Re: [Lightning] About jit_roundr_d_i

[Paolo Bonzini]

On 09/10/2010 12:21 AM, Paulo César Pereira de Andrade wrote:
>    The default implementation, assuming following the
> "round" definition and default (round to nearest) rounding mode,
> is actually wrong because of ties. The problem is that on
> ties, "round" should round away from zero, but there is
> no such rounding mode, only the default, towards zero.

True.  Let's just document that round is the same as the C function 
rint.  (Though, shouldn't it round to even?)

>    BTW, is this really correct?
> extr_d_f  o1 o2<- convert float o2 to double o1
> extr_i_d  o1 o2<- convert int o2 to double o1
>
> If extr_i_d means "int to double", then extr_d_f
> should mean "double to float", and not "float to double"

Which it does:

#define jit_extr_d_f(rd, rs)    CVTSD2SSrr((rs), (rd))

SD2SS = scalar double to scalar float.

But for x87 they're all dummy.

> (there is only FISTTP, no FISTT, so, need to
> load the value, and pop it, instead of using
> FXCH, but it should still be cheap as it does,
> correctly, rounding towards zero on ties)

FISTTP is not on all processors though.  At this point, it's easier to 
use SSE(2) for 32-bit too.

Paolo