Re: [PATCH 1/2] target/riscv/vector_helper.c: skip set tail when vta is zero

[Alistair Francis <alistair23@gmail.com>]

On Fri, Apr 28, 2023 at 6:58 AM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
>
> The function is a no-op if 'vta' is zero but we're still doing a lot of
> stuff in this function regardless. vext_set_elems_1s() will ignore every
> single time (since vta is zero) and we just wasted time.
>
> Skip it altogether in this case. Aside from the code simplification
> there's a noticeable emulation performance gain by doing it. For a
> regular C binary that does a vectors operation like this:
>
> =======
>  #define SZ 10000000
>
> int main ()
> {
>   int *a = malloc (SZ * sizeof (int));
>   int *b = malloc (SZ * sizeof (int));
>   int *c = malloc (SZ * sizeof (int));
>
>   for (int i = 0; i < SZ; i++)
>     c[i] = a[i] + b[i];
>   return c[SZ - 1];
> }
> =======
>
> Emulating it with qemu-riscv64 and RVV takes ~0.3 sec:
>
> $ time ~/work/qemu/build/qemu-riscv64 \
>     -cpu rv64,debug=false,vext_spec=v1.0,v=true,vlen=128 ./foo.out
>
> real    0m0.303s
> user    0m0.281s
> sys     0m0.023s
>
> With this skip we take ~0.275 sec:
>
> $ time ~/work/qemu/build/qemu-riscv64 \
>     -cpu rv64,debug=false,vext_spec=v1.0,v=true,vlen=128 ./foo.out
>
> real    0m0.274s
> user    0m0.252s
> sys     0m0.019s
>
> This performance gain adds up fast when executing heavy benchmarks like
> SPEC.
>
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

Acked-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  target/riscv/vector_helper.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
> index f4d0438988..8e6c99e573 100644
> --- a/target/riscv/vector_helper.c
> +++ b/target/riscv/vector_helper.c
> @@ -268,12 +268,17 @@ static void vext_set_tail_elems_1s(CPURISCVState *env, target_ulong vl,
>                                     void *vd, uint32_t desc, uint32_t nf,
>                                     uint32_t esz, uint32_t max_elems)
>  {
> -    uint32_t total_elems = vext_get_total_elems(env, desc, esz);
> -    uint32_t vlenb = riscv_cpu_cfg(env)->vlen >> 3;
> +    uint32_t total_elems, vlenb, registers_used;
>      uint32_t vta = vext_vta(desc);
> -    uint32_t registers_used;
>      int k;
>
> +    if (vta == 0) {
> +        return;
> +    }
> +
> +    total_elems = vext_get_total_elems(env, desc, esz);
> +    vlenb = riscv_cpu_cfg(env)->vlen >> 3;
> +
>      for (k = 0; k < nf; ++k) {
>          vext_set_elems_1s(vd, vta, (k * max_elems + vl) * esz,
>                            (k * max_elems + max_elems) * esz);
> --
> 2.40.0
>
>