93fc477468
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. |
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