Export of internal Abseil changes

--
5ff0c4b38386ae80b25e9f8d0e1bac07fd7ce92c by Martijn Vels <mvels@google.com>:

Add CordzUpdateTracker class

PiperOrigin-RevId: 368469046

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4de916b1ba4b5480b4dbe93d28c5eaa110449c66 by Derek Mauro <dmauro@google.com>:

Adds `ABSL_ATTRIBUTE_LIFETIME_BOUND` to `absl::Span`s initializer_list
constructor.

Compilers that support this attribute will emit a warning
if the parameter does not have sufficient lifetime.

For more information, see
https://github.com/abseil/abseil-cpp/blob/b97a1ecda869ca8754d467a56c50275cebfeb328/absl/types/span.h#L209-L248

PiperOrigin-RevId: 368429085

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bb6669016412bf5afffc02d0818a66dfe1c524cf by Martijn Vels <mvels@google.com>:

Reorganize internal cord code in CMakeLists.txt into cord_internal library

PiperOrigin-RevId: 368423765

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d1d7fce066172d5fcfa0310c4e27631d895d7e50 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 368346725
GitOrigin-RevId: 5ff0c4b38386ae80b25e9f8d0e1bac07fd7ce92c
Change-Id: Ic4627eab4f0274e400a6d12cde3341fb538de075

1 files changed, 96 insertions, 0 deletions

diff --git a/absl/strings/internal/cordz_update_tracker.h b/absl/strings/internal/cordz_update_tracker.h
new file mode 100644
index 00000000..3c617e93
--- /dev/null
+++ b/absl/strings/internal/cordz_update_tracker.h
@@ -0,0 +1,96 @@
+// Copyright 2021 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_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzUpdateTracker tracks counters for Cord update methods.
+//
+// The purpose of CordzUpdateTracker is to track the number of calls to methods
+// updating Cord data for sampled cords. The class internally uses 'lossy'
+// atomic operations: Cord is thread-compatible, so there is no need to
+// synchronize updates. However, Cordz collection threads may call 'Value()' at
+// any point, so the class needs to provide thread safe access.
+//
+// This class is thread-safe. But as per above comments, all non-const methods
+// should be used single-threaded only: updates are thread-safe but lossy.
+class CordzUpdateTracker {
+ public:
+  // Tracked update methods.
+  enum MethodIdentifier {
+    kAssignString,
+    kAssignCord,
+    kMoveAssignCord,
+    kAppendString,
+    kAppendCord,
+    kMoveAppendCord,
+    kPrependString,
+    kPrependCord,
+    kMovePrependCord,
+    kAppendExternalMemory,
+    kFlatten,
+    kGetAppendRegion,
+    kRemovePrefix,
+    kRemoveSuffic,
+    kSubCord,
+
+    // kNumMethods defines the number of entries: must be the last entry.
+    kNumMethods,
+  };
+
+  // Constructs a new instance. All counters are zero-initialized.
+  constexpr CordzUpdateTracker() noexcept : values_{} {}
+
+  // Copy constructs a new instance.
+  CordzUpdateTracker(const CordzUpdateTracker& rhs) noexcept { *this = rhs; }
+
+  // Assigns the provided value to this instance.
+  CordzUpdateTracker& operator=(const CordzUpdateTracker& rhs) noexcept {
+    for (int i = 0; i < kNumMethods; ++i) {
+      values_[i].store(rhs.values_[i].load(std::memory_order_relaxed),
+                       std::memory_order_relaxed);
+    }
+    return *this;
+  }
+
+  // Returns the value for the specified method.
+  int64_t Value(MethodIdentifier method) const {
+    return values_[method].load(std::memory_order_relaxed);
+  }
+
+  // Increases the value for the specified method by `n`
+  void LossyAdd(MethodIdentifier method, int64_t n = 1) {
+    auto& value = values_[method];
+    value.store(value.load(std::memory_order_relaxed) + n,
+                std::memory_order_relaxed);
+  }
+
+ private:
+  std::atomic<int64_t> values_[kNumMethods];
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_