Re: [Qemu-devel] [PATCH 2/2] target-mips: Implement IEEE 754-2008 functionality for R6 and MSA instructions

[Richard Henderson]

On 03/25/2016 05:50 AM, Aleksandar Markovic wrote:
> @@ -2621,9 +2621,23 @@ uint64_t helper_float_cvtl_d(CPUMIPSState *env, uint64_t 
> fdt0)
>       uint64_t dt2;
>
>       dt2 = float64_to_int64(fdt0, &env->active_fpu.fp_status);
> -    if (get_float_exception_flags(&env->active_fpu.fp_status)
> -        & (float_flag_invalid | float_flag_overflow)) {
> -        dt2 = FP_TO_INT64_OVERFLOW;
> +    if (env->active_fpu.fcr31 & (1 << FCR31_NAN2008)) {
> +        if (get_float_exception_flags(&env->active_fpu.fp_status)
> +                & (float_flag_invalid | float_flag_overflow)) {
> +            if (float64_is_any_nan(fdt0)) {
> +                dt2 = 0;
> +            } else {
> +                if (float64_is_neg(fdt0))
> +                    dt2 = INT64_MIN;
> +                else
> +                    dt2 = INT64_MAX;
> +            }
> +        }
> +    } else {
> +        if (get_float_exception_flags(&env->active_fpu.fp_status)
> +                & (float_flag_invalid | float_flag_overflow)) {
> +            dt2 = FP_TO_INT64_OVERFLOW;
> +        }

Better to swap the tests here, so that you test the exception flags first (and 
once).  That is the exceptional condition, the one that will be true least 
often.  After that, FCR31_NAN2008 will be tested only when needed.

But also, this pattern is replicated so many times you'd do well to pull this 
sequence out to helper functions (one for s, one for d).

> +uint64_t helper_float_abs_d(CPUMIPSState *env, uint64_t fdt0)
> +{
> +    uint64_t fdt1;
> +
> +    if (env->active_fpu.fcr31 & (1 << FCR31_ABS2008)) {
> +        fdt1 = float64_abs(fdt0);
> +    } else {
> +        if (float64_is_neg(fdt0)) {
> +            fdt1 = float64_sub(0, fdt0, &env->active_fpu.fp_status);
> +        } else {
> +            fdt1 = float64_add(0, fdt0, &env->active_fpu.fp_status);
> +        }
> +        update_fcr31(env, GETPC());

Here you're better off using two separate helper functions, and chose the 
correct one during translation.  Indeed, since the 2008 version is a simple 
bit-flip, you needn't actually have a helper; just expand the sequence inline.


r~