Re: Re: [PATCH V14] VECT: Add decrement IV iteration loop control by variable amount support

[钟居哲 <juzhe.zhong@rivai.ai>]

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Hi, Richard. After I fix codes, now IR is correct I think:

loop_len_34 = MIN_EXPR <ivtmp_72, 8>;
  _74 = loop_len_34 * 2;
  loop_len_48 = MIN_EXPR <_74, 4>;
  _75 = _74 - loop_len_48;
  loop_len_49 = MIN_EXPR <_75, 4>;
  _76 = _75 - loop_len_49;
  loop_len_50 = MIN_EXPR <_76, 4>;
  loop_len_51 = _76 - loop_len_50;
  ...
  vect__1.8_33 = .LEN_LOAD (_17, 16B, loop_len_34, 0);
...
  .LEN_STORE (_17, 16B, loop_len_34, vect__4.11_21, 0);
...

  vect__10.16_52 = .LEN_LOAD (_31, 32B, loop_len_48, 0);
...
  vect__10.17_54 = .LEN_LOAD (_29, 32B, loop_len_49, 0);
...
  vect__10.18_56 = .LEN_LOAD (_25, 32B, loop_len_50, 0);
...
  vect__10.19_58 = .LEN_LOAD (_80, 32B, loop_len_51, 0);


For this case:

uint64_t x2[100];
uint16_t y2[200];

void f2(int n) {
  for (int i = 0, j = 0; i < n; i += 2, j += 4) {
    x2[i + 0] += 1;
    x2[i + 1] += 2;
    y2[j + 0] += 1;
    y2[j + 1] += 2;
    y2[j + 2] += 3;
    y2[j + 3] += 4;
  }
}

The IR is like this:

  loop_len_56 = MIN_EXPR <ivtmp_64, 8>;
  _66 = loop_len_56 * 4;
  loop_len_43 = _66 + 18446744073709551614;
  ...
  vect__1.44_44 = .LEN_LOAD (_6, 64B, 2, 0);
  ...
  vect__1.45_46 = .LEN_LOAD (_14, 64B, loop_len_43, 0);
  vect__2.46_47 = vect__1.44_44 + { 1, 2 };
  vect__2.46_48 = vect__1.45_46 + { 1, 2 };
  .LEN_STORE (_6, 64B, 2, vect__2.46_47, 0);
  .LEN_STORE (_14, 64B, loop_len_43, vect__2.46_48, 0);
  ...
  vect__6.51_57 = .LEN_LOAD (_10, 16B, loop_len_56, 0);

  vect__7.52_58 = vect__6.51_57 + { 1, 2, 3, 4, 1, 2, 3, 4 };
  .LEN_STORE (_10, 16B, loop_len_56, vect__7.52_58, 0);

It seems correct too ?

>> 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.

Thanks.


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