Re: [Qemu-devel] [PATCH v2 1/4] target-ppc: Implement bcdcfsq. instruction

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Nov 24, 2016 at 01:43:18AM +0100, Richard Henderson wrote:
> On 11/23/2016 05:21 PM, Jose Ricardo Ziviani wrote:
> > bcdcfsq.: Decimal convert from signed quadword. It is not possible
> > to convert values less than 10^31-1 or greater than -10^31-1 to be
> > represented in packed decimal format.
> > 
> > Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> > ---
> >  target-ppc/helper.h                 |  1 +
> >  target-ppc/int_helper.c             | 45 +++++++++++++++++++++++++++++++++++++
> >  target-ppc/translate/vmx-impl.inc.c |  7 ++++++
> >  3 files changed, 53 insertions(+)
> > 
> > diff --git a/target-ppc/helper.h b/target-ppc/helper.h
> > index da00f0a..87f533c 100644
> > --- a/target-ppc/helper.h
> > +++ b/target-ppc/helper.h
> > @@ -382,6 +382,7 @@ DEF_HELPER_3(bcdcfn, i32, avr, avr, i32)
> >  DEF_HELPER_3(bcdctn, i32, avr, avr, i32)
> >  DEF_HELPER_3(bcdcfz, i32, avr, avr, i32)
> >  DEF_HELPER_3(bcdctz, i32, avr, avr, i32)
> > +DEF_HELPER_3(bcdcfsq, 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 8886a72..751909c 100644
> > --- a/target-ppc/int_helper.c
> > +++ b/target-ppc/int_helper.c
> > @@ -2874,6 +2874,51 @@ uint32_t helper_bcdctz(ppc_avr_t *r, ppc_avr_t *b, uint32_t ps)
> >      return cr;
> >  }
> > 
> > +uint32_t helper_bcdcfsq(ppc_avr_t *r, ppc_avr_t *b, uint32_t ps)
> > +{
> > +    int cr;
> > +    int i;
> > +    int ox_flag = 0;
> > +    uint64_t lo_value;
> > +    uint64_t hi_value;
> > +    uint64_t max = 0x38d7ea4c68000;
> 
> This is at heart a decimal number, and should be written as such.
> Also, you need ULL for a 32-bit host compile.
> 
> > +    if (divu128(&lo_value, &hi_value, max)) {
> > +        ox_flag = 1;
> > +    } else if (lo_value >= max && hi_value == 0) {
> > +        ox_flag = 1;
> > +    }
> 
> Dispense with ox_flag and set cr = CRF_SO now.
> 
> > +    for (i = 1; hi_value; hi_value /= 10, i++) {
> > +        bcd_put_digit(&ret, hi_value % 10, i);
> > +    }
> > +
> > +    for (; lo_value; lo_value /= 10, i++) {
> > +        bcd_put_digit(&ret, lo_value % 10, i);
> > +    }
> 
> How can this possibly work?  You know there are 15 digits between high and
> low, but you continue with i++?
>
> If hi_value == 1 && lo_value == 1, this should not produce 11, but
> 10000000000000001.

Ah, yes good catch, I missed that.  I think it will be clearer to use
fixed bounds on each loop, rather than testing the value of the
remaining result.

-- 
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

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