Ted and I had a fun conversation just now about bindings in generators.
Jan, I'd like to know what you think about this.
Currently, in generators, all bindings—arguments, locals, etc.—are
marked as "aliased". It's slow. Two possible fixes:
1. LOCALS ON GENERATOR
Add bytecode instructions for getting and setting locals in a
generator. Just as non-aliased locals in normal functions are
optimized into stack slots, in generators optimize them into slots
on the GeneratorObject, where these special instructions can reach
them.
The expression stack remains as-is: on yield, copy it from the stack
to the generator; on resume, copy it back.
2. FRAME ON GENERATOR
When calling a generator, create a normal interpreter/baseline stack
frame, but on the heap instead of the stack, a part of the
GeneratorObject. Copy the arguments there. That is actually going to
be our frame. It is not physically located on the stack.
In Interpreter.cpp, make `REGS.fp` point to the current stack frame
even when it's allocated in a generator, and likewise `REGS.sp`.
Same for the corresponding registers in Baseline/ blinterp. All uses
of `BaselineFrameReg` we looked at would still work if it pointed to
a heap-allocated frame.
The CPU's `rsp` would still point to the C stack as usual.
Now just remove the special case in the frontend that marks all
bindings in generators as aliased. Emit normal GetLocal, SetLocal,
GetArg, etc. instructions. "Everything" "just" works.
Approach 1 is more flexible and calls might be faster. In approach 2,
`yield` and resume are faster, and we avoid having new opcodes. However,
the complexity settles at boundaries between execution modes, which
would now have to cope with frames possibly being noncontiguous and
stored on the heap.
What do you think?
-j
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