[committed] libstdc++: Optimise std::future::wait_for and fix futex polling

[Jonathan Wakely]

On 12/11/20 23:49 +0000, Jonathan Wakely wrote:
>To poll a std::future to see if it's ready you have to call one of the
>timed waiting functions. The most obvious way is wait_for(0s) but this
>was previously very inefficient because it would turn the relative
>timeout to an absolute one by calling system_clock::now(). When the
>relative timeout is zero (or less) we're obviously going to get a time
>that has already passed, but the overhead of obtaining the current time
>can be dozens of microseconds. The alternative is to call wait_until
>with an absolute timeout that is in the past. If you know the clock's
>epoch is in the past you can use a default constructed time_point.
>Alternatively, using some_clock::time_point::min() gives the earliest
>time point supported by the clock, which should be safe to assume is in
>the past. However, using a futex wait with an absolute timeout before
>the UNIX epoch fails and sets errno=EINVAL. The new code using futex
>waits with absolute timeouts was not checking for this case, which could
>result in hangs (or killing the process if the libray is built with
>assertions enabled).
>
>This patch checks for times before the epoch before attempting to wait
>on a futex with an absolute timeout, which fixes the hangs or crashes.
>It also makes it very fast to poll using an absolute timeout before the
>epoch (because we skip the futex syscall).
>
>It also makes future::wait_for avoid waiting at all when the relative
>timeout is zero or less, to avoid the unnecessary overhead of getting
>the current time. This makes polling with wait_for(0s) take only a few
>cycles instead of dozens of milliseconds.
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/future (future::wait_for): Do not wait for
>	durations less than or equal to zero.
>	* src/c++11/futex.cc (_M_futex_wait_until)
>	(_M_futex_wait_until_steady): Do not wait for timeouts before
>	the epoch.
>	* testsuite/30_threads/future/members/poll.cc: New test.
>
>Tested powerpc64le-linux. Committed to trunk.
>
>I think the shortcut in future::wait_for is worth backporting. The
>changes in src/c++11/futex.cc are not needed because the code using
>absolute timeouts with futex waits is not present on any release
>branch.

I've committed this fix for the new test.

Tested x86_64-linux, sparc-solaris-2.11 and powerpc-aix.


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