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 <yasm...@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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