Hi, For the first piece of code ,I tried:
  unsigned int nitems_per_iter
    = dest_rgm->max_nscalars_per_iter * dest_rgm->factor;
  step = gimple_build (seq, MULT_EXPR, iv_type, step,
                       build_int_cst (iv_type, nitems_per_iter));

Then optimized IR:
loop_len_34 = MIN_EXPR <ivtmp_72, 8>;
  _74 = loop_len_34 * 4;
  loop_len_51 = _74 + 18446744073709551604;

  _16 = (void *) ivtmp.27_41;
  _17 = &MEM <vector(8) short int> [(short int *)_16];

  vect__1.7_33 = .LEN_LOAD (_17, 16B, loop_len_34, 0);

  vect__2.8_23 = VIEW_CONVERT_EXPR<vector(8) unsigned short>(vect__1.7_33);
  vect__3.9_22 = vect__2.8_23 + { 1, 2, 1, 2, 1, 2, 1, 2 };
  vect__4.10_21 = VIEW_CONVERT_EXPR<vector(8) short int>(vect__3.9_22);
  .LEN_STORE (_17, 16B, loop_len_34, vect__4.10_21, 0);
  _20 = (void *) ivtmp.28_1;
  _31 = &MEM <vector(4) int> [(int *)_20];

  vect__10.15_52 = .LEN_LOAD (_31, 32B, 4, 0);

  _30 = (void *) ivtmp.31_4;
  _29 = &MEM <vector(4) int> [(int *)_30];

  vect__10.16_54 = .LEN_LOAD (_29, 32B, 4, 0);

  _26 = (void *) ivtmp.32_8;
  _25 = &MEM <vector(4) int> [(int *)_26];

  vect__10.17_56 = .LEN_LOAD (_25, 32B, 4, 0);

  _79 = (void *) ivtmp.33_12;
  _80 = &MEM <vector(4) int> [(int *)_79];

  vect__10.18_58 = .LEN_LOAD (_80, 32B, loop_len_51, 0);

Is it correct ? It looks wierd ? 


juzhe.zhong@rivai.ai
 
From: Richard Sandiford
Date: 2023-05-25 00:00
To: 钟居哲
CC: gcc-patches; rguenther
Subject: Re: [PATCH V14] VECT: Add decrement IV iteration loop control by variable amount support
钟居哲 <juzhe.zhong@rivai.ai> writes:
> Oh. I see. Thank you so much for pointing this.
> Could you tell me what I should do in the codes?
> It seems that I should adjust it in 
> vect_adjust_loop_lens_control
>
> muliply by some factor ? Is this correct multiply by max_nscalars_per_iter
> ?
 
max_nscalars_per_iter * factor rather than just max_nscalars_per_iter
 
Note that it's possible for later max_nscalars_per_iter * factor to
be smaller, so a division might be needed in rare cases.  E.g.:
 
uint64_t x[100];
uint16_t y[200];
 
void f() {
  for (int i = 0, j = 0; i < 100; i += 2, j += 4) {
    x[i + 0] += 1;
    x[i + 1] += 2;
    y[j + 0] += 1;
    y[j + 1] += 2;
    y[j + 2] += 3;
    y[j + 3] += 4;
  }
}
 
where y has a single-control rgroup with max_nscalars_per_iter == 4
and x has a 2-control rgroup with max_nscalars_per_iter == 2
 
What gives the best code in these cases?  Is emitting a multiplication
better?  Or is using a new IV better?
 
Thanks,
Richard