Export of internal Abseil changes

--
e54b9c7bbb0c58475676c268e2e19c69f4bce48a by Jorg Brown <jorg@google.com>:

Tweak ABSL_PREDICT_TRUE slightly, for better code on some platforms and/or
optimization levels.  "false || (x)" is more verbose than "!!(x)", but
ultimately more efficient.

For example, given this code:

void InitIfNecessary() {
  if (ABSL_PREDICT_TRUE(NeedsInit())) {
    SlowInitIfNecessary();
  }
}

Clang with default optimization level will produce:

Before this CL              After this CL
InitIfNecessary:            InitIfNecessary:
  push rbp                    push rbp
  mov  rbp, rsp               mov  rbp, rsp
  call NeedsInit              call NeedsInit
  xor  al, -1
  xor  al, -1
  test al, 1                  test al, 1
  jne  .LBB2_1                jne  .LBB3_1
  jmp  .LBB2_2                jmp  .LBB3_2
.LBB2_1:                    .LBB3_1:
  call SlowInitIfNecessary    call SlowInitIfNecessary
.LBB2_2:                    .LBB3_2:
  pop  rbp                    pop  rbp
  ret                         ret
PiperOrigin-RevId: 276401386

--
0a3c4dfd8342bf2b1b11a87f1c662c883f73cab7 by Abseil Team <absl-team@google.com>:

Fix comment nit: sem_open => sem_init.

The code calls sem_init, not sem_open, to initialize an unnamed semaphore.
(sem_open creates or opens a named semaphore.)

PiperOrigin-RevId: 276344072

--
b36a664e9459057509a90e83d3482e1d3a4c44c7 by Abseil Team <absl-team@google.com>:

Fix typo in flat_hash_map.h: exchaged -> exchanged

PiperOrigin-RevId: 276295792

--
7bbd8d18276eb110c8335743e35fceb662ddf3d6 by Samuel Benzaquen <sbenza@google.com>:

Add assertions to verify use of iterators.

PiperOrigin-RevId: 276283300

--
677398a8ffcb1f59182cffe57a4fe7ff147a0404 by Laramie Leavitt <lar@google.com>:

Migrate distribution_impl.h/cc to generate_real.h/cc.

Combine the methods RandU64To<Float,Double> into a single method:
GenerateRealFromBits().

Remove rejection sampling from absl::uniform_real_distribution.

PiperOrigin-RevId: 276158675

--
c60c9d11d24b0c546329d998e78e15a84b3153f5 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 276126962

--
4c840cab6a8d86efa29b397cafaf7520eece68cc by Andy Soffer <asoffer@google.com>:

Update CMakeLists.txt to address https://github.com/abseil/abseil-cpp/issues/365.
This does not cover every platform, but it does at least address the
first-order issue of assuming gcc implies x86.

PiperOrigin-RevId: 276116253

--
98da366e6b5d51afe5d7ac6722126aca23d85ee6 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 276097452
GitOrigin-RevId: e54b9c7bbb0c58475676c268e2e19c69f4bce48a
Change-Id: I02d84454bb71ab21ad3d39650acf6cc6e36f58d7

