On 1/27/2016 12:37 PM, Yury Selivanov wrote:
MicroPython also has dictionary lookup caching, but it's a bit
different to your proposal. We do something much simpler: each opcode
that has a cache ability (eg LOAD_GLOBAL, STORE_GLOBAL, LOAD_ATTR,
etc) includes a single byte in the opcode which is an offset-guess
into the dictionary to find the desired element. Eg for LOAD_GLOBAL
we have (pseudo code):
CASE(LOAD_GLOBAL):
key = DECODE_KEY;
offset_guess = DECODE_BYTE;
if (global_dict[offset_guess].key == key) {
// found the element straight away
} else {
// not found, do a full lookup and save the offset
offset_guess = dict_lookup(global_dict, key);
UPDATE_BYTECODE(offset_guess);
}
PUSH(global_dict[offset_guess].elem);
We have found that such caching gives a massive performance increase,
on the order of 20%. The issue (for us) is that it increases bytecode
size by a considerable amount, requires writeable bytecode, and can be
non-deterministic in terms of lookup time. Those things are important
in the embedded world, but not so much on the desktop.
That's a neat idea! You're right, it does require bytecode to become
writeable.
Would it?
Remember "fixup lists"? Maybe they still exist for loading function
addresses from one DLL into the code of another at load time?
So the equivalent for bytecode requires a static table of offset_guess,
and the offsets into that table are allocated by the byte-code loader at
byte-code load time, and the byte-code is "fixed up" at load time to use
the correct offsets into the offset_guess table. It takes one more
indirection to find the guess, but if the result is a 20% improvement,
maybe you'd still get 19%...
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