Bug 826439 - use floats instead of doubles for exponential histogram statistic calculations; r=vdjeric

ipc/chromium/src/base/histogram.cc

@@ -746,7 +746,7 @@ void Histogram::SampleSet::AccumulateWithExponentialStats(Sample value,
                                                           size_t index) {
   Accumulate(value, count, index);
   DCHECK_GE(value, 0);
-  double value_log = log(static_cast<double>(value) + 1);
+  float value_log = logf(static_cast<float>(value) + 1.0f);
   log_sum_ += count * value_log;
   log_sum_squares_ += count * value_log * value_log;
 }

toolkit/components/telemetry/tests/unit/test_nsITelemetry.js

@@ -30,8 +30,11 @@ function test_histogram(histogram_type, name, min, max, bucket_count) {
   // verify properties
   do_check_eq(sum, s.sum);
   if (histogram_type == Telemetry.HISTOGRAM_EXPONENTIAL) {
-    do_check_eq(log_sum, s.log_sum);
-    do_check_eq(log_sum_squares, s.log_sum_squares);
+    // We do the log with float precision in C++ and double precision in
+    // JS, so there's bound to be tiny discrepancies.  Just check the
+    // integer part.
+    do_check_eq(Math.floor(log_sum), Math.floor(s.log_sum));
+    do_check_eq(Math.floor(log_sum_squares), Math.floor(s.log_sum_squares));
     do_check_false("sum_squares_lo" in s);
     do_check_false("sum_squares_hi" in s);
   } else {