Re: [PATCH] VECT: Add WHILE_LEN pattern for decrement IV support for auto-vectorization

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Juzhe,

on 2023/4/12 21:22, 钟居哲 wrote:
> Thanks Kewen. 
> 
> It seems that this proposal WHILE_LEN can help s390 when using --param vect-partial-vector-usage=2 compile option.
> 

Yeah, IMHO, the previous sequence vs. the proposed sequence are like:

int
foo (int *__restrict a, int *__restrict b, int n)
{
  if (n <= 0)
    return 0;

  int iv = 0;
  int len = MIN (n, 16);
  int sum = 0;
  do
    {
      sum += a[len] + b[len];
      iv += 16;
      int n1 = MIN (n, iv);                   // line A
      int n2 = n - n1;
      len = MIN (n2, 16);
    }
  while (n > iv);

  return sum;
}

vs.

int
foo (int *__restrict a, int *__restrict b, int n)
{
  if (n <= 0)
    return 0;

  int len;
  int sum = 0;
  do
    {
      len = MIN (n, 16);
      sum += a[len] + b[len];
      n -= len;
    }
  while (n > 0);

  return sum;
}

it at least saves one MIN (at line A) and one length preparation in the
last iteration (it's useless since loop ends).  But I think the concern
that this proposed IV isn't recognized as simple iv may stay.  I tried
to compile the above source files on Power, the former can adopt doloop
optimization but the latter fails to.

> Would you mind apply this patch && support WHILE_LEN in s390 backend and test it to see the overal benefits for s390
> as well as the correctness of this sequence ? 

Sure, if all of you think this approach and this revision is good enough to go forward for this kind of evaluation,
I'm happy to give it a shot, but only for rs6000. ;-)  I noticed that there are some discussions on withdrawing this
WHILE_LEN by using MIN_EXPR instead, I'll stay tuned.

btw, now we only adopt vector with length on the epilogues rather than the main vectorized loops, because of the
non-trivial extra costs for length preparation than just using the normal vector load/store (all lanes), so we don't
care about the performance with --param vect-partial-vector-usage=2 much.  Even if this new proposal can optimize
the length preparation for --param vect-partial-vector-usage=2, the extra costs for length preparation is still
unavoidable (MIN, shifting, one more GPR used), we would still stay with default --param vect-partial-vector-usage=1
(which can't benefit from this new proposal).

BR,
Kewen