[Python-ideas] Disable all peephole optimizations

[Steven D'Aprano]

On Wed, May 21, 2014 at 07:05:49AM -0400, Ned Batchelder wrote:
> ** The problem
> 
> A long-standing problem with CPython is that the peephole optimizer 
> cannot be completely disabled.  Normally, peephole optimization is a 
> good thing, it improves execution speed.  But in some situations, like 
> coverage testing, it's more important to be able to reason about the 
> code's execution.  I propose that we add a way to completely disable the 
> optimizer.

I'm not sure whether this is an argument for or against your proposal, 
but the continue statement shown below is *not* dead code and should not 
be optimized out. The assert fails if you remove the continue statement.

I don't have 3.4 on this machine to test with, but using 3.3, I can see 
no evidence that `continue` is optimized away. Later in your post, you 
say:

> It's true: the
> byte code for that statement [the continue] is not executed, because 
> the peephole optimizer has removed the jump to the jump.

But that cannot be true, because if it were, the assertion would 
fail. Here's your code again:


> To demonstrate the problem, here is continue.py:
> 
>    a = b = c = 0
>    for n in range(100):
>         if n % 2:
>             if n % 4:
>                 a += 1
>             continue
>         else:
>             b += 1
>         c += 1
>    assert a == 50 and b == 50 and c == 50

If the continue were not executed, c would equal 100 and the assertion 
would fail. Have I misunderstood something?

(By the way, as given, your indents are inconsistent: some are 4 spaces 
and some are 5.)


-- 
Steven