[Bug tree-optimization/95423] New: Failure to optimize separated multiplications by x and square of x

[Bug tree-optimization/95423] New: Failure to optimize separated multiplications by x and square of x

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

            Bug ID: 95423
           Summary: Failure to optimize separated multiplications by x and
                    square of x
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int a, int b, int x)
{
    return a * (x * x) + (b * x);
}

This can be optimized to `return (((a * x) + b) * x);`. LLVM does this
transformation, but GCC does not.

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

--- Comment #1 from Gabriel Ravier <gabravier at gmail dot com> ---
I've hit a similar thing with this function :

int f(int x)
{
    return (x * x * x) - (3 * x) - 1;
}

Which can be optimized to `return (((x * x) - 3) * x) - 1;`, but isn't

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Could there be overflows which don't happen originally?

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
We manage it with -fwrapv. This should happen late when we don't care about
overflow anymore, or it needs to introduce casts to an unsigned type.

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-02
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
We probably have duplicates - the main issue is the reassoc pass does not
run for signed integer arithmetic chains.

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Fri, 29 May 2020, pinskia at gcc dot gnu.org wrote:

> Could there be overflows which don't happen originally?

Yes.  Say a == INT_MIN, x == -1, b == 0, for example.  (This 
transformation can also remove overflows.)

[Bug tree-optimization/95423] Failure to optimize separated multiplications by x and square of x

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95423

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement