[gcc r11-7937] improve future::poll calibration loop

[Alexandre Oliva]

https://gcc.gnu.org/g:eadf009b229ed5d45d1c2d82d72ad2ba9a2e63b1

commit r11-7937-geadf009b229ed5d45d1c2d82d72ad2ba9a2e63b1
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Wed Mar 31 15:34:47 2021 -0300

    improve future::poll calibration loop
    
    The calibration loop I've recently added to the libstdc++
    future/members/poll.cc tests could still select iteration counts that
    might yield zero-time measurements for the wait_for when ready loop.
    
    Waiting for a future that has already had a value set is presumably
    uniformly faster than a zero-timed wait for a result, so I've changed
    the calibration loop to use the former.
    
    We might still be unlucky and get nonzero from the initial loop, so
    that the calibration is skipped altogether, but then get zero from the
    later when-ready loop.  I'm not dealing with this case in this patch.
    
    
    for  libstdc++-v3/ChangeLog
    
            * testsuite/30_threads/future/members/poll.cc: Use faster
            after-ready call in the calibration loop.

Diff:
---
 libstdc++-v3/testsuite/30_threads/future/members/poll.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/testsuite/30_threads/future/members/poll.cc b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
index 133dae15ac4..4c846d0b7ba 100644
--- a/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
+++ b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
@@ -55,6 +55,12 @@ int main()
      Attempt to calibrate it.  */
   if (start == stop)
     {
+      /* After set_value, wait_for is faster, so use that for the
+	 calibration to avoid zero at low clock resultions.  */
+      promise<int> pc;
+      future<int> fc = pc.get_future();
+      pc.set_value(1);
+
       /* Loop until the clock advances, so that start is right after a
 	 time increment.  */
       do
@@ -65,7 +71,7 @@ int main()
 	 after another time increment.  */
       do
 	{
-	  f.wait_for(chrono::seconds(0));
+	  fc.wait_for(chrono::seconds(0));
 	  stop = chrono::high_resolution_clock::now();
 	  i++;
 	}