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[Python-Dev] constant folding of -0

Antoine Pitrou solipsis at pitrou.net
Fri Mar 11 00:30:47 CET 2011 
On Thu, 10 Mar 2011 02:17:34 +0000 (UTC)
Eugene Toder <eltoder at gmail.com> wrote:
> > Indeed, see http://bugs.python.org/issue11244
> 
> Yes, I've noticed that too. However, if I'm not missing something, your patches
> do not address folding of -0.
> 
> Btw, there's an alternative approach to allow "recursive" constant folding.
> Instead of keeping a stack of last constants, you can keep a pointer to the
> start of the last (LOAD_CONSTs + NOPs) run and the number of LOAD_CONSTs in
> that run (called lastlc in the current version). When you want Nth constant
> from the end, you start from that pointer and skip lastlc-N constants. You
> also make a function to get next constant from that point. This approach has
> worse time complexity for searching in a long run of LOAD_CONSTs,

Yes, the stack basically acts as a cache to avoid computing all this
again and again.

> however,
> there are upsides:
> - very long runs of constants are rare in real code

True, but they might appear in generated code.

> - it uses less memory and doesn't have arbitrary limits on the size of the run

Neither does the latest patch.

> - it's book-keeping overhead is smaller, so when you don't have long runs of
> constants (common case, I believe), it should be faster

The book-keeping overhead should be quite small really, it's a simple
autogrowing array with O(1) access and amortized append time. What's
left is the overhead of the initial malloc() (and final free()).

> - I think it's simpler to implement

Feel free to propose an alternate patch, but I'm not sure that it would
be significantly simpler (and a stack is a well-known data structure).
Also, please present some benchmarks if you do.

Regards

Antoine.
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