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[PATCHv3 0/6] std::future::wait_* improvements
- From: Mike Crowe <mac at mcrowe dot com>
- To: <libstdc++ at gcc dot gnu dot org>, <gcc-patches at gcc dot gnu dot org>
- Cc: Mike Crowe <mac at mcrowe dot com>
- Date: Wed, 1 Aug 2018 14:19:07 +0100
- Subject: [PATCHv3 0/6] std::future::wait_* improvements
v2 of this series was originally posted back in January (see
Apart from minor log message tweaks, the changes since that version are:
* [1/6] Improve libstdc++-v3 async test
Speed up the tests at the risk of more sporadic failures on loaded
Use lambda rather than separate function for asynchronous routine.
* [2/6] libstdc++ futex: Use FUTEX_CLOCK_REALTIME for wait
Fall back to using gettimeofday and FUTEX_WAIT if FUTEX_WAIT_BITSET and
FUTEX_CLOCK_REALTIME are not available.
* [3/6] libstdc++ futex: Support waiting on std::chrono::steady_clock directly
Fall back to using clock_gettime (or the sycall directly if necessary)
and FUTEX_WAIT if FUTEX_WAIT_BITSET is unavailable.
* [4/6] libstdc++ atomic_futex: Use std::chrono::steady_clock as reference clock
* [5/6] libstdc++ futex: Loop when waiting against arbitrary clock
New patch. My work on std::condition_variable::wait_until made me realise
that there's a risk of indicating a timeout too early when using a
* [6/6] Extra async tests, not for merging
Use lambdas rather than separate functions for asynchronous routines.
Torvald Riegel had some objections to my design, but did not respond when I
attempted to justify it and attempted to change my implementation based on
his suggestions (see https://gcc.gnu.org/ml/libstdc++/2018-01/msg00071.html
It looks like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68519 could
apply equally well to __atomic_futex_unsigned::_M_load_when_equal_for.
I plan to look at that next.
I set up a Debian 4.0 "Etch" system (v2.6.18 kernel, glibc 2.3.6.)
Unfortunately, its GCC 4.1.2 is unable to compile GCC master (as of
5ba044fc3a443274462527eed385732f7ecee3a8) because hash-map.h appears to be
trying to put a reference in a std::pair.) The above patches cherry-pick
cleanly back to GCC 7.3, and that version does build after adding a few
constants to the system headers. I've confirmed that the lack of support
for FUTEX_WAIT_BITSET is detected correctly and that the code falls back to
using FUTEX_WAIT. This test also shows that the fallback to calling the
clock_gettime system call directly is working too. The async.cc tests all
I haven't made any attempt to add entries to the .abilist files. I'm not
sure whether I'm supposed to do that as part of the patches, or not.
Mike Crowe (6):
Improve libstdc++-v3 async test
libstdc++ futex: Use FUTEX_CLOCK_REALTIME for wait
libstdc++ futex: Support waiting on std::chrono::steady_clock directly
libstdc++ atomic_futex: Use std::chrono::steady_clock as reference
libstdc++ futex: Loop when waiting against arbitrary clock
Extra async tests, not for merging
libstdc++-v3/include/bits/atomic_futex.h | 89 ++++++++++--
libstdc++-v3/src/c++11/futex.cc | 113 +++++++++++++++
libstdc++-v3/testsuite/30_threads/async/async.cc | 172 +++++++++++++++++++++++
3 files changed, 364 insertions(+), 10 deletions(-)
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