Re: [PATCH v3] RISC-V: Introduce gcc option mrvv-vector-bits for RVV

[Kito Cheng <kito.cheng@gmail.com>]

Take one more look, I think this option should work and integrate with
--param=riscv-autovec-preference= since they have similar jobs but
slightly different.

We have 3 value for  --param=riscv-autovec-preference=: none, scalable
and fixed-vlmax

-mrvv-vector-bits=scalable is work like
--param=riscv-autovec-preference=scalable and
-mrvv-vector-bits=zvl is work like
--param=riscv-autovec-preference=fixed-vlmax.

So I think...we need to do some conflict check, like:

-mrvv-vector-bits=zvl can't work with --param=riscv-autovec-preference=scalable
-mrvv-vector-bits=scalable can't work with
--param=riscv-autovec-preference=fixed-vlmax

but it may not just alias since there is some useful combinations like:

-mrvv-vector-bits=zvl with --param=riscv-autovec-preference=none:
NO auto vectorization but intrinsic code still could benefit from the
-mrvv-vector-bits=zvl option.

-mrvv-vector-bits=scalable with --param=riscv-autovec-preference=none
Should still work for VLS code gen, but just disable auto
vectorization per the option semantic.

However here is something we need some fix, since
--param=riscv-autovec-preference=none still disable VLS code gen for
now, you can see some example here:
https://godbolt.org/z/fMTr3eW7K

But I think it's really the right behavior here, this part might need
to be fixed in vls_mode_valid_p and some other places.


Anyway I think we need to check all use sites with RVV_FIXED_VLMAX and
RVV_SCALABLE, and need to make sure all use site of RVV_FIXED_VLMAX
also checked with RVV_VECTOR_BITS_ZVL.



> -/* Return the VLEN value associated with -march.
> +static int
> +riscv_convert_vector_bits (int min_vlen)

Not sure if we really need this function, it seems it always returns min_vlen?

> +{
> +  int rvv_bits = 0;
> +
> +  switch (rvv_vector_bits)
> +    {
> +      case RVV_VECTOR_BITS_ZVL:
> +      case RVV_VECTOR_BITS_SCALABLE:
> +       rvv_bits = min_vlen;
> +       break;
> +      default:
> +       gcc_unreachable ();
> +    }
> +
> +  return rvv_bits;
> +}
> +
> +/* Return the VLEN value associated with -march and -mwrvv-vector-bits.