[Bug tree-optimization/106727] New: Missed fold / canonicalization for checking if a number is a power of 2

[Bug tree-optimization/106727] New: Missed fold / canonicalization for checking if a number is a power of 2

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

            Bug ID: 106727
           Summary: Missed fold / canonicalization for checking if a
                    number is a power of 2
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

These two are equivalent but generate different code:

bool foo(unsigned n) {
    return std::popcount(n) <= 1;
}
bool bar(unsigned n) {
    return (n & (n - 1)) == 0;
}

https://godbolt.org/z/crrxnfWo7

For systems that don't have popcount instructions it would be very beneficial
to transform foo -> bar. For systems that do have popcount instructions it
still would be beneficial to canonicalize the two.

[Bug middle-end/106727] Missed fold / canonicalization for checking if a number is a power of 2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
          Component|tree-optimization           |middle-end

[Bug middle-end/106727] Missed fold / canonicalization for checking if a number is a power of 2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-08-24

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  Expanding __builtin_popcount (n) <= 1 as (n & (n - 1)) == 0 might
be already done.  The canonicalization could be applied if .POPCOUNT is
available.

[Bug middle-end/106727] Missed fold / canonicalization for checking if a number is a power of 2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Mine.

[Bug middle-end/106727] Missed fold / canonicalization for checking if a number is a power of 2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106727

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed.  Expanding __builtin_popcount (n) <= 1 as (n & (n - 1)) == 0
> might be already done.  The canonicalization could be applied if .POPCOUNT
> is available.

No, it is not already done, expanding `__builtin_popcount (n) == 1` is done
(and including if n is known not to include 0 which is exapnded as `n & (n - 1)
== 0`)