On Mon, 13 Apr 2015, Hans-Peter Nilsson wrote: > b.cc:5:25: warning: requested alignment 16 is larger than 8 [-Wattributes] > alignas (16) char x[16]; > > which is mysterious (where does the 8 come from?), until I grep > the error string and find > c-family/c-common.c:check_cxx_fundamental_alignment_constraints. > > In there, I see target macros used, among them > BIGGEST_ALIGNMENT. This is 8 for cris-elf: the *bit alignment* > (there's a bug there already; bits not bytes) of the biggest > *required* alignment (modulo atomics) for types, not the biggest > *supported* alignment. So, the wrong macro (and unit) is used. > Similarly, BIGGEST_FIELD_ALIGNMENT is about *require*, not > *support*. Changing either macro is also an ABI change. > > Why not allow the presumably most relaxed value for types, like > for __attribute__ ((__aligned__())), i.e. MAX_OFILE_ALIGNMENT, > then a tighter alignment check when declaring an object?
When I was making sure C diagnosed unsupported extended alignments, my conclusion was that the support for realigning the stack means that MAX_OFILE_ALIGNMENT is supported everywhere (except for malloc, of course). See <https://gcc.gnu.org/ml/gcc-patches/2013-11/msg02187.html> for my analysis. -- Joseph S. Myers jos...@codesourcery.com
