On Mon, 17 Nov 2014, Jakub Jelinek wrote: > > If it is true that a type satisfying TYPE_USER_ALIGN will never be > > allocated at an address less-aligned than its TYPE_ALIGN, even if that's > > greater than BIGGEST_ALIGNMENT, then the change seems correct for C11 > > _Alignof. > > I think it depends on which target and where. > In structs (unless packed) the user aligned fields should be properly > aligned with respect to start of struct and the struct should have user > alignment in that case, automatic vars these days use alloca with > realignment if not handled better by the target, so should be fine too. > For data section vars and for common vars I think it really depends on the > target. Perhaps for TYPE_USER_ALIGN use minimum of the TYPE_ALIGN and > MAX_OFILE_ALIGNMENT ? > For heap objects, it really depends on how it has been allocated, but if > allocated through malloc, the extra alignment is never guaranteed. > So, it really depends in malloc counts or not.
The question, for both _Alignas and ubsan, is the alignment guaranteed *in valid programs*. malloc only provides sufficient alignment for types with fundamental alignment requirements (although there are various problems with the C11 wording; see DR#445). So if you use malloc to allocate a type with an extended alignment requirement (without doing extra realignment on the result of malloc), that's not a valid program. And if an alignment is larger than MAX_OFILE_ALIGNMENT, you get an error for declaring non-stack variables requiring that alignment. So I don't think there's any need to check MAX_OFILE_ALIGNMENT here. -- Joseph S. Myers jos...@codesourcery.com
