[RFC PATCH 1/4] libstdc++ futex: Use FUTEX_CLOCK_REALTIME for wait

[Mike Crowe]

The futex system call supports waiting for an absolute time if
FUTEX_WAIT_BITSET is used rather than FUTEX_WAIT. Doing so provides two
benefits:

1. The call to gettimeofday is not required in order to calculate a
   relative timeout.

2. If someone changes the system clock during the wait then the futex
   timeout will correctly expire earlier or later. Currently that only
   happens if the clock is changed prior to the call to gettimeofday.

According to futex(2), support for FUTEX_CLOCK_REALTIME was added in the
v2.6.28 Linux kernel and FUTEX_WAIT_BITSET was added in v2.6.25. There is
no attempt to detect the kernel version and fall back to the previous
method.
---
 libstdc++-v3/src/c++11/futex.cc | 21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
index 64aada06969..217aeefe005 100644
--- a/libstdc++-v3/src/c++11/futex.cc
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -35,6 +35,9 @@
 
 // Constants for the wait/wake futex syscall operations
 const unsigned futex_wait_op = 0;
+const unsigned futex_wait_bitset_op = 9;
+const unsigned futex_clock_realtime_flag = 256;
+const unsigned futex_bitset_match_any = ~0;
 const unsigned futex_wake_op = 1;
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -58,22 +61,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
     else
       {
-       struct timeval tv;
-       gettimeofday (&tv, NULL);
-       // Convert the absolute timeout value to a relative timeout
        struct timespec rt;
-       rt.tv_sec = __s.count() - tv.tv_sec;
-       rt.tv_nsec = __ns.count() - tv.tv_usec * 1000;
-       if (rt.tv_nsec < 0)
-         {
-           rt.tv_nsec += 1000000000;
-           --rt.tv_sec;
-         }
-       // Did we already time out?
-       if (rt.tv_sec < 0)
-         return false;
-
-       if (syscall (SYS_futex, __addr, futex_wait_op, __val, &rt) == -1)
+       rt.tv_sec = __s.count();
+       rt.tv_nsec = __ns.count();
+       if (syscall (SYS_futex, __addr, futex_wait_bitset_op | 
futex_clock_realtime_flag, __val, &rt, nullptr, futex_bitset_match_any) == -1)
          {
            _GLIBCXX_DEBUG_ASSERT(errno == EINTR || errno == EAGAIN
                                  || errno == ETIMEDOUT);
-- 
2.11.0