[Bug tree-optimization/56355] New: abs and multiplication

[Bug tree-optimization/56355] New: abs and multiplication

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

             Bug #: 56355
           Summary: abs and multiplication
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: glisse@gcc.gnu.org


#include <cmath>
#include <cstdlib>
typedef double T;
// typedef int T;
T f(T a, T b){
  return std::abs(a)*std::abs(b);
}
T g(T a){
  return std::abs(a)*std::abs(a);
}
T h(T a){
  return std::abs(a*a);
}

Compiled with g++ -O3 (-ffast-math doesn't help), g is properly optimized to
a*a but only at the RTL level, and the other 2 are not optimized at all. If I
make T a typedef for int, nothing is optimized.

For the first one, I would expect abs(a*b), and for the others just a*a.

Related to PR 31548.

[Bug tree-optimization/56355] abs and multiplication

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> 2013-02-16 12:07:28 UTC ---
Actually, for g/h with double, using __builtin_fabs instead of std::abs does
it, so it is just the usual lack of combine at the tree level. But there is
still f, and the builtin approach doesn't help for int.

[Bug tree-optimization/56355] abs and multiplication

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-02-18
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> 2013-02-18 10:45:23 UTC ---
We lack gimple-level standard math function combining (basically all of
fold_* in builtins.c needs to be replicated by GIMPLE functionality).

[Bug tree-optimization/56355] abs and multiplication

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> 2013-03-20 15:40:30 UTC ---
Jeff Law mentions that it would be good to add this "squares are nonnegative"
information to VRP, not just tree_binary_nonnegative_warnv_p.
http://gcc.gnu.org/ml/gcc-patches/2013-03/msg00690.html

[Bug tree-optimization/56355] abs and multiplication

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> 2013-03-20 16:19:33 UTC ---
(In reply to comment #3)
> Jeff Law mentions that it would be good to add this "squares are nonnegative"
> information to VRP, not just tree_binary_nonnegative_warnv_p.
> http://gcc.gnu.org/ml/gcc-patches/2013-03/msg00690.html

Er, actually, since my patch, VRP does manage to deduce a non-negative range
for squares, although, sadly, when computing x*x, it doesn't manage to merge
the information from the interval of x and this non-negative property.

[Bug tree-optimization/56355] abs and multiplication

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2016-07-24 00:00:00         |2021-12-25

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The only one which I think is missing is:
(if (FLOAT_TYPE_P (type))
 (simplify
  (mult (abs @0) (abs @1))
  (abs (mult @0 @1)))

But I do think we can only do it for FLOAT_TYPE_P as
((unsigned)INT_MIN)*((unsigned)-1) will be different when using abs I think.

[Bug tree-optimization/56355] abs and multiplication

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56355

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note g has been optimized since GCC 6, r6-4098-g5d3498b4e03c in fact.