Re: [Qemu-devel] [RFC PATCH 11/30] target/arm: implement half-precision F(MIN|MAX)(V|NMV)

[Richard Henderson <richard.henderson@linaro.org>]

On 10/13/2017 09:24 AM, Alex Bennée wrote:
> +/*
> + * do_reduction_op helper
> + *
> + * This mirrors the Reduce() pseudocode in the ARM ARM. It is
> + * important for correct NaN propagation that we do these
> + * operations in exactly the order specified by the pseudocode.
> + *
> + * This is a recursive function, TCG temps should be freed by the
> + * calling function once it is done with the values.
> + */
> +static TCGv_i32 do_reduction_op(DisasContext *s, int fpopcode, int rn,
> +                                int esize, int size, int vmap, TCGv_ptr fpst)
> +{
> +    if (esize == size) {
> +        int element;
> +        TCGMemOp msize = esize == 16 ? MO_16 : MO_32;
> +        TCGv_i32 tcg_elem;
> +
> +        /* We should have one register left here */
> +        assert(ctpop8(vmap) == 1);

I think you should match the ctpop to the size of vmap.  It's true you only
need uint8_t at present, so maybe use that?  At least it's self-consistent.

> +        tcg_hi = do_reduction_op(s, fpopcode, rn, esize, bits, vmap_hi, fpst);
> +        tcg_lo = do_reduction_op(s, fpopcode, rn, esize, bits, vmap_lo, fpst);
> +        tcg_res = tcg_temp_new_i32();

You can re-use one of the two inputs for the output, fwiw.

> +        /* Bit 1 of size field encodes min vs max and the actual size
> +         * depends on the encoding of the U bit. If not set (and FP16
> +         * enabled) then we do half-precision float instead of single
> +         * precision.
>           */
>          is_min = extract32(size, 1, 1);
>          is_fp = true;
> -        size = 2;
> +        if (!is_u && arm_dc_feature(s, ARM_FEATURE_V8_FP16)) {
> +            size = 1;

You do still need to check size[0] == 0.


r~