Issue 2 with "[libstdc++/65033] Give alignment info to libatomic"

[Joseph Myers]

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
joseph@codesourcery.com