Re: Questions about "Handle constant exponents." in gcc/builtins.c

[Vincent Lefevre <vincent+gcc@vinc17.org>]

On 2010-03-18 15:32:04 +0100, Michael Matz wrote:
> > So, pow(-0.0, 0.5) should return +0. But sqrt(-0.0) should return -0
> > according to the IEEE 754 standard (and F.9.4.5 from ISO C99).
> 
> Yes, and I don't know why they specified it like that.  After all 
> (-0)*(-0)==+0 (not ==-0), so the above definition is internally 
> insonsistent.  Defining sqrt(-0) as +0 would be equally inconsistent, but 
> at least agree with the pow(-0, 0.5) result.

sqrt(-0) was defined first by the IEEE 754 standard in 1985.
AFAIK, -0 was chosen to allow a hack for some convention in
interval arithmetic (there may be other reasons). pow(-0, y)
was defined by the C committee.

> But unfortunately you are right, this expansion can only be done for
> -fno-signed-zeros. (FWIW the general expandsion of pow(x,N/2) where
> N!=1 is already guarded by unsafe_math, but for N==1 we do it
> unconditionally).

If GCC is able to track range of values, the transformation could
be allowed when it can be proved that the value -0 is not possible
(e.g. when x > 0).

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Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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