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Date: Tue, 31 Dec 2013 06:51:21 -0800
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Subject: Re: [Qemu-devel] [PATCH 14/22] softfloat: Add support for ties-away
	rounding
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To: Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org
Cc: Tom Musta <tommusta@gmail.com>, Peter Crosthwaite <peter.crosthwaite@xilinx.com>, patches@linaro.org, Aurelien Jarno <aurelien@aurel32.net>, Michael Matz <matz@suse.de>, Alexander Graf <agraf@suse.de>, Claudio Fontana <claudio.fontana@linaro.org>, Dirk Mueller <dmueller@suse.de>, Will Newton <will.newton@linaro.org>, Laurent Desnogues <laurent.desnogues@gmail.com>, =?ISO-8859-1?Q?Alex_Benn=E9e?= <alex.bennee@linaro.org>, kvmarm@lists.cs.columbia.edu, Christoffer Dall <christoffer.dall@linaro.org>

[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~