On Thursday 14 July 2016 10:14, Ian Kelly wrote:
> On Wed, Jul 13, 2016 at 12:48 PM, Vijay Kumar <vijayku...@bravegnu.org>
> wrote:
>> Hi Everyone,
>> I wrote an article on Python byte code hacking. The article is available
>> from http://www.bravegnu.org/blog/python-byte-code-hacks.html The article
>> uses an incremental approach for explaining Python's code objects and how to
>> modify them. Unfortunately, I am stuck with Python 2, because Python 2 does
>> not optimize out the divide operation, at compile time, and my example
>> sequence depends on that behavior.
>
> def f():
> return g(6, 2)
>
> def g(a, b):
> return a // b
>
> Now it can't optimize out the operation.
I haven't had time to read the article yet, but is Vijay talking about Python's
peekhole optimizer? It does constant folding, so that arithmetic expressions
containing *only* constants will compile to the result:
def f():
return 6//2 # compiles to "return 3"
The simplest way to defeat that is to use a variable:
def f():
x = 6
return x//2
The peephole optimizer is simple-minded enough that it won't pre-compute that.
Let's check:
py> import dis
py> def f(): return 6//2
...
py> dis.dis(f)
1 0 LOAD_CONST 3 (3)
3 RETURN_VALUE
py> def f():
... x = 6
... return x//2
...
py> dis.dis(f)
2 0 LOAD_CONST 1 (6)
3 STORE_FAST 0 (x)
3 6 LOAD_FAST 0 (x)
9 LOAD_CONST 2 (2)
12 BINARY_FLOOR_DIVIDE
13 RETURN_VALUE
--
Steve
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