Hi,
On Mon, 15 Aug 2011, Michael Matz wrote:
> > > .LFB0:
> > > .cfi_startproc
> > > movsd .LC0(%rip), %xmm2
> > > movapd %xmm0, %xmm1
> > > andpd %xmm2, %xmm1
> > > andnpd %xmm0, %xmm2
> > > addsd .LC1(%rip), %xmm1
> > > roundsd $1, %xmm1, %xmm1
> > > orpd %xmm2, %xmm1
> > > movapd %xmm1, %xmm0
> > > ret
> >
> > Hm, why do we need the sign-copy? If I read the docs correctly
> > we can simply use roundsd directly, no?
>
> round-half-away-from-zero breaks your neck. round[ps][sd] only supports
> the usual four IEEE rounding modes.
But, you should be able to apply the sign to the 0.5, which wouldn't
require building the absolute value of input:
round(x) = trunc(x + (copysign (0.5, x)))
which should roughly be expanded to:
movsd signbits(%rip), %xmm1
andpd %xmm0, %xmm1
movsd nextof0.5(%rip), %xmm2
orpd %xmm1, %xmm2
addpd %xmm2, %xmm0
roundsd $1, %xmm0, %xmm0
ret
Which has one logical operation less (and one move because I chose a more
optimal register assignment).
Ciao,
Michael.
