[Bug middle-end/101596] New: vect_recog_mulhs_pattern could use incorrect precision to check shift count

[linkw at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101596

            Bug ID: 101596
           Summary: vect_recog_mulhs_pattern could use incorrect precision
                    to check shift count
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linkw at gcc dot gnu.org
  Target Milestone: ---

I happened to spot this when I was working to add one new pattern for Power10
divide extended. Now vect_recog_mulhs_pattern uses target_precision to check
the scale_term is expected or not, it's wrong when the precision of the actual
used new_type larger than target_precision, since we use the new_type for the
internal function call.

Test case:

```
#define N 32

typedef signed long long sLL;
signed int si_a[N], si_b[N];
signed short sh_c[N];

#define SH_CNT 48
//#define SH_CNT 32

__attribute__ ((noipa)) void
test_ui ()
{
  for (int i = 0; i < N; i++)
    sh_c[i] = ((sLL) si_a[i] * (sLL) si_b[i]) >> SH_CNT;
}

__attribute__ ((optimize ("O0"))) int
main ()
{
  for (int i = 0; i < N; i++)
    {
      si_a[i] = 0x12345678;
      si_b[i] = 0x76543210;
    }
  test_ui ();
  for (int i = 0; i < N; i++)
    {
      signed short exp = ((sLL) si_a[i] * (sLL) si_b[i]) >> SH_CNT;
      if (exp != sh_c[i])
        __builtin_abort ();
    }
  return 0;
}
```

gcc test.c -mcpu=power10 -O2 -ftree-vectorize

$ ./a.out
Aborted (core dumped)

test.c:13:21: note:   vect_recog_mulhs_pattern: detected: _6 = _5 >> 48;