[WIP][libc] Add freelist malloc

Author: PiJoules

libc/config/baremetal/riscv/entrypoints.txt

@@ -170,6 +170,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.ldiv
     libc.src.stdlib.llabs
     libc.src.stdlib.lldiv
+    libc.src.stdlib.malloc
     libc.src.stdlib.qsort
     libc.src.stdlib.rand
     libc.src.stdlib.srand

libc/src/__support/fixedvector.h

@@ -25,17 +25,17 @@ template <typename T, size_t CAPACITY> class FixedVector {
   constexpr FixedVector() = default;
 
   using iterator = typename cpp::array<T, CAPACITY>::iterator;
-  constexpr FixedVector(iterator begin, iterator end) {
+  constexpr FixedVector(iterator begin, iterator end) : store{} {
     for (; begin != end; ++begin)
       push_back(*begin);
   }
 
-  constexpr FixedVector(size_t count, const T &value) {
+  constexpr FixedVector(size_t count, const T &value) : store{} {
     for (size_t i = 0; i < count; ++i)
       push_back(value);
   }
 
-  bool push_back(const T &obj) {
+  constexpr bool push_back(const T &obj) {
     if (item_count == CAPACITY)
       return false;
     store[item_count] = obj;
@@ -54,13 +54,13 @@ template <typename T, size_t CAPACITY> class FixedVector {
     return true;
   }
 
-  T &operator[](size_t idx) { return store[idx]; }
+  constexpr T &operator[](size_t idx) { return store[idx]; }
 
-  const T &operator[](size_t idx) const { return store[idx]; }
+  constexpr const T &operator[](size_t idx) const { return store[idx]; }
 
   bool empty() const { return item_count == 0; }
 
-  size_t size() const { return item_count; }
+  constexpr size_t size() const { return item_count; }
 
   // Empties the store for all practical purposes.
   void reset() { item_count = 0; }

libc/src/__support/macros/attributes.h

@@ -30,4 +30,10 @@
 #define LIBC_THREAD_LOCAL thread_local
 #endif
 
+#if __cplusplus >= 202002L
+#define LIBC_CONSTINIT constinit
+#else
+#define LIBC_CONSTINIT __attribute__((__require_constant_initialization__))
+#endif
+
 #endif // LLVM_LIBC_SRC___SUPPORT_MACROS_ATTRIBUTES_H

libc/src/__support/threads/thread.cpp

@@ -115,7 +115,8 @@ class ThreadAtExitCallbackMgr {
 public:
   constexpr ThreadAtExitCallbackMgr()
       : mtx(/*timed=*/false, /*recursive=*/false, /*robust=*/false,
-            /*pshared=*/false) {}
+            /*pshared=*/false),
+        callback_list() {}
 
   int add_callback(AtExitCallback *callback, void *obj) {
     cpp::lock_guard lock(mtx);

libc/src/stdlib/CMakeLists.txt

@@ -401,9 +401,24 @@ else()
       libc.src.__support.CPP.cstddef
       libc.src.__support.CPP.array
       libc.src.__support.CPP.span
+      libc.src.stdio.printf
   )
-  add_entrypoint_external(
+  add_entrypoint_object(
     malloc
+    SRCS
+      freelist_malloc.cpp
+    HDRS
+      malloc.h
+    DEPENDS
+      .block
+      .freelist
+      libc.src.__support.CPP.new
+      libc.src.__support.CPP.optional
+      libc.src.__support.CPP.span
+      libc.src.__support.CPP.type_traits
+      libc.src.__support.fixedvector
+      libc.src.string.memcpy
+      libc.src.string.memset
   )
   add_entrypoint_external(
     free

libc/src/stdlib/block.h

@@ -245,7 +245,7 @@ class Block {
   void mark_free() { info_.used = 0; }
 
   /// Marks this block as the last one in the chain.
-  void mark_last() { info_.last = 1; }
+  constexpr void mark_last() { info_.last = 1; }
 
   /// Clears the last bit from this block.
   void clear_last() { info_.last = 1; }
@@ -259,15 +259,15 @@ class Block {
     return check_status() == internal::BlockStatus::VALID;
   }
 
+  constexpr Block(size_t prev_outer_size, size_t outer_size);
+
 private:
   /// Consumes the block and returns as a span of bytes.
   static ByteSpan as_bytes(Block *&&block);
 
   /// Consumes the span of bytes and uses it to construct and return a block.
   static Block *as_block(size_t prev_outer_size, ByteSpan bytes);
 
-  Block(size_t prev_outer_size, size_t outer_size);
-
   /// Returns a `BlockStatus` that is either VALID or indicates the reason why
   /// the block is invalid.
   ///
@@ -442,7 +442,9 @@ Block<OffsetType, kAlign> *Block<OffsetType, kAlign>::prev() const {
 // Private template method implementations.
 
 template <typename OffsetType, size_t kAlign>
-Block<OffsetType, kAlign>::Block(size_t prev_outer_size, size_t outer_size) {
+constexpr Block<OffsetType, kAlign>::Block(size_t prev_outer_size,
+                                           size_t outer_size)
+    : info_{} {
   prev_ = prev_outer_size / ALIGNMENT;
   next_ = outer_size / ALIGNMENT;
   info_.used = 0;

libc/src/stdlib/freelist.h

@@ -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`.
   union {
     FreeListNode *node;
     cpp::byte *bytes;
   } aliased;
 
   aliased.bytes = chunk.data();
-
-  size_t chunk_ptr = find_chunk_ptr_for_size(chunk.size(), false);
-
-  // Add it to the correct list.
-  aliased.node->size = chunk.size();
-  aliased.node->next = chunks_[chunk_ptr];
-  chunks_[chunk_ptr] = aliased.node;
+  set_freelist_node(aliased.node, chunk);
 
   return true;
 }
@@ -180,8 +190,9 @@ bool FreeList<NUM_BUCKETS>::remove_chunk(span<cpp::byte> chunk) {
 }
 
 template <size_t NUM_BUCKETS>
-size_t FreeList<NUM_BUCKETS>::find_chunk_ptr_for_size(size_t size,
-                                                      bool non_null) const {
+constexpr size_t
+FreeList<NUM_BUCKETS>::find_chunk_ptr_for_size(size_t size,
+                                               bool non_null) const {
   size_t chunk_ptr = 0;
   for (chunk_ptr = 0u; chunk_ptr < sizes_.size(); chunk_ptr++) {
     if (sizes_[chunk_ptr] >= size &&