Export of internal Abseil changes.

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00d42e3d5433aaf29c2ed293520b2ba178ae8bdb by Greg Falcon <gfalcon@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 238061818

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867a7ca318fac2991ea9a4107dbae3cc9fbf974a by Abseil Team <absl-team@google.com>:

Added a IWYU export pragma when including a standard header for the purpose of aliasing its symbols.

PiperOrigin-RevId: 238022277

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17047745058f2f151cd986ea9f649512542d3876 by Matt Armstrong <marmstrong@google.com>:

Clarify the comment discouraging WrapUnique<T>(x) calls.

PiperOrigin-RevId: 237873803

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3dcb2e4968243d33ca0ce53280c445df50f4a7ec by Samuel Benzaquen <sbenza@google.com>:

Workaround clang bug https://bugs.llvm.org/show_bug.cgi?id=38289

PiperOrigin-RevId: 237873551

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f348d2dc7087a990cbdfb95aa51fd7ff478ae40e by Samuel Benzaquen <sbenza@google.com>:

Reduce minimum capacity to 1.
This reduces memory usage for small tables.
A flat_hash_set<int> of 1 element goes from 92 bytes to 24.
A flat_hash_set<string> of 1 element goes from 512 bytes to 56.

PiperOrigin-RevId: 237859811

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9c8125be5e4e5d22a7bb62bdec8c323338385c1b by Jon Cohen <cohenjon@google.com>:

Bump to CMake 3.5. This is the oldest modern cmake being included by default in most popular OS distributions according to https://repology.org/project/cmake/versions.  Specifically, Ubuntu LTS 16.04 uses cmake 3.5 (https://packages.ubuntu.com/xenial/cmake)

PiperOrigin-RevId: 237859345

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07638d672e0a4dced986a62750cfd8318ed36ffa by Derek Mauro <dmauro@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 237714597
GitOrigin-RevId: 00d42e3d5433aaf29c2ed293520b2ba178ae8bdb
Change-Id: I5faecc45add4a5a774d4f9baf06e5519091f2ccc

2 files changed, 36 insertions, 0 deletions

diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index 33f528ce..93b62c52 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc
@@ -123,6 +123,28 @@ uint128 MakeUint128FromFloat(T v) {
 
   return MakeUint128(0, static_cast<uint64_t>(v));
 }
+
+#if defined(__clang__) && !defined(__SSE3__)
+// Workaround for clang bug: https://bugs.llvm.org/show_bug.cgi?id=38289
+// Casting from long double to uint64_t is miscompiled and drops bits.
+// It is more work, so only use when we need the workaround.
+uint128 MakeUint128FromFloat(long double v) {
+  // Go 50 bits at a time, that fits in a double
+  static_assert(std::numeric_limits<double>::digits >= 50, "");
+  static_assert(std::numeric_limits<long double>::digits <= 150, "");
+  // Undefined behavior if v is not finite or cannot fit into uint128.
+  assert(std::isfinite(v) && v > -1 && v < std::ldexp(1.0L, 128));
+
+  v = std::ldexp(v, -100);
+  uint64_t w0 = static_cast<uint64_t>(static_cast<double>(std::trunc(v)));
+  v = std::ldexp(v - static_cast<double>(w0), 50);
+  uint64_t w1 = static_cast<uint64_t>(static_cast<double>(std::trunc(v)));
+  v = std::ldexp(v - static_cast<double>(w1), 50);
+  uint64_t w2 = static_cast<uint64_t>(static_cast<double>(std::trunc(v)));
+  return (static_cast<uint128>(w0) << 100) | (static_cast<uint128>(w1) << 50) |
+         static_cast<uint128>(w2);
+}
+#endif  // __clang__ && !__SSE3__
 }  // namespace
 
 uint128::uint128(float v) : uint128(MakeUint128FromFloat(v)) {}
diff --git a/absl/numeric/int128_test.cc b/absl/numeric/int128_test.cc
index 216ec50c..5e1b5ec3 100644
--- a/absl/numeric/int128_test.cc
+++ b/absl/numeric/int128_test.cc
@@ -271,6 +271,20 @@ TEST(Uint128, ConversionTests) {
   EXPECT_EQ(static_cast<absl::uint128>(round_to_zero), 0);
   EXPECT_EQ(static_cast<absl::uint128>(round_to_five), 5);
   EXPECT_EQ(static_cast<absl::uint128>(round_to_nine), 9);
+
+  absl::uint128 highest_precision_in_long_double =
+      ~absl::uint128{} >> (128 - std::numeric_limits<long double>::digits);
+  EXPECT_EQ(highest_precision_in_long_double,
+            static_cast<absl::uint128>(
+                static_cast<long double>(highest_precision_in_long_double)));
+  // Apply a mask just to make sure all the bits are the right place.
+  const absl::uint128 arbitrary_mask =
+      absl::MakeUint128(0xa29f622677ded751, 0xf8ca66add076f468);
+  EXPECT_EQ(highest_precision_in_long_double & arbitrary_mask,
+            static_cast<absl::uint128>(static_cast<long double>(
+                highest_precision_in_long_double & arbitrary_mask)));
+
+  EXPECT_EQ(static_cast<absl::uint128>(-0.1L), 0);
 }
 
 TEST(Uint128, OperatorAssignReturnRef) {