[PATCH] Returns NaN for sqrt with negative fp argument
resistor at mac.com
Wed Jun 11 09:30:40 PDT 2014
On Jun 11, 2014, at 7:49 AM, Tim Northover <t.p.northover at gmail.com> wrote:
>> Is there any performance advantage of using the explicitly incorrect 0.0?
> There's probably no significant difference between 0.0 and NaN, but
> there could well be between either and undef if it allows future
> optimisations. If we make any change here, it should be that one.
0.0 is generally a cheaper immediate to materialize than NaN. Most RISC targets have direct support for materializing a zero, but not for materializing a NaN.
That said, I agree with Tim that undef is the proper solution. I suspect the 0.0 result was trying to work around an imagined constraint that sqrt(x) must return a positive value. However, it’s not clear to me that that is something we really care about.
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