[Python-Dev] pyc files, constant folding and borderline portability issues (original) (raw)

Adam Olsen rhamph at gmail.com
Tue Apr 7 00:05:58 CEST 2009

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**DBLMANTDIG (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

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