Re: [PATCHv3 0/6] std::future::wait_* improvements

[Jonathan Wakely <jwakely at redhat dot com>]

On 01/08/18 14:19 +0100, Mike Crowe wrote:
> v2 of this series was originally posted back in January (see
> https://gcc.gnu.org/ml/libstdc++/2018-01/msg00035.html )
>
> 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
>  machines.
>
>  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
>
>  No changes
>
> * [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
>  non-standard clock.
>
> * [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.)

That's https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86739

> 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
> passed.
>
> 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.

Changes to gnu.ver linker script are needed, which should be in the patch.

Changes to the baseline_symbols.txt files should not be included in
the patch, those are regenerated later.


> 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
>    clock
>  libstdc++ futex: Loop when waiting against arbitrary clock
>  Extra async tests, not for merging

Most of these patches fail to apply because they use spaces where the
GCC code has tabs. I can fix it, but I'm not sure why the tabs aren't
in your patches.