Re: [Qemu-devel] [PATCH 2/2] fpu: Bound increment for scalbn

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@linaro.org> writes:

> Without bounding the increment, we can overflow exp either here
> in scalbn_decomposed or when adding the bias in round_canonical.
> This can result in e.g. underflowing to 0 instead of overflowing
> to infinity.
>
> The old softfloat code did bound the increment.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  fpu/softfloat.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index ba6e654050..a589f328c9 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -1883,6 +1883,12 @@ static FloatParts scalbn_decomposed(FloatParts a, int n, float_status *s)
>          return return_nan(a, s);
>      }
>      if (a.cls == float_class_normal) {
> +        /* The largest float type (even though not supported by FloatParts)
> +         * is float128, which has a 15 bit exponent.  Bounding N to 16 bits
> +         * still allows rounding to infinity, without allowing overflow
> +         * within the int32_t that backs FloatParts.exp.
> +         */
> +        n = MIN(MAX(n, -0x10000), 0x10000);
>          a.exp += n;
>      }
>      return a;

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>

(risu FWIW although it obviously didn't catch this failure ;-)

--
Alex Bennée