On Mon, May 26, 2025 at 2:23 PM John Paul Adrian Glaubitz <[email protected]> wrote: > > On Mon, 2025-05-26 at 14:16 -0400, Jeffrey Walton wrote: > > I don't know about the 64 byte alignment on NetBSD. Maybe it was > > copied/pasted/modified from amd64? amd64 has a BIGGEST_ALIGNMENT of 64 > > due to AVX512. Prior to that, I believe amd64 had a BIGGEST_ALIGNMENT > > of 32 due to AVX. (These were due to the aligned loads, like > > _mm256_load_epi32 (32-byte requirement) or _mm512_load_epi32 (64-byte > > requirement)). > > > > For PowerPC with Altivec, I believe BIGGEST_ALIGNMENT should be 16. > > > > I don't know if __ALTIVEC__ plays a part in BIGGEST_ALIGNMENT. That > > is, should BIGGEST_ALIGNMENT be 4 (for int) or 8 (for doubles) unless > > __ALTIVEC__ is defined; and if __ALTIVEC__ is defined, then > > BIGGEST_ALIGNMENT should be 16. > > The alignment is specified in bits here, not bytes. Hence a BIGGEST_ALIGNMENT > of 64 means 8 bytes, not 64 bytes.
Derp... Maybe the 64-bits/8-bytes is coming from limitations in the linker. See the discussion of __BIGGEST_ALIGNMENT__ at <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>: On some systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8-byte alignment, then specifying aligned(16) in an __attribute__ still only provides you with 8-byte alignment. Jeff
