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

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

From-SVN: r273421

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 688615f48c9285129c81cd62692469f57ff9ff25..69f5b86f30132287b8f1ef3402c400214b1ee47c 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -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

diff --git a/libstdc++-v3/doc/xml/manual/configure.xml b/libstdc++-v3/doc/xml/manual/configure.xml
index d296c8d8a49e8efd5ea17fedb6e3ed9368110740..58587e858a406142fc5cca8b6d21328399f1d4e9 100644 (file)
--- a/libstdc++-v3/doc/xml/manual/configure.xml
+++ b/libstdc++-v3/doc/xml/manual/configure.xml
@@ -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>