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.
