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

[Paolo Savini <paolo.savini@embecosm.com>]

Thanks for the feedback Richard, I'm working on the endianness. Could 
you please give me more details about the atomicity issues you are 
referring to?

Best wishes

Paolo

On 7/27/24 08:15, Richard Henderson wrote:
> 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~
>