Re: Re: [PATCH 2/3 V2] RISC-V: Enable basic auto-vectorization for RVV

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

Hi Robin:

Share with you more context that I've discussed with Ju-Zhe, and look
for comments from you :)

There is 3 different auto vectorization flavor:
- VLA
- VLS fixed-vlmax (Name TBD)
- (Traditional) VLS

I think I don't need to explain too much on VLA.
So let we focus on second and third:

VLS fixed-vlmax, that's something like -mriscv-vector-bits= or
-msve-vector-bits, assume VLEN is a static length, and evaluate
scalable vector mode as fixed length vector mode (e.g. evaluate (8x +
8) byte to 16 byte), so that stack allocation could be done by static
instead computed by vlenb register, and vlvmax could be evaluate to a
static value too, but the code generated by this mode is not portable,
when you compile with -mriscv-vector-bits=128, then the code can't run
on machine which VLEN is not exactly equal to 128.

(Traditional) VLS, vectorized to something like int32x4_t, stack
allocation can be determined in static too since the size is fixed,
but size of vector register size is still a poly_int16 value (scalable
vector), not evaluated to fixed length vector like VLS fixed-vlmax
mode, this mode could be useful to handle those loop can't vectorized
by VLA mode, or used by SLP vectorizor, and this mode is more portable
than VLS fixed-vlmax mode since it only require VLEN is larger than
specific length, rather than require must equal to specific length.





On Thu, Apr 20, 2023 at 5:47 PM juzhe.zhong@rivai.ai
<juzhe.zhong@rivai.ai> wrote:
>
> Ahhh. These compile options are not finalized.
> I just ask kito provide me some compile option that I can specify LMUL && auto-vectorization mode && vector-length (scalable or fixed-length)
> in order to have chances test auto-vectorizaiton fully for example: fully testing LMUL = 1/2/4/8 auto-vectorization (You can see the codes in rvv.exp).
> Then, he let me add --param options.
>
> I can change compile option as you suggested.
>
> Thanks.
>
>
> juzhe.zhong@rivai.ai
>
> From: Robin Dapp
> Date: 2023-04-20 17:42
> To: juzhe.zhong@rivai.ai; kito.cheng
> CC: gcc-patches; palmer; jeffreyalaw
> Subject: Re: [PATCH 2/3 V2] RISC-V: Enable basic auto-vectorization for RVV
> > Can you give more comments about Robin's opinion that he want to change into
> > "fixed" vs "varying" or "fixed vector size" vs "dynamic vector size" ?
>
> It's not necessary to decide on this now as --params are not supposed
> to be stable and can be changed quickly.  I was just curious if this had
> already been discussed or finalized elsewhere.
>
> Regards
> Robin
>