[libc-commits] [libc] [libc] Add regex AST and ExprPool (PR #198728)

[Jeff Bailey via libc-commits]

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@@ -0,0 +1,109 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Pool for Regular Expression AST nodes.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_REGEX_REGEX_EXPR_POOL_H
+#define LLVM_LIBC_SRC___SUPPORT_REGEX_REGEX_EXPR_POOL_H
+
+#include "src/__support/CPP/expected.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/regex/regex_ast.h"
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+/// An arena-based pool for Regular Expression AST nodes.
+///
+/// This class manages the allocation and hash-consing of Expr nodes. All
+/// nodes created through this pool are owned by it and will be freed when
+/// the pool is destroyed. Hash-consing ensures that identical expressions
+/// are represented by the same pointer, enabling fast comparison and
+/// derivative normalization.
+class ExprPool {
+  /// Internal storage block for AST nodes.
+  ///
+  /// Blocks are allocated on demand to avoid large contiguous allocations
+  /// and are linked together in a list for cleanup.
+  /// TODO: Consider adopting cpp::forward_list for block management once
+  /// it is available in LLVM-libc.
+  struct Block {
+    /// Number of Expr nodes stored in each block.
+    static constexpr size_t SIZE = 256;
+    /// The actual storage for Expr nodes.
+    Expr nodes[SIZE];
+    /// Pointer to the next block in the chain.
+    Block *next = nullptr;
+    /// Number of nodes currently used in this block.
+    size_t used = 0;
+
+    /// Initializes an empty block.
+    Block();
+  };
+
+  /// The first block in the allocation chain.
+  Block *head = nullptr;
+  /// The block currently being used for new node allocations.
+  Block *current = nullptr;
+  /// Total number of nodes allocated across all blocks.
+  size_t node_count = 0;
+
+  /// Size of the hash table used for hash-consing.
+  /// Must be significantly larger than MAX_NODES to maintain efficiency.
+  static constexpr size_t HASH_SIZE = 16384;
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kaladron wrote:

I added an explanation as well here.

https://github.com/llvm/llvm-project/pull/198728