[LLVMdev] Signed NaNs in APFloat arithmetic

[Venkatraman Govindaraju]

On Fri, Aug 8, 2014 at 8:22 AM, David Jones <djones at xtreme-eda.com> wrote:
> Is there any intention of making floating absolute and negate primitive IR
> instructions?
>
> I ask because only a few days ago I was also faced with the task of
> implementing negate in my compiler, and finding no suitable IR instruction,
> simply subtracted from zero. But this is wrong.
>
> I could change my code to do the bit casting and fiddling, but I wonder:
> would that be lowered appropriately on all architectures?
>
> A quick survey from my various manuals:
> - m68k has negate and absolute value instructions.
> - so does x87
> - so does PA-RISC
> - but SPARC does not.

AFAIK, SPARC has absolute and negation instructions (fabss, fnegs) and
SPARCv9 has fabsd and fnegd instructions for double precision floating
points.

> - and neither does x86 SSE (although bit fiddling might be real cheap here)
>
> On some of the architectures, moving a value from a floating-point register
> to integer and back could impact performance, so it pays to use the
> processor's native instructions for negation and absolute value if they
> exist.
>
>
>
> On Fri, Aug 8, 2014 at 10:36 AM, Stephen Canon <scanon at apple.com> wrote:
>>
>> Worth noting that -0.0 - x isn't actually correct either, since it fails
>> to flip the sign of -0 if the rounding mode is round toward negative (for
>> platforms that support IEEE-754 rounding modes), and it raises invalid if x
>> is a signaling NaN.  As Owen noted, FP negation really "ought" to be treated
>> as a bitwise operation, rather than mapped into arithmetic.
>>
>> - Steve
>>
>> On Aug 8, 2014, at 4:34 AM, Tim Northover <t.p.northover at gmail.com> wrote:
>>
>> > On 7 August 2014 20:52, Keno Fischer <kfischer at college.harvard.edu>
>> > wrote:
>> >> One more update: Since the code generated by the bitcast wasn't ideal
>> >> and we were afraid to loose vectorization, etc., we ended up going
>> >> with fsub -0.0, x, which for some reason unlike fsub 0.0, x, seems to
>> >> be have appropriately at all optimization levels.
>> >
>> > That's because "fsub 0.0, x" is incorrect for x=+0.0. Took me a while
>> > to work out why the "obvious" choice didn't work the first time I
>> > encountered it too.
>> >
>> > Cheers.
>> >
>> > Tim.
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