[Vm-dev] VM Maker: VMMaker.oscog-eem.790.mcz

[David T. Lewis]

On Tue, Jul 01, 2014 at 09:21:00PM +0200, Nicolas Cellier wrote:
>  
> 2014-07-01 4:22 GMT+02:00 <commits at source.squeak.org>:
> 
> >
> > Item was changed:
> >   ----- Method: LargeIntegersPlugin>>cDigitSub:len:with:len:into: (in
> > category 'C core') -----
> > + cDigitSub: pByteSmall len: smallLen with: pByteLarge len: largeLen into:
> > pByteRes
> > +       | z |
> > - cDigitSub: pByteSmall
> > -               len: smallLen
> > -               with: pByteLarge
> > -               len: largeLen
> > -               into: pByteRes
> > -       | z limit |
> >         <var: #pByteSmall type: 'unsigned char * '>
> >         <var: #pByteLarge type: 'unsigned char * '>
> >         <var: #pByteRes type: 'unsigned char * '>
> >
> > +       z := 0. "Loop invariant is -1<=z<=1"
> > +       0 to: smallLen - 1 do:
> > -       z := 0.
> > -       "Loop invariant is -1<=z<=1"
> > -       limit := smallLen - 1.
> > -       0 to: limit do:
> >                 [:i |
> >                 z := z + (pByteLarge at: i) - (pByteSmall at: i).
> > +               pByteRes at: i put: z - (z // 256 * 256). "sign-tolerant
> > form of (z bitAnd: 255)"
> >
> 
> Frankly, having z declared unsigned int and just doing  pByteRes at: i put:
> (z bitAnd: 16rFF) as I suggested would be way way simpler and will ALWAYS
> work.
> Why the hell invoke the complications of signed arithmetic when the content
> pByteRes is unsigned???

Nicolas,

This sounds right to me.

Do you have a final version of #cDigitSub:len:with:len:into: that you can
recommend? The version in VMMaker-nice.342 produces failures in the
KernelTests-Numbers tests when compiled in 64-bit mode, but I think that
you have since suggested some other improvements in this email thread.

Dave