Re: [PATCH v2 08/12] target/riscv/vector_helper.c: use vlenb in HELPER(vsetvl)

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

On 1/16/24 09:25, Daniel Henrique Barboza wrote:
> Use the new 'vlenb' CPU config to validate fractional LMUL. The original
> comparison is done with 'vlen' and 'sew', both in bits. Adjust the shift
> to use vlenb.
> 
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>   target/riscv/vector_helper.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
> index cb944229b0..9e3ae4b5d3 100644
> --- a/target/riscv/vector_helper.c
> +++ b/target/riscv/vector_helper.c
> @@ -45,9 +45,13 @@ target_ulong HELPER(vsetvl)(CPURISCVState *env, target_ulong s1,
>                                               xlen - 1 - R_VTYPE_RESERVED_SHIFT);
>   
>       if (lmul & 4) {
> -        /* Fractional LMUL - check LMUL * VLEN >= SEW */
> +        /*
> +         * Fractional LMUL: check VLEN * LMUL >= SEW,
> +         * or VLEN * (8 - lmul) >= SEW. Using VLENB we
> +         * need 3 less shifts rights.

The last sentence is structured oddly.  Perhaps

   Using VLENB, we decrease the right shift by 3

or perhaps just show the expansion:

/*
  * Fractional LMUL, check
  *
  *    VLEN * LMUL >= SEW
  *    VLEN >> (8 - lmul) >= sew
  *    (vlenb << 3) >> (8 - lmul) >= sew
  *    vlenb >> (8 - 3 - lmul) >= sew
  */

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

r~


> +         */
>           if (lmul == 4 ||
> -            cpu->cfg.vlen >> (8 - lmul) < sew) {
> +            cpu->cfg.vlenb >> (8 - 3 - lmul) < sew) {
>               vill = true;
>           }
>       }