PR #1632: inlined_vector: Use trivial relocation for `erase`

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1632

Prior art for the `vector::erase` optimization:
https://github.com/AmadeusITGroup/amc/blob/efcb7be/include/amc/vectorcommon.hpp#L176-L180 https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslalg/bslalg_arrayprimitives.h#L3787-L3799 https://github.com/facebook/folly/blob/d24bf04/folly/FBVector.h#L1254-L1262 https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qarraydataops.h#L856-L861

Merge 6ce011079ccf945ae95434ce45ea6c5e3a088af8 into 55d28d4b3b82f9a47b3fa9b811b675a032820621

Merging this change closes #1632

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1632 from Quuxplusone:trivial-erase 6ce011079ccf945ae95434ce45ea6c5e3a088af8
PiperOrigin-RevId: 612278964
Change-Id: I327ace8a38292b4610c6be031cc334e77c76fd35

1 files changed, 22 insertions, 8 deletions

diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index 90a74dc7..a1575328 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -893,16 +893,30 @@ auto Storage<T, N, A>::Erase(ConstIterator<A> from,
       std::distance(ConstIterator<A>(storage_view.data), from));
   SizeType<A> erase_end_index = erase_index + erase_size;
 
-  IteratorValueAdapter<A, MoveIterator<A>> move_values(
-      MoveIterator<A>(storage_view.data + erase_end_index));
-
-  AssignElements<A>(storage_view.data + erase_index, move_values,
-                    storage_view.size - erase_end_index);
+  // Fast path: if the value type is trivially relocatable and we know
+  // the allocator doesn't do anything fancy, then we know it is legal for us to
+  // simply destroy the elements in the "erasure window" (which cannot throw)
+  // and then memcpy downward to close the window.
+  if (absl::is_trivially_relocatable<ValueType<A>>::value &&
+      std::is_nothrow_destructible<ValueType<A>>::value &&
+      std::is_same<A, std::allocator<ValueType<A>>>::value) {
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + erase_index, erase_size);
+    std::memmove(
+        reinterpret_cast<char*>(storage_view.data + erase_index),
+        reinterpret_cast<const char*>(storage_view.data + erase_end_index),
+        (storage_view.size - erase_end_index) * sizeof(ValueType<A>));
+  } else {
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        MoveIterator<A>(storage_view.data + erase_end_index));
 
-  DestroyAdapter<A>::DestroyElements(
-      GetAllocator(), storage_view.data + (storage_view.size - erase_size),
-      erase_size);
+    AssignElements<A>(storage_view.data + erase_index, move_values,
+                      storage_view.size - erase_end_index);
 
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + (storage_view.size - erase_size),
+        erase_size);
+  }
   SubtractSize(erase_size);
   return Iterator<A>(storage_view.data + erase_index);
 }