Re: [Qemu-devel] [PATCH 4/5] tricore: add QSEED instruction

[Bastian Koppelmann <kbastian@mail.uni-paderborn.de>]

Hi,

On 6/5/19 8:11 AM, David Brenken wrote:
> +/*
> + * Target TriCore QSEED.F significand Lookup Table
> + *
> + * The QSEED.F output significand depends on the least-significant
> + * exponent bit and the 6 most-significant significand bits.
> + *
> + * IEEE 754 float datatype
> + * partitioned into Sign (S), Exponent (E) and Significand (M):
> + *
> + * S   E E E E E E E E   M M M M M M ...
> + *    |             |               |
> + *    +------+------+-------+-------+
> + *           |              |
> + *          for        lookup table
> + *      calculating     index for
> + *        output E       output M
> + */
> +static const uint8_t target_qseed_significand_table[128] = {
> +    253, 252, 245, 244, 239, 238, 231, 230, 225, 224, 217, 216,
> +    211, 210, 205, 204, 201, 200, 195, 194, 189, 188, 185, 184,
> +    179, 178, 175, 174, 169, 168, 165, 164, 161, 160, 157, 156,
> +    153, 152, 149, 148, 145, 144, 141, 140, 137, 136, 133, 132,
> +    131, 130, 127, 126, 123, 122, 121, 120, 117, 116, 115, 114,
> +    111, 110, 109, 108, 103, 102, 99, 98, 93, 92, 89, 88, 83,
> +    82, 79, 78, 75, 74, 71, 70, 67, 66, 63, 62, 59, 58, 55,
> +    54, 53, 52, 49, 48, 45, 44, 43, 42, 39, 38, 37, 36, 33,
> +    32, 31, 30, 27, 26, 25, 24, 23, 22, 19, 18, 17, 16, 15,
> +    14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2
> +};


Can you explain in a comment how you arrived at this lookup table?


> +    } else if (float32_is_neg(arg1)) {
> +        result = float32_sqrt_nan;
> +        env->FPU_FI = 1;
[...]
> +
> +    flags = f_get_excp_flags(env);
> +    if (flags) {
> +        if (flags & float_flag_invalid) {
> +            f_update_psw_flags(env, flags);
> +        } else {
> +            env->FPU_FS = 0;
> +        }
> +    } else {
> +        env->FPU_FS = 0;

You are setting FPU_FS to 0, even though FPU_FI might have been set in 
case of a NaN. I think it's best to remove the whole softfloat check, as 
none of the softfloat functions you call, can raise any flags.

Cheers,

Bastian