Re: [Qemu-devel] [PATCH] target/ppc: bcdsub fix sign when result is zero

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

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On Mon, Jul 16, 2018 at 07:03:12PM +0000, Yasmin Beatriz wrote:
> When the result of bcdsub is equal to zero, the result sign may be
> set to negative in some cases, and this does not follow the Power ISA
> specifications as to decimal integer arithmetic instructions.
> 
> Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>

The fix looks correct, but since this has been broken forever and
we're in hard freeze I'm going to apply for 3.1, not 3.0.

Fwiw, I think the surrounding logic can probably be simplified a
little as well - I think this is now the only case with a zero result,
so I think the 'zero' local can probably be removed.

> ---
>  target/ppc/int_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
> index 03d37da..fa18e6e 100644
> --- a/target/ppc/int_helper.c
> +++ b/target/ppc/int_helper.c
> @@ -2747,6 +2747,9 @@ uint32_t helper_bcdadd(ppc_avr_t *r,  ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
>              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
>              zero = 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);
>          } else {
>              result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
>              zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);

-- 
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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