New submission from Xavier Morel <xavier.mo...@masklinn.net>:
I was looking at the disassembly of a fairly straightforward listcomp:
[e for e in s if e[0]]
1 0 BUILD_LIST 0
2 LOAD_FAST 0 (.0)
>> 4 FOR_ITER 16 (to 22)
6 STORE_FAST 1 (e)
8 LOAD_FAST 1 (e)
10 LOAD_CONST 0 (0)
12 BINARY_SUBSCR
14 POP_JUMP_IF_FALSE 4
16 LOAD_FAST 1 (e)
18 LIST_APPEND 2
20 JUMP_ABSOLUTE 4
>> 22 RETURN_VALUE
6, 8 bothered me because STORE_FAST is documented as
> Stores TOS into the local co_varnames[var_num].
So it seems like it leaves TOS and thus the LOAD is unnecessary, However
looking at ceval.c:
case TARGET(STORE_FAST): {
PREDICTED(STORE_FAST);
PyObject *value = POP();
SETLOCAL(oparg, value);
FAST_DISPATCH();
}
so STORE_FAST does pop the TOS and the LOAD_FAST is necessary. This is
confusing because there are instructions which literally have POP in their name
whose stack behaviour is documented explicitly.
Should all bytecode instructions have their stack behaviour explicitly
documented, or only those with an *odd* stack behaviour (e.g.
JUMP_IF_FALSE_OR_POP) and the rest maybe covered by a note saying that they
will pop their parameters and push back their result or somesuch?
--
Furthermore, maybe optimising `STORE_LOCAL a; LOAD_LOCAL a` to `DUP_TOP;
STORE_LOCAL a` would be useful? It obviously would have no effect on bytecode
size since wordcode, and `fastlocals[i]` would be in cache and the conditional
check likely predicted, but it seems like skipping them entirely would still be
more reliable? This idea is somewhat supported by expression assignments
already generating the latter:
>>> @dis.dis
... def foo():
... if a := thing():
... do(a)
...
3 0 LOAD_GLOBAL 0 (thing)
2 CALL_FUNCTION 0
4 DUP_TOP
6 STORE_FAST 0 (a)
8 POP_JUMP_IF_FALSE 18
4 10 LOAD_GLOBAL 1 (do)
12 LOAD_FAST 0 (a)
14 CALL_FUNCTION 1
16 POP_TOP
>> 18 LOAD_CONST 0 (None)
20 RETURN_VALUE
This optimisation would also trigger for e.g.
[x[i] for x in xs]
or
a = foo()
if a:
# do thing
making the latter generate bytecode identical to walrus assignments at least
for the trivial case: currently the only difference (aside from line numbers)
is that the normal assignment generates STORE_FAST;LOAD_FAST while expression
assignments generate DUP_TOP;STORE_FAST.
----------
assignee: docs@python
components: Documentation
messages: 385762
nosy: docs@python, xmorel
priority: normal
severity: normal
status: open
title: TOS-behaviour documentation is
type: enhancement
versions: Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43036>
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