* doc/xml/manual/configure.xml: Improve documentation of
--enable-libstdcxx-time option.
Committed to trunk.
commit 6a7d6b5ea112fd5c14d5af2394817178293ac934
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Thu Jul 11 19:43:32 2019 +0000
Improve docs for --enable-libstdcxx-time=rt
* doc/xml/manual/configure.xml: Improve documentation of
--enable-libstdcxx-time option.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@273421
138bc75d-0d04-0410-961f-82ee72b054a4
diff --git a/libstdc++-v3/doc/xml/manual/configure.xml
b/libstdc++-v3/doc/xml/manual/configure.xml
index d296c8d8a49..58587e858a4 100644
--- 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>
