> On Oct 25, 2015, at 12:33 PM, Raymond Hettinger <raymond.hettin...@gmail.com>
> wrote:
>
>> On Oct 22, 2015, at 10:02 AM, Brett Cannon <br...@python.org> wrote:
>>
>> So my question is, the byte code generator removes the unused functions,
>> variables etc…, is it right?
>>
>> Technically the peepholer removes the dead branch, but since the peepholer
>> is run on all bytecode you can't avoid it.
>
> IIRC, the code was never generated in the first place (before the peephole
> pass). This used to be true before the AST branch was added and I think it
> may still be true.
I just verified this. So Brett's post was incorrect and misleading.
Raymond
----------- Verify by turning-off the optimizations ----------
cpython $ hg diff Python/peephole.c
diff --git a/Python/peephole.c b/Python/peephole.c
--- a/Python/peephole.c
+++ b/Python/peephole.c
@@ -383,7 +383,7 @@
/* Avoid situations where jump retargeting could overflow */
assert(PyBytes_Check(code));
codelen = PyBytes_GET_SIZE(code);
- if (codelen > 32700)
+ if (codelen > 0)
goto exitUnchanged;
-------- Then run a simple disassembly -----------------------
from dis import dis
def f(x):
if 0:
print('First')
print('Second')
dis(f)
-------- The output is ---------------------------------------------------
$ py tmp.py
6 0 LOAD_GLOBAL 0 (print)
3 LOAD_CONST 1 ('Second')
6 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
9 POP_TOP
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
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