Hi, Richard. 
After several tries with your testcases (I already added into V15 patch).
I think "using a new IV" would be better than "multiplication"

Now:
 loop_len_34 = MIN_EXPR <ivtmp_72, 8>;
  _74 = MIN_EXPR <ivtmp_75, 16>;   ------> multiplication approach will changed into  _74 = loop_len_34  * 2;
  loop_len_48 = MIN_EXPR <_74, 4>;
  _77 = _74 - loop_len_48;
  loop_len_49 = MIN_EXPR <_77, 4>;
  _78 = _77 - loop_len_49;
  loop_len_50 = MIN_EXPR <_78, 4>;
  loop_len_51 = _78 - loop_len_50;

I prefer "new IV" since it looks more reasonable and better codegen.
Could you take a look at it:
V15 patch:
https://gcc.gnu.org/pipermail/gcc-patches/2023-May/619534.html 
  
Thanks.


juzhe.zhong@rivai.ai
 
From: Richard Sandiford
Date: 2023-05-25 04:05
To: 钟居哲
CC: gcc-patches; rguenther
Subject: Re: [PATCH V14] VECT: Add decrement IV iteration loop control by variable amount support
I'll look at the samples tomorrow, but just to address one thing:
 
钟居哲 <juzhe.zhong@rivai.ai> writes:
>>> What gives the best code in these cases?  Is emitting a multiplication
>>> better?  Or is using a new IV better?
> Could you give me more detail information about "new refresh IV" approach.
> I'd like to try that.
 
By “using a new IV” I meant calling vect_set_loop_controls_directly
for every rgroup, not just the first.  So in the earlier example,
there would be one decrementing IV for x and one decrementing IV for y.
 
Thanks,
Richard