Fix #382 - MinGW often reports negative CPU times. (#475)

When stopping a timer, the current time is subtracted from the start time. However, when the times are identical, or sufficiently close together, the subtraction can result in a negative number. For some reason MinGW is the only platform where this problem manifests. I suspect it's due to MinGW specific behavior in either the CPU timing code, floating point model, or printf formatting. Either way, the fix for MinGW should be correct across all platforms.

Fix #382 - MinGW often reports negative CPU times. (#475)
72a4581c · Eric · Dominic Hamon · f65c6d9a · 72a4581c
Commit 72a4581c authored Nov 07, 2017 by Eric Committed by Dominic Hamon Nov 07, 2017
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benchmark.cc src/benchmark.cc +3 -1

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--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -175,7 +175,9 @@ class ThreadTimer {
    CHECK(running_);
    running_ = false;
    real_time_used_ += ChronoClockNow() - start_real_time_;
-    cpu_time_used_ += ThreadCPUUsage() - start_cpu_time_;
+    // Floating point error can result in the subtraction producing a negative
+    // time. Guard against that.
+    cpu_time_used_ += std::max<double>(ThreadCPUUsage() - start_cpu_time_, 0);
  }
  // Called by each thread