Make Abseil unit tests tolerate fused multiply/add contraction

Apply a patch from upstream to make tests pass when GCC replaces double
addition and multiplication with fused multiply/add instructions.

2 files changed, 64 insertions, 0 deletions

diff --git a/debian/patches/fma-contraction.diff b/debian/patches/fma-contraction.diff
new file mode 100644
index 00000000..95181a36
--- /dev/null
+++ b/debian/patches/fma-contraction.diff
@@ -0,0 +1,63 @@
+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);
+   }
+ }
+ 
diff --git a/debian/patches/series b/debian/patches/series
index ceceb694..c07c5a22 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -7,3 +7,4 @@ nan-narrowing.diff
 endian-hash.diff
 endian-random.diff
 ppc-float-conversion.diff
+fma-contraction.diff