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

Guido van Rossum guido at python.org
Mon Apr 6 23:28:41 CEST 2009

On Mon, Apr 6, 2009 at 1:22 PM, Mark Dickinson <dickinsm at gmail.com> wrote:

On Mon, Apr 6, 2009 at 9:05 PM, Raymond Hettinger <python at rcn.com> wrote:

The code for the lsum() recipe is more readable with a line like:

 exp = long(mant * 2.0 ** 53) than with  exp = long(mant * 9007199254740992.0) It would be ashamed if code written like the former suddenly started doing the exponentation in the inner-loop or if the code got rewritten by hand as shown.

Do you have any evidence that people write lots of inner loops with constant expressions? In real-world code these just don't exist that much. The case of constant folding in Python is much weaker than in C because Python doesn't have real compile-time constants, so named "constants" are variables to the compiler.

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).

So true.

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.

Right. Over the years the peephole optimizer and constant folding have been a constant (though small) source of bugs. I'm not sure that there is much real-world value in it, and it is certainly not right to choose speed over correctness.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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