[Bug target/112817] RISC-V: RVV: provide attribute riscv_rvv_vector_bits for VLS codegen

["vineetg at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112817

--- Comment #15 from Vineet Gupta <vineetg at gcc dot gnu.org> ---
(In reply to Vineet Gupta from comment #14)
> 2. implement gcc toggle -mrvv-vector-bits=zvl which essentially copies the
> xxx from -march string

Done:

commit 0a01d1232ff0a8b094270fbf45c9fd0ea46df19f
Author: Pan Li <pan2.li@intel.com>
Date:   Fri Feb 23 15:37:28 2024 +0800

    RISC-V: Introduce gcc option mrvv-vector-bits for RVV

    This patch would like to introduce one new gcc option for RVV. To
    appoint the bits size of one RVV vector register. Valid arguments to
    '-mrvv-vector-bits=' are:

    * scalable
    * zvl

    The scalable will pick up the zvl*b in the march as the minimal vlen.
    For example, the minimal vlen will be 512 when march=rv64gcv_zvl512b
    and mrvv-vector-bits=scalable.

    The zvl will pick up the zvl*b in the march as exactly vlen.
    For example, the vlen will be 1024 exactly when march=rv64gcv_zvl1024b
    and mrvv-vector-bits=zvl.

    The internal option --param=riscv-autovec-preference will be replaced
    by option -mrvv-vector-bits. Aka:

    * -mrvv-vector-bits=scalable indicates
--param=riscv-autovec-preference=scalable
    * -mrvv-vector-bits=zvl indicates
--param=riscv-autovec-preference=fixed-vlmax

    You can also take -fno-tree-vectorize for
--param=riscv-autovec-preference=none.
    The internal option --param=riscv-autovec-preference is unavailable after
this
    patch.