1 files changed, 144 insertions, 0 deletions

diff --git a/absl/random/internal/generate_real.h b/absl/random/internal/generate_real.h
new file mode 100644
index 00000000..246d863e
--- /dev/null
+++ b/absl/random/internal/generate_real.h
@@ -0,0 +1,144 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+#define ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+
+// This file contains some implementation details which are used by one or more
+// of the absl random number distributions.
+
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/internal/bits.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/traits.h"
+
+namespace absl {
+namespace random_internal {
+
+// Tristate tag types controlling the output of GenerateRealFromBits.
+struct GeneratePositiveTag {};
+struct GenerateNegativeTag {};
+struct GenerateSignedTag {};
+
+// GenerateRealFromBits generates a single real value from a single 64-bit
+// `bits` with template fields controlling the output.
+//
+// The `SignedTag` parameter controls whether positive, negative,
+// or either signed/unsigned may be returned.
+//   When SignedTag == GeneratePositiveTag, range is U(0, 1)
+//   When SignedTag == GenerateNegativeTag, range is U(-1, 0)
+//   When SignedTag == GenerateSignedTag, range is U(-1, 1)
+//
+// When the `IncludeZero` parameter is true, the function may return 0 for some
+// inputs, otherwise it never returns 0.
+//
+// When a value in U(0,1) is required, use:
+//   Uniform64ToReal<double, PositiveValueT, true>;
+//
+// When a value in U(-1,1) is required, use:
+//   Uniform64ToReal<double, SignedValueT, false>;
+//
+//   This generates more distinct values than the mathematical equivalent
+//   `U(0, 1) * 2.0 - 1.0`.
+//
+// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
+//   GenerateRealFromBits<double>(..., -1);  => U(0, 0.5)
+//   GenerateRealFromBits<double>(..., 1);   => U(0, 2)
+//
+template <typename RealType,  // Real type, either float or double.
+          typename SignedTag = GeneratePositiveTag,  // Whether a positive,
+                                                     // negative, or signed
+                                                     // value is generated.
+          bool IncludeZero = true>
+inline RealType GenerateRealFromBits(uint64_t bits, int exp_bias = 0) {
+  using real_type = RealType;
+  using uint_type = absl::conditional_t<std::is_same<real_type, float>::value,
+                                        uint32_t, uint64_t>;
+
+  static_assert(
+      (std::is_same<double, real_type>::value ||
+       std::is_same<float, real_type>::value),
+      "GenerateRealFromBits must be parameterized by either float or double.");
+
+  static_assert(sizeof(uint_type) == sizeof(real_type),
+                "Mismatched unsinged and real types.");
+
+  static_assert((std::numeric_limits<real_type>::is_iec559 &&
+                 std::numeric_limits<real_type>::radix == 2),
+                "RealType representation is not IEEE 754 binary.");
+
+  static_assert((std::is_same<SignedTag, GeneratePositiveTag>::value ||
+                 std::is_same<SignedTag, GenerateNegativeTag>::value ||
+                 std::is_same<SignedTag, GenerateSignedTag>::value),
+                "");
+
+  static constexpr int kExp = std::numeric_limits<real_type>::digits - 1;
+  static constexpr uint_type kMask = (static_cast<uint_type>(1) << kExp) - 1u;
+  static constexpr int kUintBits = sizeof(uint_type) * 8;
+
+  int exp = exp_bias + int{std::numeric_limits<real_type>::max_exponent - 2};
+
+  // Determine the sign bit.
+  // Depending on the SignedTag, this may use the left-most bit
+  // or it may be a constant value.
+  uint_type sign = std::is_same<SignedTag, GenerateNegativeTag>::value
+                       ? (static_cast<uint_type>(1) << (kUintBits - 1))
+                       : 0;
+  if (std::is_same<SignedTag, GenerateSignedTag>::value) {
+    if (std::is_same<uint_type, uint64_t>::value) {
+      sign = bits & uint64_t{0x8000000000000000};
+    }
+    if (std::is_same<uint_type, uint32_t>::value) {
+      const uint64_t tmp = bits & uint64_t{0x8000000000000000};
+      sign = static_cast<uint32_t>(tmp >> 32);
+    }
+    // adjust the bits and the exponent to account for removing
+    // the leading bit.
+    bits = bits & uint64_t{0x7FFFFFFFFFFFFFFF};
+    exp++;
+  }
+  if (IncludeZero) {
+    if (bits == 0u) return 0;
+  }
+
+  // Number of leading zeros is mapped to the exponent: 2^-clz
+  // bits is 0..01xxxxxx. After shifting, we're left with 1xxx...0..0
+  int clz = base_internal::CountLeadingZeros64(bits);
+  bits <<= (IncludeZero ? clz : (clz & 63));  // remove 0-bits.
+  exp -= clz;                                 // set the exponent.
+  bits >>= (63 - kExp);
+
+  // Construct the 32-bit or 64-bit IEEE 754 floating-point value from
+  // the individual fields: sign, exp, mantissa(bits).
+  uint_type val =
+      (std::is_same<SignedTag, GeneratePositiveTag>::value ? 0u : sign) |
+      (static_cast<uint_type>(exp) << kExp) |
+      (static_cast<uint_type>(bits) & kMask);
+
+  // bit_cast to the output-type
+  real_type result;
+  memcpy(static_cast<void*>(&result), static_cast<const void*>(&val),
+         sizeof(result));
+  return result;
+}
+
+}  // namespace random_internal
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_