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Wed Jan 27 15:46:15 EST 2016 · https://mail.python.org/pipermail/python-dev/2016-January/142957.html

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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[Python-Dev] Speeding up CPython 5-10%