-namespace factor
-{
+namespace factor {
-struct callback {
- cell size;
- code_block *compiled;
- void *code() { return (void *)(this + 1); }
-};
+// The callback heap is used to store the machine code that alien-callbacks
+// actually jump to when C code invokes them.
+
+// The callback heap has entries that look like code_blocks from the code heap, but
+// callback heap entries are allocated contiguously, never deallocated, and all
+// fields but the owner are set to false_object. The owner points to the callback
+// bottom word, whose entry point is the callback body itself, generated by the
+// optimizing compiler. The machine code that follows a callback stub consists of a
+// single CALLBACK_STUB machine code template, which performs a jump to a "far"
+// address (on PowerPC and x86-64, its loaded into a register first).
+
+// GC updates the CALLBACK_STUB code if the code block of the callback bottom word
+// is ever moved. The callback stub itself won't move, though, and is never
+// deallocated. This means that the callback stub itself is a stable function
+// pointer that C code can hold on to until the associated Factor VM exits.
+
+// Since callback stubs are GC roots, and are never deallocated, the associated
+// callback code in the code heap is also never deallocated.
+
+// The callback heap is not saved in the image. Running GC in a new session after
+// saving the image will deallocate any code heap entries that were only reachable
+// from the callback heap in the previous session when the image was saved.
struct callback_heap {
- segment *seg;
- cell here;
- factor_vm *parent;
-
- explicit callback_heap(cell size, factor_vm *parent);
- ~callback_heap();
-
- callback *add(code_block *compiled);
- void update(callback *stub);
-
- callback *next(callback *stub)
- {
- return (callback *)((cell)stub + stub->size + sizeof(callback));
- }
-
- template<typename Iterator> void iterate(Iterator &iter)
- {
- callback *scan = (callback *)seg->start;
- callback *end = (callback *)here;
- while(scan < end)
- {
- iter(scan);
- scan = next(scan);
- }
- }
+ segment* seg;
+ free_list_allocator<code_block>* allocator;
+ factor_vm* parent;
+
+ callback_heap(cell size, factor_vm* parent);
+ ~callback_heap();
+
+ instruction_operand callback_operand(code_block* stub, cell index);
+ void store_callback_operand(code_block* stub, cell index, cell value);
+ void update(code_block* stub);
+ code_block* add(cell owner, cell return_rewind);
};
}