[gcc r11-6686] calibrate intervals to avoid zero in futures poll test

[Alexandre Oliva]

https://gcc.gnu.org/g:3651c1b5c9c0960e50f00ca5b59d144b8a586b5d

commit r11-6686-g3651c1b5c9c0960e50f00ca5b59d144b8a586b5d
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Jan 14 16:12:22 2021 -0300

    calibrate intervals to avoid zero in futures poll test
    
    We get occasional failures of 30_threads/future/members/poll.cc
    on some platforms whose high resolution clock doesn't have such a high
    resolution; wait_for_0 ends up as 0, and then some asserts fail as
    intervals measured as longer than zero are tested for less than
    several times zero.
    
    This patch adds some calibration in the iteration count to set a
    measurable base time interval with some additional margin.
    
    
    for  libstdc++-v3/ChangeLog
    
            * testsuite/30_threads/future/members/poll.cc: Calibrate
            iteration count.

Diff:
---
 .../testsuite/30_threads/future/members/poll.cc    | 33 +++++++++++++++++++++-
 1 file changed, 32 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 91f685b172d..133dae15ac4 100644
--- a/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
+++ b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
@@ -25,7 +25,7 @@
 #include <iostream>
 #include <testsuite_hooks.h>
 
-const int iterations = 200;
+int iterations = 200;
 
 using namespace std;
 
@@ -45,10 +45,41 @@ int main()
   promise<int> p;
   future<int> f = p.get_future();
 
+ start_over:
   auto start = chrono::high_resolution_clock::now();
   for(int i = 0; i < iterations; i++)
     f.wait_for(chrono::seconds(0));
   auto stop = chrono::high_resolution_clock::now();
+
+  /* We've run too few iterations for the clock resolution.
+     Attempt to calibrate it.  */
+  if (start == stop)
+    {
+      /* Loop until the clock advances, so that start is right after a
+	 time increment.  */
+      do
+	start = chrono::high_resolution_clock::now();
+      while (start == stop);
+      int i = 0;
+      /* Now until the clock advances again, so that stop is right
+	 after another time increment.  */
+      do
+	{
+	  f.wait_for(chrono::seconds(0));
+	  stop = chrono::high_resolution_clock::now();
+	  i++;
+	}
+      while (start == stop);
+      /* Go for some 10 cycles, but if we're already past that and
+	 still get into the calibration loop, double the iteration
+	 count and try again.  */
+      if (iterations < i * 10)
+	iterations = i * 10;
+      else
+	iterations *= 2;
+      goto start_over;
+    }
+
   double wait_for_0 = print("wait_for(0s)", stop - start);
 
   start = chrono::high_resolution_clock::now();