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From: Benjamin Barenblat <bbaren@google.com>
Subject: Round floats using round(x), not static_cast<int>(x + 0.5)
Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/d96e287417766deddbff2d01b96321288c59491e
Adding 0.5 to an IEEE float may cause one bit of precision loss, which
is enough to change the result in certain cases. For example,
static_cast<int>(std::round(0.49999999999999994)) == 0
static_cast<int>(0.49999999999999994 + 0.5) == 1
The author works at Google. Upstream applied this patch as Piper
revision 369926519 and exported it to GitHub; the Applied-Upstream URL
above points to the exported commit.
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -1320,7 +1320,7 @@ TEST(Duration, SmallConversions) {
EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(0));
// TODO(bww): Is the next one OK?
- EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(0.124999999e-9));
+ EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(std::nextafter(0.125e-9, 0)));
EXPECT_EQ(absl::Nanoseconds(1) / 4, absl::Seconds(0.125e-9));
EXPECT_EQ(absl::Nanoseconds(1) / 4, absl::Seconds(0.250e-9));
EXPECT_EQ(absl::Nanoseconds(1) / 2, absl::Seconds(0.375e-9));
@@ -1330,7 +1330,7 @@ TEST(Duration, SmallConversions) {
EXPECT_EQ(absl::Nanoseconds(1), absl::Seconds(0.875e-9));
EXPECT_EQ(absl::Nanoseconds(1), absl::Seconds(1.000e-9));
- EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(-0.124999999e-9));
+ EXPECT_EQ(absl::ZeroDuration(), absl::Seconds(std::nextafter(-0.125e-9, 0)));
EXPECT_EQ(-absl::Nanoseconds(1) / 4, absl::Seconds(-0.125e-9));
EXPECT_EQ(-absl::Nanoseconds(1) / 4, absl::Seconds(-0.250e-9));
EXPECT_EQ(-absl::Nanoseconds(1) / 2, absl::Seconds(-0.375e-9));
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -1352,7 +1352,7 @@
inline Duration MakePosDoubleDuration(double n) {
const int64_t int_secs = static_cast<int64_t>(n);
const uint32_t ticks = static_cast<uint32_t>(
- (n - static_cast<double>(int_secs)) * kTicksPerSecond + 0.5);
+ std::round((n - static_cast<double>(int_secs)) * kTicksPerSecond));
return ticks < kTicksPerSecond
? MakeDuration(int_secs, ticks)
: MakeDuration(int_secs + 1, ticks - kTicksPerSecond);
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