From f8e0ff7f33338c2874b75e45e4ea5abbfafb954c Mon Sep 17 00:00:00 2001
From: Evan Brown <ezb@google.com>
Date: Mon, 3 Oct 2022 10:51:40 -0700
Subject: [PATCH] Use trivial relocation for transfers in swisstable and
 b-tree.

PiperOrigin-RevId: 478547898
Change-Id: Ie20cd0a49df042be912888ee238333a5f5fa0404
---
 absl/container/internal/common_policy_traits.h    | 15 ++++++++++++++-
 .../internal/common_policy_traits_test.cc         |  9 +++++----
 absl/container/internal/container_memory.h        | 13 ++++++++++++-
 3 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/absl/container/internal/common_policy_traits.h b/absl/container/internal/common_policy_traits.h
index cc2e89ba..c99e68f4 100644
--- a/absl/container/internal/common_policy_traits.h
+++ b/absl/container/internal/common_policy_traits.h
@@ -16,6 +16,7 @@
 #define ABSL_CONTAINER_INTERNAL_COMMON_POLICY_TRAITS_H_
 
 #include <cstddef>
+#include <cstring>
 #include <memory>
 #include <new>
 #include <type_traits>
@@ -32,7 +33,8 @@ template <class Policy, class = void>
 struct common_policy_traits {
   // The actual object stored in the container.
   using slot_type = typename Policy::slot_type;
-
+  using reference = decltype(Policy::element(std::declval<slot_type*>()));
+  using value_type = typename std::remove_reference<reference>::type;
 
   // PRECONDITION: `slot` is UNINITIALIZED
   // POSTCONDITION: `slot` is INITIALIZED
@@ -89,6 +91,17 @@ struct common_policy_traits {
   template <class Alloc>
   static void transfer_impl(Alloc* alloc, slot_type* new_slot,
                             slot_type* old_slot, char) {
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606
+    if (absl::is_trivially_relocatable<value_type>()) {
+      // TODO(b/247130232): remove cast after fixing class-memaccess warning.
+      std::memcpy(static_cast<void*>(
+                      std::launder(const_cast<std::remove_const_t<value_type>*>(
+                          &element(new_slot)))),
+                  &element(old_slot), sizeof(value_type));
+      return;
+    }
+#endif
+
     construct(alloc, new_slot, std::move(element(old_slot)));
     destroy(alloc, old_slot);
   }
diff --git a/absl/container/internal/common_policy_traits_test.cc b/absl/container/internal/common_policy_traits_test.cc
index 768d870e..5eaa4aae 100644
--- a/absl/container/internal/common_policy_traits_test.cc
+++ b/absl/container/internal/common_policy_traits_test.cc
@@ -27,7 +27,7 @@ namespace container_internal {
 namespace {
 
 using ::testing::MockFunction;
-using ::testing::Return;
+using ::testing::AnyNumber;
 using ::testing::ReturnRef;
 
 using Slot = int;
@@ -101,9 +101,10 @@ TEST_F(Test, element) {
 
 TEST_F(Test, without_transfer) {
   int b = 42;
-  EXPECT_CALL(element, Call(&b)).WillOnce(::testing::ReturnRef(b));
-  EXPECT_CALL(construct, Call(&alloc, &a, b));
-  EXPECT_CALL(destroy, Call(&alloc, &b));
+  EXPECT_CALL(element, Call(&a)).Times(AnyNumber()).WillOnce(ReturnRef(a));
+  EXPECT_CALL(element, Call(&b)).WillOnce(ReturnRef(b));
+  EXPECT_CALL(construct, Call(&alloc, &a, b)).Times(AnyNumber());
+  EXPECT_CALL(destroy, Call(&alloc, &b)).Times(AnyNumber());
   common_policy_traits<PolicyWithoutOptionalOps>::transfer(&alloc, &a, &b);
 }
 
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index 00e9f6d7..c29c533b 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -17,6 +17,7 @@
 
 #include <cassert>
 #include <cstddef>
+#include <cstring>
 #include <memory>
 #include <new>
 #include <tuple>
@@ -340,7 +341,8 @@ template <class K, class V>
 struct map_slot_policy {
   using slot_type = map_slot_type<K, V>;
   using value_type = std::pair<const K, V>;
-  using mutable_value_type = std::pair<K, V>;
+  using mutable_value_type =
+      std::pair<absl::remove_const_t<K>, absl::remove_const_t<V>>;
 
  private:
   static void emplace(slot_type* slot) {
@@ -424,6 +426,15 @@ struct map_slot_policy {
   static void transfer(Allocator* alloc, slot_type* new_slot,
                        slot_type* old_slot) {
     emplace(new_slot);
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606
+    if (absl::is_trivially_relocatable<value_type>()) {
+      // TODO(b/247130232): remove cast after fixing class-memaccess warning.
+      std::memcpy(static_cast<void*>(std::launder(&new_slot->value)),
+                  &old_slot->value, sizeof(value_type));
+      return;
+    }
+#endif
+
     if (kMutableKeys::value) {
       absl::allocator_traits<Allocator>::construct(
           *alloc, &new_slot->mutable_value, std::move(old_slot->mutable_value));