Re: [PATCH 1/3] improve detection of attribute conflicts (PR 81544)

[Joseph Myers <joseph at codesourcery dot com>]

On Wed, 9 Aug 2017, Martin Sebor wrote:

> the same function with the other of this pair attributes.  I'd
> also be okay with not diagnosing this combination if I could
> convince myself that it's safe (or can be made safe) and treated
> consistently.

I'd expect it to be safe; it might simply happen that some calls are 
optimized based on incomplete information (and even that is doubtful, as 
all optimizations except front-end folding should be taking place after 
all the declarations have been seen).

> The problem whose subset the diagnostic detects is declaring
> the function const and calling it before the pure declaration
> that corresponds to its definition is seen.  I.e., the inverse
> of what the ssa-ccp-2.c test does.  I think a mismatch between
> the attributes is as suspect as a mismatch in the function's
> signature.

In the ssa-ccp-2.c case, it's not clear to me the test intends to use the 
same function at all; maybe one of the foo9 functions should be renamed.

I think it's actually like having a non-prototype declaration and a 
prototype one, where a composite type is formed from two compatible types.  
(We have a warning in the very specific case of a non-prototype 
*definition* followed by a prototype declaration.)

> In any event, it sounds to me that conceptually, it might be
> useful to be able to specify which of a pair of mutually
> exclusive (or redundant) attributes to retain and/or accept:
> the first or the second, and not just whether to accept or
> drop the most recent one.  That will make it possible to make
> the most appropriate decision about each specific pair of
> attributes without impacting any of the others.

If they conflict, I'm not sure there's any basis for making a choice 
specific to a particular pair of attributes.

In cases like const and pure where one is a subset of the other, it makes 
sense to choose based on the pair of attributes - and not necessarily to 
warn under the same conditions as the warnings for conflicting attributes.

-- 
Joseph S. Myers
joseph@codesourcery.com