Re: [PATCH v2 4/4] target/m68k: fix FPSR quotient byte for frem instruction

[Richard Henderson <richard.henderson@linaro.org>]

On 1/4/23 05:45, Mark Cave-Ayland wrote:
> The FPSR quotient byte should be set to the value of the quotient and not the
> result. Manually calculate the quotient in the frem helper in round to nearest
> even mode (note this is different from the quotient calculated internally for
> fmod), and use it to set the quotient byte accordingly.
> 
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1314
> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
> ---
>   target/m68k/fpu_helper.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/target/m68k/fpu_helper.c b/target/m68k/fpu_helper.c
> index 5fd094a33c..56f7400140 100644
> --- a/target/m68k/fpu_helper.c
> +++ b/target/m68k/fpu_helper.c
> @@ -538,17 +538,25 @@ void HELPER(fmod)(CPUM68KState *env, FPReg *res, FPReg *val0, FPReg *val1)
>   
>   void HELPER(frem)(CPUM68KState *env, FPReg *res, FPReg *val0, FPReg *val1)
>   {
> +    float_status fp_status;
> +    FPReg fp_quot;
>       uint32_t quotient;
>       int sign;
>   
> +    /* Calculate quotient directly using round to nearest mode */
> +    set_float_rounding_mode(float_round_nearest_even, &fp_status);
> +    set_floatx80_rounding_precision(
> +        get_floatx80_rounding_precision(&env->fp_status), &fp_status);
> +    fp_quot.d = floatx80_div(val1->d, val0->d, &fp_status);
> +
>       res->d = floatx80_rem(val1->d, val0->d, &env->fp_status);
>   
> -    if (floatx80_is_any_nan(res->d)) {
> +    if (floatx80_is_any_nan(fp_quot.d)) {

I think you should leave this line unchanged, and move the div afterward.
I also think you should completely initialize the local fp_status = { }.

With that,
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~