New submission from Raymond Hettinger <raymond.hettin...@gmail.com>:
In the show code below, the STORE_FAST x is FOLLOWED by LOAD_FAST x. This is
a common word code pairing. Perhaps a new combined opcode would help:
case TARGET(LOAD_AND_STORE_FAST): {
PyObject *value = GETLOCAL(oparg);
if (value == NULL) {
format_exc_check_arg(tstate, PyExc_UnboundLocalError,
UNBOUNDLOCAL_ERROR_MSG,
PyTuple_GetItem(co->co_varnames, oparg));
goto error;
}
Py_INCREF(value);
SETLOCAL(oparg, value);
FAST_DISPATCH();
}
The combined opcode saves one one trip around the eval-loop and it saves
unnecessary stack manipulations (a PUSH() immediately followed by a POP()).
The code generation would likely need to be a compiler or AST step because it
crosses basic block boundaries. Care would need to be taken to not adversely
affect tracing the code in a debugger.
Note in the following code, the "x" is never used after the STORE/LOAD pair.
In theory, the two opcodes could be dropped entirely; however, would affect a
call to "locals()".
-------- Code disassembly ------
>>> def f(s):
for x in g:
yield x**2
>>> dis(f)
2 0 LOAD_GLOBAL 0 (g)
2 GET_ITER
>> 4 FOR_ITER 14 (to 20)
6 STORE_FAST 1 (x)
3 8 LOAD_FAST 1 (x)
10 LOAD_CONST 1 (2)
12 BINARY_POWER
14 YIELD_VALUE
16 POP_TOP
18 JUMP_ABSOLUTE 4
>> 20 LOAD_CONST 0 (None)
22 RETURN_VALUE
----------
components: Interpreter Core
messages: 354024
nosy: rhettinger
priority: normal
severity: normal
status: open
title: Possible wordcode optimization for STORE/LOAD pairs
type: performance
versions: Python 3.9
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue38381>
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