GCC tries to align a vector on its natural boundary, i.e., that given by its size, up to MAX_OBJECT_ALIGNMENT. Vectors that are bigger than that are either silently [mis]aligned on that same maximum boundary (PR 89798), silently truncated (and misaligned), or cause an ICE (PR 89797). Compiling the following:
__attribute__ ((vector_size (N))) char v; _Static_assert (sizeof (v) == N, "size"); _Static_assert (__alignof__ (v) == N, "alignment"); with N set to 1LLU << I shows these failures: I < 29 succeeds I < 31 fails alignment I < 32 ICE I >= 32 fails alignment and size Attribute aligned doesn't seem to have any effect on types or variables declared with attribute vector_size. The alignment set by the latter prevails. This happens no matter what scope the vector is defined in (i.e., file or local). I have some questions: 1) Is there some reason to align vectors on the same boundary as their size no matter how big it is? I can't find such a requirement in the ABIs I looked at. Or would it be more appropriate to align the big ones on the preferred boundary for the target? For instance, does it make more sense to align a 64KB vector on a 64KB boundary than on, say, a 64-byte boundary (or some other boundary less than 64K?) 2) If not, is it then appropriate to underalign very large vectors on a boundary less than their size? 3) Should the aligned attribute not override the default vector alignment? I would like to think the answer to (1) is that vectors should be aligned on the preferred boundary for the target/ABI. If that's feasible, it should also obviate question (2). I believe the answer to (3) is yes. If not, GCC should issue a warning that it doesn't honor the aligned attribute. Thanks Martin
