On 9 January 2018 at 12:22, Alex Bennée <alex.ben...@linaro.org> wrote:
> While a comparison between a QNaN and a number will return the number
> it is not the same with a signaling NaN. In this case the SNaN will
> "win" and after potentially raising an exception it will be quietened.
>
> Signed-off-by: Alex Bennée <alex.ben...@linaro.org>
> Reviewed-by: Richard Henderson <richard.hender...@linaro.org>
> ---
> v2
> - added return for propageFloat
> ---
> fpu/softfloat.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 3a4ab1355f..44c043924e 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -7683,6 +7683,7 @@ int float128_compare_quiet(float128 a, float128 b,
> float_status *status)
> * minnum() and maxnum() functions. These are similar to the min()
> * and max() functions but if one of the arguments is a QNaN and
> * the other is numerical then the numerical argument is returned.
> + * SNaNs will get quietened before being returned.
> * minnum() and maxnum correspond to the IEEE 754-2008 minNum()
> * and maxNum() operations. min() and max() are the typical min/max
> * semantics provided by many CPUs which predate that specification.
> @@ -7703,11 +7704,14 @@ static inline float ## s float ## s ## _minmax(float
> ## s a, float ## s b, \
> if (float ## s ## _is_any_nan(a) || \
> float ## s ## _is_any_nan(b)) { \
> if (isieee) { \
> - if (float ## s ## _is_quiet_nan(a, status) && \
> + if (float ## s ## _is_signaling_nan(a, status) || \
> + float ## s ## _is_signaling_nan(b, status)) { \
> + return propagateFloat ## s ## NaN(a, b, status); \
> + } else if (float ## s ## _is_quiet_nan(a, status) && \
> !float ## s ##_is_any_nan(b)) { \
> return b; \
> } else if (float ## s ## _is_quiet_nan(b, status) && \
> - !float ## s ## _is_any_nan(a)) { \
> + !float ## s ## _is_any_nan(a)) { \
> return a; \
> } \
> } \
> return propagateFloat ## s ## NaN(a, b, status); \
> } \
[added a couple of extra lines of context at the end for clarity]
Am I misreading this patch? I can't see in what case it makes a
difference to the result. The code change adds an explicit "if
either A or B is an SNaN then return the propagateFloat*NaN() result".
But if either A or B is an SNaN then we won't take either of the
previously existing branches in this if() ("if A is a QNaN and B is
not a NaN" and "if B is a QNaN and A is not a NaN"), and so we'll
end up falling through to the "return propagateFloat*NaN" line after
the end of the "is (ieee) {...}".
thanks
-- PMM
