Re: [PATCH] Optimize abs(x) < 0.0

[Toon Moene <toon at moene dot indiv dot nluug dot nl>]

Roger Sayle wrote:

> > Now computer hardware is forced to project that three valued logic onto a
> > two valued logical "true, false" dichotomy.  The solution chosen is to have
> > both comparisons above return "false".  That means that to check for a NaN
> > and use a > or <= comparison, you have to use *both*, else the NaN will
> > just escape your check.
> 
> When applying a code transformation it is sufficient to guarantee that
> the behaviour of the transformed code is the same as the original.
> In this case, I don't have to explicitly "check for a NaN" with two
> comparisons iff the optimized code produces the same results in the
> presence of NaNs and finite values.

Yep, rethinking this, it is the original code writer who should make sure
that abs(a) < 0.0 is the correct check, i.e.:

> [You might also appreciate that "x == x" is a better explicit check
> for finite vs. NaN that would require only a single comparison]

He/she should have done the above check.

The code in the patch is OK because it doesn't introduce a hole where there
wasn't one before :-)

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
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