Re: [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions

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

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On Thu, Jul 26, 2018 at 04:44:43PM -0300, Yasmin Beatriz wrote:
> On Thu, Jul 26, 2018 at 11:40:20AM +1000, David Gibson wrote:
> > On Tue, Jul 24, 2018 at 12:13:04PM +0000, Yasmin Beatriz wrote:
> > > After solving a corner case in bcdsub, this patch simplifies the logic
> > > of both bcdadd/sub instructions by removing some unnecessary local flags.
> > > 
> > > Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
> > > ---
> > >  target/ppc/int_helper.c | 33 +++++++++------------------------
> > >  1 file changed, 9 insertions(+), 24 deletions(-)
> > > 
> > > diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
> > > index fa18e6e..b8ac4bb 100644
> > > --- a/target/ppc/int_helper.c
> > > +++ b/target/ppc/int_helper.c
> > > @@ -2671,16 +2671,14 @@ static int bcd_cmp_mag(ppc_avr_t *a, ppc_avr_t *b)
> > >      return 0;
> > >  }
> > >  
> > > -static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > +static void bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > >                         int *overflow)
> > >  {
> > >      int carry = 0;
> > >      int i;
> > > -    int is_zero = 1;
> > >      for (i = 1; i <= 31; i++) {
> > >          uint8_t digit = bcd_get_digit(a, i, invalid) +
> > >                          bcd_get_digit(b, i, invalid) + carry;
> > > -        is_zero &= (digit == 0);
> > >          if (digit > 9) {
> > >              carry = 1;
> > >              digit -= 10;
> > > @@ -2689,26 +2687,20 @@ static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > >          }
> > >  
> > >          bcd_put_digit(t, digit, i);
> > > -
> > > -        if (unlikely(*invalid)) {
> > > -            return -1;
> > > -        }
> > >      }
> > >  
> > >      *overflow = carry;
> > > -    return is_zero;
> > >  }
> > >  
> > > -static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > +static void bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > >                         int *overflow)
> > >  {
> > >      int carry = 0;
> > >      int i;
> > > -    int is_zero = 1;
> > > +
> > >      for (i = 1; i <= 31; i++) {
> > >          uint8_t digit = bcd_get_digit(a, i, invalid) -
> > >                          bcd_get_digit(b, i, invalid) + carry;
> > > -        is_zero &= (digit == 0);
> > >          if (digit & 0x80) {
> > >              carry = -1;
> > >              digit += 10;
> > > @@ -2717,14 +2709,9 @@ static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > >          }
> > >  
> > >          bcd_put_digit(t, digit, i);
> > > -
> > > -        if (unlikely(*invalid)) {
> > > -            return -1;
> > > -        }
> > >      }
> > >  
> > >      *overflow = carry;
> > > -    return is_zero;
> > >  }
> > >  
> > >  uint32_t helper_bcdadd(ppc_avr_t *r,  ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > > @@ -2734,25 +2721,25 @@ uint32_t helper_bcdadd(ppc_avr_t *r,  ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > >      int sgnb = bcd_get_sgn(b);
> > >      int invalid = (sgna == 0) || (sgnb == 0);
> > >      int overflow = 0;
> > > -    int zero = 0;
> > >      uint32_t cr = 0;
> > >      ppc_avr_t result = { .u64 = { 0, 0 } };
> > >  
> > >      if (!invalid) {
> > >          if (sgna == sgnb) {
> > >              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > > -            zero = bcd_add_mag(&result, a, b, &invalid, &overflow);
> > > -            cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > > +            bcd_add_mag(&result, a, b, &invalid, &overflow);
> > > +            cr = bcd_cmp_zero(&result);
> > >          } else if (bcd_cmp_mag(a, b) > 0) {
> > >              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > > -            zero = bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > > +            bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > >              cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > >          } else if (bcd_cmp_mag(a, b) == 0) {
> > >              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(0, ps);
> > > -            zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > > +            bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > 
> > I don't think you actually need the sub here, since you know the
> > result is going to be zero.
> 
> Right. Will fix this in v2.
> 
> > Although.. in all of the different-sign cases aren't we effectively
> > doing the subtraction twice - once in bcd_cmp_mag() then again in
> > bcd_sub_mag()?
> 
> Actually no, bcd_cmp_mag() compares the magnitude between 'a' and 'b'
> starting from the most significant digit and returns as soon as it finds
> a difference between the two of them. It helps to decide whether we're
> going to perform a - b or b - a.

Ah, good point.

> Anyway, I think the following code can make it easier to understand:
> 
> int magnitude;
> if (sgna == sgnb) {
>     // same thing
> } else {
>     magnitude = bdc_cmp_mag(a, b);
>     if (magnitude > 0) {
>         // do a - b
>     } else if (magnitude < 0) {
>         // do b - a
>     } else {
>         // 0
>     }
> }

Yes, I think that's a good idea.

> 
> > > +            cr = CRF_EQ;
> > >          } else {
> > >              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
> > > -            zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > > +            bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > >              cr = (sgnb > 0) ? CRF_GT : CRF_LT;
> > >          }
> > >      }
> > > @@ -2762,8 +2749,6 @@ uint32_t helper_bcdadd(ppc_avr_t *r,  ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > >          cr = CRF_SO;
> > >      } else if (overflow) {
> > >          cr |= CRF_SO;
> > > -    } else if (zero) {
> > > -        cr = CRF_EQ;
> > >      }
> > >  
> > >      *r = 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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