Export of internal Abseil changes

--
2e894f3c2fadc789abf9011222526d5da1e0433e by Gennadiy Rozental <rogeeff@google.com>:

Internal change

PiperOrigin-RevId: 267557172

--
535be36d401a556156223ecc1aabd73a271f0f05 by Abseil Team <absl-team@google.com>:

Internal change.

PiperOrigin-RevId: 267456795
GitOrigin-RevId: 2e894f3c2fadc789abf9011222526d5da1e0433e
Change-Id: I95d29cbde5cd8342ae71b77728baa61b7cf6d440

1 files changed, 18 insertions, 2 deletions

diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index a84f7fa8d304..16330380f548 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -16,12 +16,16 @@
 #ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
 #define ABSL_FLAGS_INTERNAL_FLAG_H_
 
+#include <cstring>
+
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/registry.h"
 
 namespace absl {
 namespace flags_internal {
 
+constexpr int64_t AtomicInit() { return 0xababababababababll; }
+
 // Signature for the mutation callback used by watched Flags
 // The callback is noexcept.
 // TODO(rogeeff): add noexcept after C++17 support is added.
@@ -44,6 +48,7 @@ class Flag final : public flags_internal::CommandLineFlag {
             initial_value_gen,
             /*def=*/nullptr,
             /*cur=*/nullptr),
+        atomic_(flags_internal::AtomicInit()),
         callback_(nullptr) {}
 
   T Get() const {
@@ -76,8 +81,8 @@ class Flag final : public flags_internal::CommandLineFlag {
 
   bool AtomicGet(T* v) const {
     const int64_t r = atomic_.load(std::memory_order_acquire);
-    if (r != flags_internal::CommandLineFlag::kAtomicInit) {
-      memcpy(v, &r, sizeof(T));
+    if (r != flags_internal::AtomicInit()) {
+      std::memcpy(v, &r, sizeof(T));
       return true;
     }
 
@@ -108,7 +113,18 @@ class Flag final : public flags_internal::CommandLineFlag {
     delete locks_;
   }
 
+  void StoreAtomic() override {
+    if (sizeof(T) <= sizeof(int64_t)) {
+      int64_t t = 0;
+      std::memcpy(&t, cur_, (std::min)(sizeof(T), sizeof(int64_t)));
+      atomic_.store(t, std::memory_order_release);
+    }
+  }
+
   // Flag's data
+  // For some types, a copy of the current value is kept in an atomically
+  // accessible field.
+  std::atomic<int64_t> atomic_;
   FlagCallback callback_;  // Mutation callback
 };