Re: [Qemu-devel] [PATCH] Softfloat: Add support to softfloat to return floatxx_default_nan when the corresponding target status flag is set.

[Peter Maydell <peter.maydell@linaro.org>]

On 4 February 2011 14:01, Christophe Lyon <christophe.lyon@st.com> wrote:
> Some CPUs have a status flag imposing to return floatxx_default_nan
> whatever the input value, when converting from one FP format to
> another. Implement this, using the already existing default_nan_mode
> status flag, only for ARM at the moment, though other architectures
> may have the same feature. This patch only modifies the
> commonNaNToFloat32 and commonNaNToFloat64 conversion functions, as ARM
> only uses these.

> -static float32 commonNaNToFloat32( commonNaNT a )
> +static float32 commonNaNToFloat32( commonNaNT a STATUS_PARAM )
>  {
>     bits32 mantissa = a.high>>41;
> +
> +#if defined(TARGET_ARM)
> +    if ( STATUS(default_nan_mode) ) {
> +        return float32_default_nan;
> +    }
> +#endif

The target-specific #ifdef is pretty ugly.
Fortunately the only architecture except ARM that sets
default_nan_mode is sh4, so all we have to do is find out
what it does with its float-to-float conversions. It looks
to me from
http://documentation.renesas.com/eng/products/mpumcu/rej09b0318_sh_4sm.pdf
that it also returns the default NaN for converting a NaN
from one float format to another. So you can just drop the
#ifdef.

(cc'ing Aurelien to check since he's the SH4 maintainer.)

I also think the change is simple enough that we ought to do
it consistently for the floatx80 and float128 functions even if
neither ARM nor sh4 use them.

-- PMM