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.

Gbp-Pq: Name float-rounding.diff

diff --git a/absl/time/duration_test.cc b/absl/time/duration_test.cc
index fb28fa987f65ffc774e3952cd2a841e81ae6da1f..a3617e744ae456a21769aca9c36bd4248f57db16 100644 (file)
--- 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));

diff --git a/absl/time/time.h b/absl/time/time.h
index 2df68581d1b08bdcb57248d1d91d0ad76b1160fb..48982df45a3ce2d29f1af12be28fb8e6c8e00252 100644 (file)
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -1352,7 +1352,7 @@ constexpr Duration MakeDuration(int64_t hi, int64_t lo) {
 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);