On 09/29/2015 06:00 AM, Jonathan Wakely wrote:
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?
This is OK, as far as it goes, but while we're at it, can we do
something to fix the description of the weak parameter?
@@ -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.
What is "weak compare_exchange", and what is "the strong variation", and
how do they differ in terms of behavior?
-Sandra
