Hi, Kewen.

I have tried to implement "decrement IV" feature and incorporate into "vect_set_loop_controls_directly".
Since the implementation is quite different from vect_set_loop_controls_directly, it will make  vect_set_loop_controls_directly
very complicated sometimes it makes me very hard to debug when I am testing. 

I am not sure but I can try again.

This patch isolated those implementation into a single function "vect_set_loop_controls_by_select_vl" which makes Richards
easier to review codes. Well, I think I can try again to incorporate those codes into "vect_set_loop_controls_directly" when they 
finish the review process of  "vect_set_loop_controls_by_select_vl".

Thanks.


juzhe.zhong@rivai.ai
 
From: Kewen.Lin
Date: 2023-05-08 15:55
To: juzhe.zhong@rivai.ai
CC: gcc-patches; rguenther; richard.sandiford
Subject: Re: [PATCH V4] VECT: Add decrement IV iteration loop control by variable amount support
Hi Juzhe,
 
> Hi, Kewen.
> 
>>> Sorry for chiming in, I had some concern here.
>>> We already have some handlings for partial vector in length in vect_set_loop_controls_directly
>>>(actually it deals with both mask and length), the proposed vect_set_loop_controls_by_select_vl
>>>for select_vl looks like a variant for partial vector in length (comparing to the normal MIN),
>>>and also adopts decrement IV.  IMHO, it seems better to modify/extend the current handling in
>>>vect_set_loop_controls_directly for length, or factor out the handlings for length there and
>>>extend the factored one.  Otherwise, it means we have two sets of handlings for partial vector
>>>in lengths, it looks bad to maintain.  As the previous discussion, adopting decrement IV is an
>>>enhancement for loop control, it's good for both cases w/ or w/o select_vl.  If the above
>>>understanding is correct, the possible steps seem to be:
>>>  - factor out the handling for length (* optional)
>>>  - modify it with decrement IV
>>>  - extend it with select_vl.
>>>In future if some RVV vendor wants to degenerate select_vl to min, it can just adopt the same
>>>handlings with min by not defining select_vl optab.
> 
> You mean like this:
> doing this inside vect_set_loop_controls_directly ?
>         if (use_while_len_p)
>           return vect_set_loop_controls_by_while_len(...)
 
No, I meant either factoring out those handlings for partial vector in length in function
vect_set_loop_controls_directly to one separated function like: vect_set_loop_controls_directly_length
and rename the existing one to vect_set_loop_controls_directly_mask, or keep the existing
vect_set_loop_controls_directly for both mask and length but modify/extend the part for length.
If there is no much code to share between mask and length, the former may be better.
 
BR,
Kewen