{success_memorder}.
next reply other threads:[~2015-09-29 12:00 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-29 12:42 Jonathan Wakely [this message]
2015-09-29 16:00 ` Sandra Loosemore
2015-10-01 11:28 ` Andrew Haley
2015-10-01 17:32 ` Jonathan Wakely
2015-10-01 17:34 ` Andrew Haley
2015-10-01 17:42 ` Jonathan Wakely
2015-10-01 17:58 ` Sandra Loosemore
2015-10-01 18:35 ` Jonathan Wakely
2015-10-01 18:40 ` Jonathan Wakely
2015-10-03 13:00 ` Jonathan Wakely
2016-01-13 14:27 ` Jonathan Wakely
2016-01-13 17:16 ` Sandra Loosemore
2016-01-13 19:26 ` Jonathan Wakely
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