[Bug c/111848] New: RISC-V: RVV cost model pick unexpected big LMUL

[juzhe.zhong at rivai dot ai via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111848

            Bug ID: 111848
           Summary: RISC-V: RVV cost model pick unexpected big LMUL
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juzhe.zhong at rivai dot ai
  Target Milestone: ---

#include <stdint.h>
void
f3 (uint8_t *restrict a, uint8_t *restrict b,
   uint8_t *restrict c, uint8_t *restrict d,
   int n)
{
  for (int i = 0; i < n; ++i)
    {
      a[i * 8] = c[i * 8] + d[i * 8];
      a[i * 8 + 1] = c[i * 8] + d[i * 8 + 1];
      a[i * 8 + 2] = c[i * 8 + 2] + d[i * 8 + 2];
      a[i * 8 + 3] = c[i * 8 + 2] + d[i * 8 + 3];
      a[i * 8 + 4] = c[i * 8 + 4] + d[i * 8 + 4];
      a[i * 8 + 5] = c[i * 8 + 4] + d[i * 8 + 5];
      a[i * 8 + 6] = c[i * 8 + 6] + d[i * 8 + 6];
      a[i * 8 + 7] = c[i * 8 + 6] + d[i * 8 + 7];
      b[i * 8] = c[i * 8 + 1] + d[i * 8];
      b[i * 8 + 1] = c[i * 8 + 1] + d[i * 8 + 1];
      b[i * 8 + 2] = c[i * 8 + 3] + d[i * 8 + 2];
      b[i * 8 + 3] = c[i * 8 + 3] + d[i * 8 + 3];
      b[i * 8 + 4] = c[i * 8 + 5] + d[i * 8 + 4];
      b[i * 8 + 5] = c[i * 8 + 5] + d[i * 8 + 5];
      b[i * 8 + 6] = c[i * 8 + 7] + d[i * 8 + 6];
      b[i * 8 + 7] = c[i * 8 + 7] + d[i * 8 + 7];
    }
}

This case pick LMUL = 8 which causes horrible vector register spillings.

After experiment, the ideal LMUL should be 2.