https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88955
Bug ID: 88955
Summary: transparent_union for vector types not accepted
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: amonakov at gcc dot gnu.org
Target Milestone: ---
GCC (and Clang) rejects an attempt to create a transparent union corresponding
to an SSE register:
typedef unsigned long u64x2 __attribute__ ((vector_size (16)));
typedef union
__attribute__((transparent_union))
{
u64x2 u64;
} v128;
The diagnostic is not very clear about the reason:
union.i:3:9: warning: union cannot be made transparent
Looking in gdb, it appears that the union has mode TI, while its TYPE_FIELDS
has V2DI mode.
Adding a dummy __int128 field makes GCC accept the code (but such workaround
won't work for wider vectors, or on 32-bit).
The intended usecase for this is a vector union type for easier interop between
Intel intrinsics and user code written with generic vectors, as suggested by
Richard in https://gcc.gnu.org/ml/gcc-patches/2019-01/msg01186.html
