[Python-Dev] Possible optimization for LOAD_FAST ? (original) (raw)

Lukas Lueg lukas.lueg at googlemail.com
Tue Dec 28 18:08:58 CET 2010

Consider the following code:

def foobar(x): for i in range(5): x[i] = i

The bytecode in python 2.7 is the following:

2 0 SETUP_LOOP 30 (to 33) 3 LOAD_GLOBAL 0 (range) 6 LOAD_CONST 1 (5) 9 CALL_FUNCTION 1 12 GET_ITER >> 13 FOR_ITER 16 (to 32) 16 STORE_FAST 1 (i)

3 19 LOAD_FAST 1 (i) 22 LOAD_FAST 0 (x) 25 LOAD_FAST 1 (i) 28 STORE_SUBSCR 29 JUMP_ABSOLUTE 13 >> 32 POP_BLOCK >> 33 LOAD_CONST 0 (None) 36 RETURN_VALUE

Can't we optimize the LOAD_FAST in lines 19 and 25 to a single load and put the reference twice on the stack? There is no way that the reference of i might change in between the two lines. Also, the load_fast in lne 22 to reference x could be taken out of the loop as x will always point to the same object....

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