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From: Benjamin Barenblat <bbaren@google.com>
Subject: Make tests tolerant of FMA contraction
Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/b0735979d778a768caee207f01f327535cbd2140
Weaken Duration.ToDoubleSecondsCheckEdgeCases and
Duration.ToDoubleSecondsCheckRandom to make them less sensitive to fused
multiply/add contraction.
The author works at Google. Upstream applied this patch as Piper
revision 360297653 and exported it to GitHub; the the Applied-Upstream URL
above points to the exported commit.
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -1369,10 +1369,13 @@
EXPECT_THAT(ToTimeval(absl::Nanoseconds(2000)), TimevalMatcher(tv));
}
-void VerifySameAsMul(double time_as_seconds, int* const misses) {
+void VerifyApproxSameAsMul(double time_as_seconds, int* const misses) {
auto direct_seconds = absl::Seconds(time_as_seconds);
auto mul_by_one_second = time_as_seconds * absl::Seconds(1);
- if (direct_seconds != mul_by_one_second) {
+ // These are expected to differ by up to one tick due to fused multiply/add
+ // contraction.
+ if (absl::AbsDuration(direct_seconds - mul_by_one_second) >
+ absl::time_internal::MakeDuration(0, 1u)) {
if (*misses > 10) return;
ASSERT_LE(++(*misses), 10) << "Too many errors, not reporting more.";
EXPECT_EQ(direct_seconds, mul_by_one_second)
@@ -1384,7 +1387,8 @@
// For a variety of interesting durations, we find the exact point
// where one double converts to that duration, and the very next double
// converts to the next duration. For both of those points, verify that
-// Seconds(point) returns the same duration as point * Seconds(1.0)
+// Seconds(point) returns a duration near point * Seconds(1.0). (They may
+// not be exactly equal due to fused multiply/add contraction.)
TEST(Duration, ToDoubleSecondsCheckEdgeCases) {
constexpr uint32_t kTicksPerSecond = absl::time_internal::kTicksPerSecond;
constexpr auto duration_tick = absl::time_internal::MakeDuration(0, 1u);
@@ -1423,8 +1427,8 @@
}
// Now low_edge is the highest double that converts to Duration d,
// and high_edge is the lowest double that converts to Duration after_d.
- VerifySameAsMul(low_edge, &misses);
- VerifySameAsMul(high_edge, &misses);
+ VerifyApproxSameAsMul(low_edge, &misses);
+ VerifyApproxSameAsMul(high_edge, &misses);
}
}
}
@@ -1444,8 +1448,8 @@
int misses = 0;
for (int i = 0; i < 1000000; ++i) {
double d = std::exp(uniform(gen));
- VerifySameAsMul(d, &misses);
- VerifySameAsMul(-d, &misses);
+ VerifyApproxSameAsMul(d, &misses);
+ VerifyApproxSameAsMul(-d, &misses);
}
}
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