https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109543
Bug ID: 109543
Summary: Avoid using BLKmode for unions with a non-BLKmode
member when possible
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: avieira at gcc dot gnu.org
Target Milestone: ---
Hi,
So with the following C-code:
$ cat t.c
#include <arm_neon.h>
#ifdef GOOD
typedef float64x2x2_t TYPE;
#else
typedef union
{
float64x2x2_t v;
double d[4];
} TYPE;
#endif
void foo (TYPE *a, TYPE *b, TYPE *c, unsigned n)
{
TYPE X = a[0];
TYPE Y = b[0];
TYPE Z = c[0];
for (unsigned i = 0; i < n; ++n)
{
TYPE temp = X;
X = Y;
Y = Z;
Z = temp;
}
a[0] = X;
b[0] = Y;
c[0] = Z;
}
If compiled for aarch64 with -DGOOD the compiler will use vector register moves
in the loop, whereas without -DGOOD it will use the stack with memmoves.
The reason for this is because when picking the mode to address a UNION with
gcc will always choose BLKmode as soon as any member of a UNION is BLKmode. In
such cases I think it would be safe to go with not-BLKmode of members that have
the same size as the entire UNION?
