Export of internal Abseil changes.

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fd86c60bac6c41f1629ce1ab7dc1c8edff398a59 by Alex Strelnikov <strel@google.com>:

Import PR: https://github.com/abseil/abseil-cpp/pull/243
Fix Windows ARM64 intrinsic use.

PiperOrigin-RevId: 228535649

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a0ca663f606a3b31493683e405be2b1cff450894 by CJ Johnson <johnsoncj@google.com>:

Fixes issue of mixed signedness comparison

PiperOrigin-RevId: 228535623

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d71aaa1705d7303b43fe02088fe07b153e647796 by Shaindel Schwartz <shaindel@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 228534365

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c1b49d361aa880198e071f93997724bddbcd4760 by Samuel Benzaquen <sbenza@google.com>:

Internal cleanup

PiperOrigin-RevId: 228406627

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0c4b1c2bed107698e209055b3431771d7a1bdba1 by Dave Walker <dawalker@google.com>:

Add comments about the purpose of container_internal::slot_type.

PiperOrigin-RevId: 228264537

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060aa6077d2f3a0a129149e0644d19f2f521b241 by Abseil Team <absl-team@google.com>:

#include <cmath> in hashtablez_sampler.cc

Expected to fix the android build.

PiperOrigin-RevId: 228222550
GitOrigin-RevId: fd86c60bac6c41f1629ce1ab7dc1c8edff398a59
Change-Id: I26339fd4548c1a81b037cb52c26910d1bd850ea8

1 files changed, 23 insertions, 4 deletions

diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index 56c5d2d..35b691c 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -286,11 +286,30 @@ struct IsLayoutCompatible {
 
 }  // namespace memory_internal
 
-// If kMutableKeys is false, only the value member is accessed.
+// The internal storage type for key-value containers like flat_hash_map.
 //
-// If kMutableKeys is true, key is accessed through all slots while value and
-// mutable_value are accessed only via INITIALIZED slots. Slots are created and
-// destroyed via mutable_value so that the key can be moved later.
+// It is convenient for the value_type of a flat_hash_map<K, V> to be
+// pair<const K, V>; the "const K" prevents accidental modification of the key
+// when dealing with the reference returned from find() and similar methods.
+// However, this creates other problems; we want to be able to emplace(K, V)
+// efficiently with move operations, and similarly be able to move a
+// pair<K, V> in insert().
+//
+// The solution is this union, which aliases the const and non-const versions
+// of the pair. This also allows flat_hash_map<const K, V> to work, even though
+// that has the same efficiency issues with move in emplace() and insert() -
+// but people do it anyway.
+//
+// If kMutableKeys is false, only the value member can be accessed.
+//
+// If kMutableKeys is true, key can be accessed through all slots while value
+// and mutable_value must be accessed only via INITIALIZED slots. Slots are
+// created and destroyed via mutable_value so that the key can be moved later.
+//
+// Accessing one of the union fields while the other is active is safe as
+// long as they are layout-compatible, which is guaranteed by the definition of
+// kMutableKeys. For C++11, the relevant section of the standard is
+// https://timsong-cpp.github.io/cppwp/n3337/class.mem#19 (9.2.19)
 template <class K, class V>
 union slot_type {
  private: