Re: [Qemu-devel] [PATCH 14/22] softfloat: Add support for ties-away rounding

[Richard Henderson <rth@twiddle.net>]

[Tom, this is exactly what you need to fix FRIN rounding.]

On 12/31/2013 05:35 AM, Peter Maydell wrote:
> -    float_round_to_zero      = 3
> +    float_round_to_zero      = 3,
> +    float_round_ties_away    = 4,

I'm not keen on the name.  Does anyone else think float_round_nearest_inf is a
better name?

> +++ b/fpu/softfloat.c
> @@ -107,7 +107,7 @@ static int32 roundAndPackInt32( flag zSign, uint64_t absZ STATUS_PARAM)
>      roundingMode = STATUS(float_rounding_mode);
>      roundNearestEven = ( roundingMode == float_round_nearest_even );
>      roundIncrement = 0x40;
> -    if ( ! roundNearestEven ) {
> +    if (!roundNearestEven && roundingMode != float_round_ties_away) {
>          if ( roundingMode == float_round_to_zero ) {
>              roundIncrement = 0;
>          }

This whole section of code is now a mess.  I know you're looking for minimal
changes here, but perhaps I can convince you that

    switch (roundingMode) {
    case float_round_nearest_even:
    case float_round_ties_away:
        roundIncrement = 0x40;
        break;
    case float_round_to_zero:
        roundIncrement = 0;
        break;
    case float_round_up:
        roundIncrement = zSign ? 0 : 0x7f;
        break;
    case float_round_down:
        roundIncrement = zSign ? 0x7f : 0;
        break;
    default:
        abort();
    }

is easier to follow?

Otherwise, I don't see anything actually wrong in the patch.


r~