[Python-Dev] pyc files, constant folding and borderline portability issues

[Adam Olsen]

On Mon, Apr 6, 2009 at 2:22 PM, Mark Dickinson <dickinsm at gmail.com> wrote:
> Well, I'd say that the obvious solution here is to compute
> the constant 2.0**53 just once, somewhere outside the
> inner loop.  In any case, that value would probably be better
> written as 2.0**DBL_MANT_DIG (or something similar).
>
> As Antoine reported, the constant-folding caused quite
> a confusing bug report (issue #5593):  the problem (when
> we eventually tracked it down) was that the folded
> constant was in a .pyc file, and so wasn't updated when
> the compiler flags changed.

Another way of looking at this is that we have a ./configure option
which affects .pyc output.  Therefor, we should add a flag to the
magic number, causing it to be regenerated as needed.

Whether that's better or worse than removing constant folding I
haven't decided.  I have such low expectations of floating point that
I'm not surprised by bugs like this.  I'm more surprised that people
expect consistent, deterministic results...


-- 
Adam Olsen, aka Rhamphoryncus