[Bug tree-optimization/103633] New: Missed popcount recognition

[Bug tree-optimization/103633] New: Missed popcount recognition

[acoplan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103633

            Bug ID: 103633
           Summary: Missed popcount recognition
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

For:

int f1 (unsigned long b) {
    int c = 0;
    while (b) {
        b &= b - 1;
        c++;
    }
    return c;
}

int f2(unsigned long x) {
    int c = 0;
    for (; x; x >>= 1) {
        if (x & 1)
            c++;
    }
    return c;
}

GCC recognizes f1 as popcount but not the more naive f2.

[Bug tree-optimization/103633] Missed popcount recognition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103633

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org
   Last reconfirmed|                            |2021-12-09
           Severity|normal                      |enhancement
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
We don't recongize this either:
int f21(unsigned long x) {
    int c = 0;
    for (; x; x >>= 1) {
        c+= (x & 1);
    }
    return c;
}

Confirmed.

[Bug tree-optimization/103633] Missed popcount recognition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103633

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
A few extra testcase:
```
unsigned short fs(unsigned int execs)
{
  unsigned i;
  unsigned short num_algorithms = 0;
    for (i=0; i<32; i++) {
        if ((1<<i) & execs)
          num_algorithms++;
    }
  return num_algorithms;
}

unsigned short fi(unsigned int execs)
{
  unsigned i;
  unsigned int num_algorithms = 0;
    for (i=0; i<32; i++) {
        if ((1<<i) & execs)
          num_algorithms++;
    }
  return num_algorithms;
}

```
These testcases comes from coremarks but from the startup code so it has no
performance impact ...
You will also notice that fi has better code gen than fs with the vectorizer.