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Improve docs for --enable-libstdcxx-time=rt

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* doc/xml/manual/configure.xml: Improve documentation of
	--enable-libstdcxx-time option.

From-SVN: r273421
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Jonathan Wakely 2019-07-11 20:43:32 +01:00 committed by Jonathan Wakely
parent a90fe12c80
commit f6822be73a
2 changed files with 16 additions and 7 deletions
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3
libstdc++-v3/ChangeLog
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@ -1,5 +1,8 @@
2019-07-11 Jonathan Wakely <jwakely@redhat.com>
* doc/xml/manual/configure.xml: Improve documentation of
--enable-libstdcxx-time option.
* include/bits/atomic_base.h (__atomic_impl): New namespace for
wrappers around atomic built-ins.
(__atomic_float, __atomic_ref): New class templates for use as base

20
libstdc++-v3/doc/xml/manual/configure.xml
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@ -166,18 +166,24 @@
<varlistentry><term><code>--enable-libstdcxx-time=OPTION</code></term>
<listitem><para>Enables link-type checks for the availability of the
clock_gettime clocks, used in the implementation of [time.clock],
and of the nanosleep and sched_yield functions, used in the
<function>clock_gettime</function> clocks, used in the implementation
of [time.clock], and of the <function>nanosleep</function> and
<function>sched_yield</function> functions, used in the
implementation of [thread.thread.this] of the 2011 ISO C++ standard.
The choice OPTION=yes checks for the availability of the facilities
in libc and libposix4. In case it's needed the latter is also linked
to libstdc++ as part of the build process. OPTION=rt also searches
(and, if needed, links) librt. Note that the latter is not always
desirable because, in glibc, for example, in turn it triggers the
linking of libpthread too, which activates locking, a large overhead
for single-thread programs. OPTION=no skips the tests completely.
to libstdc++ as part of the build process. OPTION=rt also checks in
librt (and, if it's needed, links to it). Note that linking to librt
is not always desirable because for glibc it requires linking to
libpthread too, which causes all reference counting to use atomic
operations, resulting in a potentially large overhead for
single-threaded programs. OPTION=no skips the tests completely.
The default is OPTION=auto, which skips the checks and enables the
features only for targets known to support them.
For Linux targets, if <function>clock_gettime</function> is not used
then the [time.clock] implementation will use a system call to access
the realtime and monotonic clocks, which is significantly slower than
the C library's <function>clock_gettime</function> function.
</para>
</listitem></varlistentry>