calibrate intervals to avoid zero in futures poll test

[Alexandre Oliva <oliva@adacore.com>]

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.

Regstrapped on x86_64-linux-gnu, and also tested on
x-arm-wrs-vxworks7r2.  Ok to install?


for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/future/members/poll.cc: Calibrate
	iteration count.
---
 .../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 fff9bea899c90..7b41411a54386 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();


-- 
Alexandre Oliva, happy hacker  https://FSFLA.org/blogs/lxo/
   Free Software Activist         GNU Toolchain Engineer
        Vim, Vi, Voltei pro Emacs -- GNUlius Caesar