[pypy-dev] Contributing to pypy [especially numpy]

[David Cournapeau]

On Sun, Oct 16, 2011 at 10:21 PM, Armin Rigo <arigo at tunes.org> wrote:
> Hi David,
>
> On Sun, Oct 16, 2011 at 19:13, David Cournapeau <cournape at gmail.com> wrote:
>> (...) and there is also the issue
>> of correctness in floating point code generation. Given that
>> decade-old compilers get it wrong, I would expect pypy jit to have
>> quite a few funky corner cases as well.
>
> No, we should not have corner cases, because we don't go there at all.
>  We know very well that rewriting operations on floats can slightly
> change their results, so we don't do it.  In other words the JIT
> produces a sequence of residual operations that has bit-wise the same
> effect as the original sequence of Python operations.
>
> (More precisely, it seems that we only replace FLOAT_MUL(x, 1.0) by
> "x" and FLOAT_MUL(x, -1.0) by "-x", as well as simplify repeated
> FLOAT_NEG's and assume that FLOAT_MUL's are commutative.  As far as I
> can tell these trivial optimizations are all bit-wise correct, at
> least on modern FPUs.)

Interesting to know. But then, wouldn't this limit the speed gains to
be expected from the JIT ? And I am not sure I understand how you can
"not go there" if you want to vectorize code to use SIMD instruction
sets ?

David