[gcc(refs/users/aoliva/heads/testme)] calibrate intervals to avoid zero in futures poll test

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:64c7b51992d9b738ba5c245ba9474e733e6c7bf4

commit 64c7b51992d9b738ba5c245ba9474e733e6c7bf4
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Dec 31 21:38:29 2020 -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 fff9bea899c..7b41411a543 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);
+      /* Got for some 10 cycles, but 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();