Re: [PATCH v2] target/ppc: Fix 64-bit decrementer

[Richard Henderson]

On 9/14/21 2:47 AM, Cédric Le Goater wrote:
> On 9/14/21 11:19 AM, Peter Maydell wrote:
> > >       /* Truncate value to decr_width and sign extend for simplicity */
> > > -    value &= ((1ULL << nr_bits) - 1);
> > > +    value = sextract64(value, 0, nr_bits);
> > >       negative = !!(value & (1ULL << (nr_bits - 1)));
> > >       if (negative) {
> > >           value |= (0xFFFFFFFFULL << nr_bits);
> >
> > I think these lines that say "if negative then force all the
> > high bits to one" are also no longer required. That is, this
> > entire section of code:
> >      value &= ((1ULL << nr_bits) - 1);
> >      negative = !!(value & (1ULL << (nr_bits - 1)));
> >      if (negative) {
> >          value |= (0xFFFFFFFFULL << nr_bits);
> >      }
> >
> > is an open-coded sign-extension, which can all be replaced with
> > the single line
> >      value = sextract64(value, 0, nr_bits);
>
> 'negative' is used for more tests afterwards but you are right. I will respin
> with more changes.

After the sign-extension, negative needs no complicated expression.

  value = sextract64(value, 0, nr_bits);
  negative = (target_long)value < 0;


r~