Re: [RFC 2/2] target/riscv: rvv: improve performance of RISC-V vector loads and stores on large amounts of data.

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

On 7/18/24 01:30, Paolo Savini wrote:
> This patch optimizes the emulation of unit-stride load/store RVV instructions
> when the data being loaded/stored per iteration amounts to 64 bytes or more.
> The optimization consists of calling __builtin_memcpy on chunks of data of 128
> and 256 bytes between the memory address of the simulated vector register and
> the destination memory address and vice versa.
> This is done only if we have direct access to the RAM of the host machine.
> 
> Signed-off-by: Paolo Savini <paolo.savini@embecosm.com>
> ---
>   target/riscv/vector_helper.c | 17 ++++++++++++++++-
>   1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
> index 4b444c6bc5..7674972784 100644
> --- a/target/riscv/vector_helper.c
> +++ b/target/riscv/vector_helper.c
> @@ -486,7 +486,22 @@ vext_group_ldst_host(CPURISCVState *env, void *vd, uint32_t byte_end,
>       }
>   
>       fn = fns[is_load][group_size];
> -    fn(vd, byte_offset, host + byte_offset);
> +
> +    if (byte_offset + 32 < byte_end) {
> +      group_size = MO_256;
> +      if (is_load)
> +        __builtin_memcpy((uint8_t *)(vd + byte_offset), (uint8_t *)(host + byte_offset), 32);
> +      else
> +        __builtin_memcpy((uint8_t *)(host + byte_offset), (uint8_t *)(vd + byte_offset), 32);
> +    } else if (byte_offset + 16 < byte_end) {
> +      group_size = MO_128;
> +      if (is_load)
> +        __builtin_memcpy((uint8_t *)(vd + byte_offset), (uint8_t *)(host + byte_offset), 16);
> +      else
> +        __builtin_memcpy((uint8_t *)(host + byte_offset), (uint8_t *)(vd + byte_offset), 16);
> +    } else {
> +      fn(vd, byte_offset, host + byte_offset);
> +    }
>   

This will not work for big-endian hosts.

This may have atomicity issues, depending on the spec, the compiler options, and the host 
capabilities.


r~