[llvm] Introduce paged vector (PR #66430)

Jakub Kuderski via llvm-commits llvm-commits at lists.llvm.org
Fri Sep 15 10:17:18 PDT 2023

@@ -0,0 +1,133 @@
+//===- llvm/ADT/PagedVector.h - 'Lazyly allocated' vectors --------*- C++
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// This file defines the PagedVector class.
+#include <cassert>
+#include <vector>
+namespace llvm {
+// A vector that allocates memory in pages.
+// Order is kept, but memory is allocated only when one element of the page is
+// accessed. This introduces a level of indirection, but it is useful when you
+// have a sparsely initialised vector where the full size is allocated upfront
+// with the default constructor and elements are initialised later, on first
+// access.
+// Notice that this does not have iterators, because if you
+// have iterators it probably means you are going to touch
+// all the memory in any case, so better use a std::vector in
+// the first place.
+template <typename T, int PAGE_SIZE = 1024 / sizeof(T)> class PagedVector {
+  // The actual number of element in the vector which can be accessed.
+  std::size_t Size = 0;
+  // The position of the initial element of the page in the Data vector.
+  // Pages are allocated contiguously in the Data vector.
+  mutable std::vector<int> Lookup;
kuhar wrote:

The comment makes sense to me but I wonder if we could pick more descriptive name. Maybe `PageToDataIdx` or `PageToOffset`.


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