Re: [Qemu-devel] [Qemu-ppc] [PATCH] 4/4] target-ppc: Implement bcdctz. instruction

[joserz]

On Thu, Oct 27, 2016 at 12:47:34PM +1100, David Gibson wrote:
> On Wed, Oct 26, 2016 at 11:18:58AM -0200, Jose Ricardo Ziviani wrote:
> > bcdctz. converts from BCD to Zoned numeric format. Zoned format uses
> > a byte to represent a digit where the most significant nibble is 0x3
> > or 0xf, depending on the preferred signal.
> > 
> > Signed-off-by: Jose Ricardo Ziviani <address@hidden>
> > ---
> >  target-ppc/helper.h                 |  1 +
> >  target-ppc/int_helper.c             | 54 
> > +++++++++++++++++++++++++++++++++++++
> >  target-ppc/translate/vmx-impl.inc.c |  7 +++--
> >  3 files changed, 60 insertions(+), 2 deletions(-)
> > 
> > diff --git a/target-ppc/helper.h b/target-ppc/helper.h
> > index f460635..3b928b8 100644
> > --- a/target-ppc/helper.h
> > +++ b/target-ppc/helper.h
> > @@ -372,6 +372,7 @@ DEF_HELPER_4(bcdsub, i32, avr, avr, avr, i32)
> >  DEF_HELPER_3(bcdcfn, i32, avr, avr, i32)
> >  DEF_HELPER_2(bcdctn, i32, avr, avr)
> >  DEF_HELPER_3(bcdcfz, i32, avr, avr, i32)
> > +DEF_HELPER_3(bcdctz, i32, avr, avr, i32)
> >  
> >  DEF_HELPER_2(xsadddp, void, env, i32)
> >  DEF_HELPER_2(xssubdp, void, env, i32)
> > diff --git a/target-ppc/int_helper.c b/target-ppc/int_helper.c
> > index 8cbbdfc..e0a84bb 100644
> > --- a/target-ppc/int_helper.c
> > +++ b/target-ppc/int_helper.c
> > @@ -2768,6 +2768,60 @@ uint32_t helper_bcdcfz(ppc_avr_t *r, ppc_avr_t *b, 
> > uint32_t ps)
> >      *r = ret;
> >      return cr;
> >  }
> > +
> > +uint32_t helper_bcdctz(ppc_avr_t *r, ppc_avr_t *b, uint32_t ps)
> > +{
> > +    int i;
> > +    int j;
> > +    int cr = 0;
> > +    int invalid = 0;
> > +    uint8_t digit = 0;
> > +    int sgnb = bcd_get_sgn(b);
> > +    int zone_lead = (ps) ? 0xF0 : 0x30;
> > +    ppc_avr_t ret = { .u64 = { 0, 0 } };
> > +
> > +    int eq_flag = (b->u64[HI_IDX] == 0) && ((b->u64[LO_IDX] >> 4) == 0);
> > +    int ox_flag = (b->u64[HI_IDX] != 0);
> 
> This doesn't look right.  The zoned format can accomodate 16 digits.
> The low word of the packed format can accomodate 15 digits + sign.  So
> it should be possible to convert a packed for with a value in the low
> nibble of the upper word.

David, it seems that zoned format can accommodate 15 digits + sign at most. 
Suppose I have this 16 bytes zoned number: f9f9f9f9f9f9f9f9f9f9f9f9f9f9f9xx. 
Converting it to packed bcd will give 999999999999999x. Do you agree? can I 
keep that ox_flag logic?

> 
> > +    for (i = 31, j = 16; i > 1; i -= 2, j--) {
> 
> Again not seeing any point for the reverse order.
> 
> > +        digit = get_nibble(b, j);
> > +        ret.u8[BCD_DIG_BYTE(i)] = zone_lead + digit;
> > +
> > +        if (unlikely(digit > 9)) {
> > +            invalid = 1;
> > +        }
> > +    }
> > +
> > +    if (unlikely(!sgnb)) {
> > +        sgnb = (!get_nibble(b, 0)) ? 1 : -1;
> > +    }
> 
> Doesn't bcd_get_sgn() returning 0 indicate a badly encoded sign
> point.  In which case you should set invalid, rather than making up a
> sign according to this apparently arbitrary scheme.
> 
> > +
> > +    if (ps) {
> > +        bcd_put_digit(&ret, (sgnb == 1) ? 0xC : 0xD, 1);
> > +    } else {
> > +        bcd_put_digit(&ret, (sgnb == 1) ? 0x3 : 0x7, 1);
> > +    }
> > +    bcd_put_digit(&ret, get_nibble(b, 1), 0);
> > +
> > +    if (!eq_flag) {
> > +        cr = (sgnb == 1) ? 1 << CRF_GT : 1 << CRF_LT;
> > +    } else {
> > +        cr = 1 << CRF_EQ;
> > +    }
> > +
> > +    if (ox_flag) {
> > +        cr |= 1 << CRF_SO;
> > +    }
> > +
> > +    if (unlikely(invalid)) {
> > +        cr = 1 << CRF_SO;
> > +    }
> > +
> > +    *r = ret;
> > +
> > +    return cr;
> > +}
> > +
> >  void helper_vsbox(ppc_avr_t *r, ppc_avr_t *a)
> >  {
> >      int i;
> > diff --git a/target-ppc/translate/vmx-impl.inc.c 
> > b/target-ppc/translate/vmx-impl.inc.c
> > index 9192f8f..62c44f0 100644
> > --- a/target-ppc/translate/vmx-impl.inc.c
> > +++ b/target-ppc/translate/vmx-impl.inc.c
> > @@ -918,6 +918,7 @@ GEN_BCD(bcdsub)
> >  GEN_BCD2(bcdcfn)
> >  GEN_BCD3(bcdctn)
> >  GEN_BCD2(bcdcfz)
> > +GEN_BCD2(bcdctz)
> >  
> >  static void gen_xpnd04_1(DisasContext *ctx)
> >  {
> > @@ -927,7 +928,8 @@ static void gen_xpnd04_1(DisasContext *ctx)
> >      case 2:
> >          break; /* bcdcfsq. */
> >      case 4:
> > -        break; /* bcdctz. */
> > +        gen_bcdctz(ctx);
> > +        break;
> >      case 5:
> >          gen_bcdctn(ctx);
> >          break;
> > @@ -952,7 +954,8 @@ static void gen_xpnd04_2(DisasContext *ctx)
> >      case 2:
> >          break; /* bcdcfsq. */
> >      case 4:
> > -        break; /* bcdctz. */
> > +        gen_bcdctz(ctx);
> > +        break;
> >      case 6:
> >          gen_bcdcfz(ctx);
> >          break;
> 
> -- 
> David Gibson                  | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au        | minimalist, thank you.  NOT _the_ 
> _other_
>                               | _way_ _around_!
> http://www.ozlabs.org/~dgibson