Re: [REPOST] Invalid Code when reading from unaligned zero-sized array

["Joseph S. Myers" <joseph at codesourcery dot com>]

On Mon, 16 Dec 2013, Eric Botcazou wrote:

> which of course blatantly violates the do-not-rely-on-mode rule.  Although the 
> layout change apparently occurs very rarely, I think that this rules out the 
> direct mode change in stor-layout.c... <snip>

Well - makes such a change unsuitable for 4.9.  Longer-term we still want 
to avoid ABIs depending on modes of structs / unions / arrays (and 
depending on modes of vectors can also have undesired effects in some 
cases - I took care when working on the ARM hard-float ABI variant to 
avoid the ABI for argument passing of GCC generic vectors depending on 
whether NEON vector support was enables).  That suggests to me:

* Produce a list of all ABI-affecting target macros and hooks that might, 
wrongly, use modes to determine the ABI (struct layout, alignment, 
argument passing, function return, etc.).

* Put that on a wiki page along with a description of the problem (when 
it's OK to use modes in determining the ABI and when it isn't).  Also make 
sure the documentation of the macros / hooks in the internals manual is 
clear about not using modes inappropriately.

* Put a table of target architectures on the wiki page with a column for 
architecture maintainers to record whether they have checked for and fixed 
any problematic uses of modes.

* Ask architecture maintainers to do the checks and fixes.  Fixes may be 
nontrivial in some cases, as compatibility means that if the ABI did 
depend on the mode you may need to replicate the relevant bits of 
stor-layout.c logic that determined the mode inside the back end.

* After a while (pinging maintainers as needed), obsolete then remove any 
architectures that have not been checked.

-- 
Joseph S. Myers
joseph@codesourcery.com