[Bug tree-optimization/105451] New: miss optimizations due to inconsistency in complex numbers associativity

[Bug tree-optimization/105451] New: miss optimizations due to inconsistency in complex numbers associativity

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105451

            Bug ID: 105451
           Summary: miss optimizations due to inconsistency in complex
                    numbers associativity
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: tnfchris at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---

The two functions

#include <complex.h>

void f (complex double * restrict a, complex double *restrict b,
        complex double *restrict c, complex double *res, int n)
{
  for (int i = 0; i < n; i++)
   res[i] = a[i] * (b[i] * c[i]);
}

and

void g (complex double * restrict a, complex double *restrict b,
        complex double *restrict c, complex double *res, int n)
{
  for (int i = 0; i < n; i++)
   res[i] = (a[i] * b[i]) * c[i];
}

At -Ofast produce the same code, but internally they get there using different
SLP trees.

The former creates a chain of VEC_PERM_EXPR nodes as is expected
tinyurl.com/cmulslp1 however the latter avoids the need of the permutes by
duplicating the elements of the complex number https://tinyurl.com/cmulslp2

The former we can detect as back to back complex multiplication but the latter
we can't.

Not sure what the best way to get consistency here is.

[Bug tree-optimization/105451] miss optimizations due to inconsistency in complex numbers associativity

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105451

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-05-02

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's difficult to even spot the difference in the SLP tree, but the
associatable multiplications make it somewhat random which of the valid SLP
tree representations we'll pick.  All of the SLP discovery is a heuristically
greedy search for the _first_ match, not for the "best" even ("best"
interpreted as "largest" in case of BB vectorization).

The only thing that reliably happens at the moment is associating until the
leafs are all from one load group (until we support multiple load groups in
one leaf).