New submission from Pablo Galindo Salgado <[email protected]>:
The peephole optimizer currently does not optimize the result of its own
optimization when its possible. For example, consider the following code:
if a:
if b:
if (c or d):
foo()
else:
bar()
else:
baz()
The bytecode for this is:
2 0 LOAD_GLOBAL 0 (a)
2 POP_JUMP_IF_FALSE 32
4 4 LOAD_GLOBAL 1 (b)
6 POP_JUMP_IF_FALSE 24
5 8 LOAD_GLOBAL 2 (c)
10 POP_JUMP_IF_TRUE 16
6 12 LOAD_GLOBAL 3 (d)
14 POP_JUMP_IF_FALSE 30
7 >> 16 LOAD_GLOBAL 4 (foo)
18 CALL_FUNCTION 0
20 POP_TOP
22 JUMP_ABSOLUTE 38
9 >> 24 LOAD_GLOBAL 5 (bar)
26 CALL_FUNCTION 0
28 POP_TOP
>> 30 JUMP_FORWARD 6 (to 38)
11 >> 32 LOAD_GLOBAL 6 (baz)
34 CALL_FUNCTION 0
36 POP_TOP
>> 38 LOAD_CONST 0 (None)
40 RETURN_VALUE
Notice that the 14 POP_JUMP_IF_FALSE 30 jumps to another jump (30 JUMP_FORWARD
6 (to 38)). If we repeat the optimizations until the resulting bytecode does
not change more we get:
2 0 LOAD_GLOBAL 0 (a)
2 POP_JUMP_IF_FALSE 32
4 4 LOAD_GLOBAL 1 (b)
6 POP_JUMP_IF_FALSE 24
5 8 LOAD_GLOBAL 2 (c)
10 POP_JUMP_IF_TRUE 16
6 12 LOAD_GLOBAL 3 (d)
5 14 POP_JUMP_IF_FALSE 38
7 >> 16 LOAD_GLOBAL 4 (foo)
18 CALL_FUNCTION 0
20 POP_TOP
22 JUMP_ABSOLUTE 38
9 >> 24 LOAD_GLOBAL 5 (bar)
26 CALL_FUNCTION 0
28 POP_TOP
30 JUMP_FORWARD 6 (to 38)
11 >> 32 LOAD_GLOBAL 6 (baz)
34 CALL_FUNCTION 0
36 POP_TOP
>> 38 LOAD_CONST
Notice that in this example the original instruction now is (14
POP_JUMP_IF_FALSE 38).
----------
components: Interpreter Core
messages: 345543
nosy: pablogsal
priority: normal
severity: normal
status: open
title: Make multiple passes of the peephole optimizer until bytecode cannot be
optimized further
versions: Python 3.9
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue37271>
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