[LLVMdev] Signed NaNs in APFloat arithmetic

[Owen Anderson]

As another data point, every GPU I’m aware of directly supports fabs and fneg operations.

I would support a proposal to move fabs and fneg either to intrinsics or instructions, and remove the current practice of using fsub.

—Owen

On 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.
> - 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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