Allow querying the allocator for the buffer size (#1404)

mlx/allocator.cpp

@@ -23,11 +23,22 @@ void free(Buffer buffer) {
 }
 
 Buffer CommonAllocator::malloc(size_t size, bool) {
-  return Buffer{std::malloc(size)};
+  void* ptr = std::malloc(size + sizeof(size_t));
+  if (ptr != nullptr) {
+    *static_cast<size_t*>(ptr) = size;
+  }
+  return Buffer{ptr};
 }
 
 void CommonAllocator::free(Buffer buffer) {
-  std::free(buffer.raw_ptr());
+  std::free(buffer.ptr());
+}
+
+size_t CommonAllocator::size(Buffer buffer) const {
+  if (buffer.ptr() == nullptr) {
+    return 0;
+  }
+  return *static_cast<size_t*>(buffer.ptr());
 }
 
 Buffer malloc_or_wait(size_t size) {

mlx/allocator.h

@@ -41,6 +41,7 @@ class Allocator {
  public:
   virtual Buffer malloc(size_t size, bool allow_swap = false) = 0;
   virtual void free(Buffer buffer) = 0;
+  virtual size_t size(Buffer buffer) const = 0;
 
   Allocator() = default;
   Allocator(const Allocator& other) = delete;
@@ -57,6 +58,7 @@ class CommonAllocator : public Allocator {
  public:
   virtual Buffer malloc(size_t size, bool allow_swap = false) override;
   virtual void free(Buffer buffer) override;
+  virtual size_t size(Buffer buffer) const override;
 
  private:
   CommonAllocator() = default;

mlx/array.h

@@ -324,6 +324,10 @@ class array {
     return array_desc_->data->buffer;
   }
 
+  size_t buffer_size() const {
+    return allocator::allocator().size(buffer());
+  }
+
   // Return a copy of the shared pointer
   // to the array::Data struct
   std::shared_ptr<Data> data_shared_ptr() const {

mlx/backend/common/binary.h

@@ -43,13 +43,15 @@ void set_binary_op_output_data(
     array& out,
     BinaryOpType bopt,
     bool donate_with_move = false) {
+  bool b_donatable = is_donatable(b, out);
+  bool a_donatable = is_donatable(a, out);
   switch (bopt) {
     case BinaryOpType::ScalarScalar:
       out.set_data(
           allocator::malloc_or_wait(out.itemsize()), 1, a.strides(), a.flags());
       break;
     case BinaryOpType::ScalarVector:
-      if (b.is_donatable() && b.itemsize() == out.itemsize()) {
+      if (b_donatable) {
         if (donate_with_move) {
           out.move_shared_buffer(b);
         } else {
@@ -64,7 +66,7 @@ void set_binary_op_output_data(
       }
       break;
     case BinaryOpType::VectorScalar:
-      if (a.is_donatable() && a.itemsize() == out.itemsize()) {
+      if (a_donatable) {
         if (donate_with_move) {
           out.move_shared_buffer(a);
         } else {
@@ -79,13 +81,13 @@ void set_binary_op_output_data(
       }
       break;
     case BinaryOpType::VectorVector:
-      if (a.is_donatable() && a.itemsize() == out.itemsize()) {
+      if (a_donatable) {
         if (donate_with_move) {
           out.move_shared_buffer(a);
         } else {
           out.copy_shared_buffer(a);
         }
-      } else if (b.is_donatable() && b.itemsize() == out.itemsize()) {
+      } else if (b_donatable) {
         if (donate_with_move) {
           out.move_shared_buffer(b);
         } else {
@@ -100,16 +102,14 @@ void set_binary_op_output_data(
       }
       break;
     case BinaryOpType::General:
-      if (a.is_donatable() && a.flags().row_contiguous &&
-          a.itemsize() == out.itemsize() && a.size() == out.size()) {
+      if (a_donatable && a.flags().row_contiguous && a.size() == out.size()) {
         if (donate_with_move) {
           out.move_shared_buffer(a);
         } else {
           out.copy_shared_buffer(a);
         }
       } else if (
-          b.is_donatable() && b.flags().row_contiguous &&
-          b.itemsize() == out.itemsize() && b.size() == out.size()) {
+          b_donatable && b.flags().row_contiguous && b.size() == out.size()) {
         if (donate_with_move) {
           out.move_shared_buffer(b);
         } else {

mlx/backend/common/ternary.h

@@ -41,7 +41,7 @@ void set_ternary_op_output_data(
     TernaryOpType topt,
     bool donate_with_move = false) {
   auto maybe_donate = [&out, donate_with_move](const array& x) {
-    if (x.is_donatable() && x.itemsize() == out.itemsize()) {
+    if (is_donatable(x, out)) {
       if (donate_with_move) {
         out.move_shared_buffer(x);
       } else {

mlx/backend/common/unary.h

@@ -12,7 +12,7 @@ namespace mlx::core {
 namespace {
 
 void set_unary_output_data(const array& in, array& out) {
-  if (in.is_donatable() && in.itemsize() == out.itemsize()) {
+  if (is_donatable(in, out)) {
     out.copy_shared_buffer(in);
   } else {
     auto size = in.data_size();

mlx/backend/common/utils.h

@@ -155,4 +155,11 @@ inline auto check_contiguity(
       no_broadcast_data_size, is_row_contiguous, is_col_contiguous);
 }
 
+inline bool is_donatable(const array& in, const array& out) {
+  constexpr size_t donation_extra = 16384;
+
+  return in.is_donatable() && in.itemsize() == out.itemsize() &&
+      in.buffer_size() <= out.nbytes() + donation_extra;
+}
+
 } // namespace mlx::core

mlx/backend/metal/allocator.cpp

@@ -241,6 +241,10 @@ void MetalAllocator::free(Buffer buffer) {
   }
 }
 
+size_t MetalAllocator::size(Buffer buffer) const {
+  return static_cast<MTL::Buffer*>(buffer.ptr())->length();
+}
+
 MetalAllocator& allocator() {
   // By creating the |allocator_| on heap, the destructor of MetalAllocator will
   // not be called on exit and all the buffers will be leaked. This is necessary

mlx/backend/metal/allocator.h

@@ -56,6 +56,7 @@ class MetalAllocator : public allocator::Allocator {
  public:
   virtual Buffer malloc(size_t size, bool allow_swap = false) override;
   virtual void free(Buffer buffer) override;
+  virtual size_t size(Buffer buffer) const override;
   size_t get_active_memory() {
     return active_memory_;
   };

mlx/backend/no_metal/allocator.cpp

@@ -10,7 +10,7 @@ Allocator& allocator() {
 }
 
 void* Buffer::raw_ptr() {
-  return ptr_;
+  return static_cast<size_t*>(ptr_) + 1;
 }
 
 } // namespace mlx::core::allocator