Ideally, LMUL should be dynamically picked by compiler according to the user codes.

However, GCC doesn't support it yet and it's not a easy feature to be supported in the future.

My plan is that we let LMUL picked statically according to compile option 
and we fully support and test LMUL = 1/2/4/8 so far.
Then we can support the feature of picking LMUL during auto-vectorization in the future when we figure out how to do that.



juzhe.zhong@rivai.ai
 
From: Jeff Law
Date: 2023-04-25 14:00
To: juzhe.zhong; gcc-patches
CC: kito.cheng; palmer
Subject: Re: [PATCH] RISC-V: Add RVV auto-vectorization compile option
 
 
On 4/6/23 19:21, juzhe.zhong@rivai.ai wrote:
> From: Juzhe-Zhong <juzhe.zhong@rivai.ai>
> 
> The next patch to enable basic RVV auto-vectorization of
> VLA auto-vectorization (RVV_SCALABLE) and fixed-length VLS auto-vectorization (RVV_FIXED_VLMAX).
> 
> We will support RVV_FIXED_VLMIN in the future.
> 
> gcc/ChangeLog:
> 
>          * config/riscv/riscv-opts.h (enum riscv_autovec_preference_enum): Add RVV auto-vectorization compile option.
>          (enum riscv_autovec_lmul_enum): Ditto.
>          * config/riscv/riscv.opt: Ditto.
No real objection here.  Just a question.   What's the rationale behind 
exposing lmul settings to the user?  I'd think that should largely be 
compiler managed.  But maybe I'm missing something.
 
jeff