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