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

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

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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.

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()?

> +            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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