Hi Pablo,

On 29/10/2020 11:08 am, Pablo Galindo Salgado wrote:
>  > The new semantics may well result in some slowdowns. That's stated in
> the PEP.I don't think I can reliably isolate the effects of the (very
> slight)
> change in the behavior of f_lineno.
>
> Ok, then let's make at least we measure the general slowdowns.

Except that we can't measure the performance of a specification.
We can only measure the performance of entire implementations.

I can make an implementation that conforms to PEP 626 that is slower
than master, or I can make one that's faster :)
It doesn't change the value of the PEP itself.

Let me give you a toy example.

def f():
     while 1:
         body()

3.9 compiles this to:

(The trailing, implicit return has been stripped for clarity)

   3     >>    0 LOAD_GLOBAL              0 (body)
               2 CALL_FUNCTION            0
               4 POP_TOP
               6 JUMP_ABSOLUTE            0

A naive implementation that conforms to PEP 626 would this compile to:

   2     >>    0 NOP
   3           2 LOAD_GLOBAL              0 (body)
               4 CALL_FUNCTION            0
               6 POP_TOP
               8 JUMP_ABSOLUTE            0

But a better implementation could produce this:

   2           0 NOP
   3     >>    2 LOAD_GLOBAL              0 (body)
               4 CALL_FUNCTION            0
               6 POP_TOP
   2           8 JUMP_ABSOLUTE            2

Which has the same bytecodes as 3.9 in the loop, and has the correct
line numbers.

Cheers,
Mark.