Re: [RFC 1/2] target/riscv: rvv: reduce the overhead for simple RISC-V vector unit-stride loads and stores

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

On 7/18/24 01:30, Paolo Savini wrote:
> From: Helene CHELIN <helene.chelin@embecosm.com>
> 
> This patch improves the performance of the emulation of the RVV unit-stride
> loads and stores in the following cases:
> 
> - when the data being loaded/stored per iteration amounts to 8 bytes or less.
> - when the vector length is 16 bytes (VLEN=128) and there's no grouping of the
>    vector registers (LMUL=1).
> 
> The optimization consists of avoiding the overhead of probing the RAM of the
> host machine and doing a loop load/store on the input data grouped in chunks
> of as many bytes as possible (8,4,2,1 bytes).
> 
> Co-authored-by: Helene CHELIN <helene.chelin@embecosm.com>
> Co-authored-by: Paolo Savini <paolo.savini@embecosm.com>
> 
> Signed-off-by: Helene CHELIN <helene.chelin@embecosm.com>
> ---
>   target/riscv/vector_helper.c | 46 ++++++++++++++++++++++++++++++++++++
>   1 file changed, 46 insertions(+)
> 
> diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
> index 29849a8b66..4b444c6bc5 100644
> --- a/target/riscv/vector_helper.c
> +++ b/target/riscv/vector_helper.c
> @@ -633,6 +633,52 @@ vext_ldst_us(void *vd, target_ulong base, CPURISCVState *env, uint32_t desc,
>   
>       VSTART_CHECK_EARLY_EXIT(env);
>   
> +    /* For data sizes <= 64 bits and for LMUL=1 with VLEN=128 bits we get a
> +     * better performance by doing a simple simulation of the load/store
> +     * without the overhead of prodding the host RAM */
> +    if ((nf == 1) && ((evl << log2_esz) <= 8 ||
> +	((vext_lmul(desc) == 0) && (simd_maxsz(desc) == 16)))) {
> +
> +	uint32_t evl_b = evl << log2_esz;
> +
> +        for (uint32_t j = env->vstart; j < evl_b;) {
> +	    addr = base + j;
> +            if ((evl_b - j) >= 8) {
> +                if (is_load)
> +                    lde_d_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                else
> +                    ste_d_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                j += 8;
> +            }
> +            else if ((evl_b - j) >= 4) {
> +                if (is_load)
> +                    lde_w_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                else
> +                    ste_w_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                j += 4;
> +            }
> +            else if ((evl_b - j) >= 2) {
> +                if (is_load)
> +                    lde_h_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                else
> +                    ste_h_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                j += 2;
> +            }
> +            else {
> +                if (is_load)
> +                    lde_b_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                else
> +                    ste_b_tlb(env, adjust_addr(env, addr), j, vd, ra);
> +                j += 1;
> +            }
> +        }

For system mode, this performs the tlb lookup N times, and so will not be an improvement.

This will not work on a big-endian host.


r~