Export of internal Abseil changes

--
a5af5874c1c5cc02bd2a748d455321f82b6f2a93 by Andy Getzendanner <durandal@google.com>:

fix compile fails with asan and -Wredundant-decls

Import of https://github.com/abseil/abseil-cpp/pull/801

PiperOrigin-RevId: 336693223

--
ed9df42ab2b742386c6692c2bed015374c919d9c by Derek Mauro <dmauro@google.com>:

Fix integer conversion warning

Fixes #814

PiperOrigin-RevId: 336651814

--
0ab4c23884e72dce17b67c1eb520f9dbb802565d by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 336585378

--
eba0e3dccd52a6e91bcff84075bef0affc650b74 by Matt Kulukundis <kfm@google.com>:

Add bitset operations to Futex helper.

PiperOrigin-RevId: 336409368

--
8b0709a8b4500bf5f0af4b602d76a298d81645e8 by Abseil Team <absl-team@google.com>:

Fix code indentation in a comment.

PiperOrigin-RevId: 336368167

--
bc3961c87a7e7760c10319a5b0349c279f7ae3ad by Samuel Benzaquen <sbenza@google.com>:

Improve performance of the registry:
 - Reduce contention
 - Reduce memory usage for each flag by `6*sizeof(void*)`.
 - Replace one immortal allocation per-flag with a single one for all the flags
 - Slightly improve single-threaded performance by avoiding the std::map indirections.

PiperOrigin-RevId: 336365904

--
264ad9f28f935aad8b6b1437f8bf804fa9104346 by Abseil Team <absl-team@google.com>:

Fix typo in comment on absl::Condition.

PiperOrigin-RevId: 336311680

--
b5b808a8c75ca0df7b09eff9a423ec171d80f771 by Derek Mauro <dmauro@google.com>:

Add missing Apache license headers

PiperOrigin-RevId: 336294980

--
89446c3a4793df8b95060385cf3e219357c3db1d by Andy Soffer <asoffer@google.com>:

Internal changes

PiperOrigin-RevId: 336287465

--
57c8be4e294881bc79a6a44b8e4bf7ecbb19b9b9 by Matt Kulukundis <kfm@google.com>:

Extract Futex from an implementation detail of Wait to a private interface.

PiperOrigin-RevId: 336123209
GitOrigin-RevId: a5af5874c1c5cc02bd2a748d455321f82b6f2a93
Change-Id: Ie5a0ebe28e571814e3e11d4c05ca308523ccf311

1 files changed, 139 insertions, 0 deletions

diff --git a/absl/synchronization/internal/futex.h b/absl/synchronization/internal/futex.h
new file mode 100644
index 00000000..3539f491
--- /dev/null
+++ b/absl/synchronization/internal/futex.h
@@ -0,0 +1,139 @@
+// Copyright 2020 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_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+
+#include "absl/base/config.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <sys/time.h>
+#include <unistd.h>
+#endif
+
+#ifdef __linux__
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#endif
+
+#include <errno.h>
+#include <stdio.h>
+#include <time.h>
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// Some Android headers are missing these definitions even though they
+// support these futex operations.
+#ifdef __BIONIC__
+#ifndef SYS_futex
+#define SYS_futex __NR_futex
+#endif
+#ifndef FUTEX_WAIT_BITSET
+#define FUTEX_WAIT_BITSET 9
+#endif
+#ifndef FUTEX_PRIVATE_FLAG
+#define FUTEX_PRIVATE_FLAG 128
+#endif
+#ifndef FUTEX_CLOCK_REALTIME
+#define FUTEX_CLOCK_REALTIME 256
+#endif
+#ifndef FUTEX_BITSET_MATCH_ANY
+#define FUTEX_BITSET_MATCH_ANY 0xFFFFFFFF
+#endif
+#endif
+
+#if defined(__NR_futex_time64) && !defined(SYS_futex_time64)
+#define SYS_futex_time64 __NR_futex_time64
+#endif
+
+#if defined(SYS_futex_time64) && !defined(SYS_futex)
+#define SYS_futex SYS_futex_time64
+#endif
+
+class FutexImpl {
+ public:
+  static int WaitUntil(std::atomic<int32_t> *v, int32_t val,
+                       KernelTimeout t) {
+    int err = 0;
+    if (t.has_timeout()) {
+      // https://locklessinc.com/articles/futex_cheat_sheet/
+      // Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
+      struct timespec abs_timeout = t.MakeAbsTimespec();
+      // Atomically check that the futex value is still 0, and if it
+      // is, sleep until abs_timeout or until woken by FUTEX_WAKE.
+      err = syscall(
+          SYS_futex, reinterpret_cast<int32_t *>(v),
+          FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
+          &abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
+    } else {
+      // Atomically check that the futex value is still 0, and if it
+      // is, sleep until woken by FUTEX_WAKE.
+      err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                    FUTEX_WAIT | FUTEX_PRIVATE_FLAG, val, nullptr);
+    }
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      err = -errno;
+    }
+    return err;
+  }
+
+  static int WaitBitsetAbsoluteTimeout(std::atomic<int32_t> *v, int32_t val,
+                                       int32_t bits,
+                                       const struct timespec *abstime) {
+    int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                      FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG, val, abstime,
+                      nullptr, bits);
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      err = -errno;
+    }
+    return err;
+  }
+
+  static int Wake(std::atomic<int32_t> *v, int32_t count) {
+    int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                      FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
+    if (ABSL_PREDICT_FALSE(err < 0)) {
+      err = -errno;
+    }
+    return err;
+  }
+
+  // FUTEX_WAKE_BITSET
+  static int WakeBitset(std::atomic<int32_t> *v, int32_t count, int32_t bits) {
+    int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                      FUTEX_WAKE_BITSET | FUTEX_PRIVATE_FLAG, count, nullptr,
+                      nullptr, bits);
+    if (ABSL_PREDICT_FALSE(err < 0)) {
+      err = -errno;
+    }
+    return err;
+  }
+};
+
+class Futex : public FutexImpl {};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_