Re: [PATCH for-10.0 11/25] target/x86: Set FloatInfZeroNaNRule explicitly

[Richard Henderson]

On 11/28/24 04:42, Peter Maydell wrote:
> Set the FloatInfZeroNaNRule explicitly for the x86 target.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   target/i386/tcg/fpu_helper.c   | 7 +++++++
>   fpu/softfloat-specialize.c.inc | 2 +-
>   2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
> index 53b49bb2977..e9de084a96d 100644
> --- a/target/i386/tcg/fpu_helper.c
> +++ b/target/i386/tcg/fpu_helper.c
> @@ -173,6 +173,13 @@ void cpu_init_fp_statuses(CPUX86State *env)
>        */
>       set_float_2nan_prop_rule(float_2nan_prop_x87, &env->mmx_status);
>       set_float_2nan_prop_rule(float_2nan_prop_x87, &env->sse_status);
> +    /*
> +     * Only SSE has multiply-add instructions.
> +     * TODO: this might be wrong, as we never implemented any x86-specific
> +     * handling for the NaN case for multiply-add. This needs to be checked
> +     * against the manual.
> +     */
> +    set_float_infzeronan_rule(float_infzeronan_dnan_never, &env->sse_status);

This is correct.

Section 14.5.2 Fused-Multiply-ADD (FMA) Numeric Behavior has table 14-17 FMA Numeric 
Behavior.  It boils down to selecting the first NaN from A * B + C.  Row 3 specifically 
documents C as the result for A and B both in {0, Finite, Inf}, which includes the infzero 
case.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~