On Mon, Apr 6, 2009 at 5:10 PM, Steven D'Aprano <steve@pearwood.info> wrote:
On Tue, 7 Apr 2009 07:27:29 am Guido van Rossum wrote:
Unfortunately the language reference is not the only thing we have to worry about. Unlike languages like C++, where compiler writers have the moral right to modify the compiler as long as they stay within the weasel-words of the standard, in Python, users' expectations carry value. Since the language is inherently not that fast, users are not all that focused on performance (if they were, they wouldn't be using Python). Unsurprising behavior OTOH is valued tremendously.
Speaking as a user, Python's slowness is *not* a feature. Anything reasonable which can increase performance is a Good Thing.
One of the better aspects of Python programming is that (in general) you can write code in the most natural way possible, with the least amount of scaffolding getting in the way. I'm with Raymond: I think it would be sad if "exp = long(mant * 2.0 ** 53)" did the exponentiation in the inner-loop. Pre-computing that value outside the loop counts as scaffolding, and gets in the way of readability and beauty.
On the other hand, I'm with Guido when he wrote "it is certainly not right to choose speed over correctness". This is especially a problem for floating point optimizations, and I urge Cesare to be conservative in any f.p. optimizations he introduces, including constant folding.
So... +1 on the general principle of constant folding, -0.5 on any such optimizations which change the semantics of a f.p. operation. The only reason it's -0.5 rather than -1 is that (presumably) anyone who cares about floating point correctness already knows to never trust the compiler.
Unfortunately, historically well-meaning attempts at adding constant-folding have more than once introduced obscure bugs that were hard to reproduce and only discovered one or two releases later. This has little to do with caring about float correctness. It's more about the difficulty of debugging Heisenbugs. For all these reasons should be super risk averse in this area. -- --Guido van Rossum (home page: http://www.python.org/~guido/)