[libc-commits] [libc] [libc][stdlib] Make the FreeListHeap constant-initializable (PR #95453)

[via libc-commits]

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@@ -68,43 +68,53 @@ template <size_t NUM_BUCKETS = 6> class FreeList {
   /// Removes a chunk from this freelist.
   bool remove_chunk(cpp::span<cpp::byte> chunk);
 
-private:
-  // For a given size, find which index into chunks_ the node should be written
-  // to.
-  size_t find_chunk_ptr_for_size(size_t size, bool non_null) const;
+  /// For a given size, find which index into chunks_ the node should be written
+  /// to.
+  constexpr size_t find_chunk_ptr_for_size(size_t size, bool non_null) const;
 
   struct FreeListNode {
     FreeListNode *next;
     size_t size;
   };
 
-public:
-  explicit FreeList(cpp::array<size_t, NUM_BUCKETS> sizes)
+  constexpr void set_freelist_node(FreeListNode *node,
+                                   cpp::span<cpp::byte> chunk);
+
+  constexpr explicit FreeList(cpp::array<size_t, NUM_BUCKETS> sizes)
       : chunks_(NUM_BUCKETS + 1, 0), sizes_(sizes.begin(), sizes.end()) {}
 
+private:
   FixedVector<FreeList::FreeListNode *, NUM_BUCKETS + 1> chunks_;
   FixedVector<size_t, NUM_BUCKETS> sizes_;
 };
 
+template <size_t NUM_BUCKETS>
+constexpr void FreeList<NUM_BUCKETS>::set_freelist_node(FreeListNode *node,
+                                                        span<cpp::byte> chunk) {
+  // Add it to the correct list.
+  size_t chunk_ptr = find_chunk_ptr_for_size(chunk.size(), false);
+  node->size = chunk.size();
+  node->next = chunks_[chunk_ptr];
+  chunks_[chunk_ptr] = node;
+}
+
 template <size_t NUM_BUCKETS>
 bool FreeList<NUM_BUCKETS>::add_chunk(span<cpp::byte> chunk) {
   // Check that the size is enough to actually store what we need
   if (chunk.size() < sizeof(FreeListNode))
     return false;
 
+  // FIXME: This is UB since type punning is not allowed in C++. THe idea here
+  // is that we write the FreeListNode `size` and `next` onto the start of the
+  // buffer. Unless the original underlying bytes were a `FreeListNode`, the
+  // only safe way to do this is with `memcpy`.
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PiJoules wrote:

Note this is resolved in https://github.com/llvm/llvm-project/pull/95330, so this will go away once that lands.

https://github.com/llvm/llvm-project/pull/95453