Re: [PATCH] RISC-V: Support TARGET_VECTORIZE_PREFERRED_VECTOR_ALIGNMENT to optimize codegen of RVV auto-vectorization

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

> we need to discern what we want to achieve here.  The goal might
> be to prevent the vectorizer from performing peeling or versioning
> for alignment.  I realize the peeling code looks ugly but it's
> actually for a good cause when the target does not support
> misaligned vector access or only with severe penalty.

Vector spec says it should support element alignment, so my
understanding is default behavior should be just aligned to
vector-spec said :)

I guess Ju-Zhe might have different thoughts on that, we might need
some more comment from him.


> So I'd much rather prefer that over the current approach as it
> is more localized and will need an mtune-related approach later
> anyway.

I know there is some HW implementation that might be faster if the
address is aligned to 128 bit or 256 bit, and some HW implementation
might only get a few penalties from the first iteration if not aligned
to some alignment.

Anyway those are all mtune-related, so I guess eventually both
riscv_builtin_vectorization_cost and
riscv_vectorize_preferred_vector_alignment should get info from mtune.


>
> Regards
>  Robin