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Re: [PATCH] handle expressions in __builtin_has_attribute (PR 88383)


On Thu, Dec 13, 2018 at 02:05:29PM -0700, Martin Sebor wrote:
> > So how is the builtin defined then?  Is the argument always an expression
> > and you only return whether its type has the attribute, or do something
> > different if the expression is of certain kind?
> 
> For a DECL the built-in looks at both the DECL_ATTRIBUTES and
> the DECL's type's TYPE_ATTRIBUTES.  For expressions, it just
> looks at TYPE_ATTRIBUTES of the expression's type.  I'm not
> sure that this is necessarily the cleanest design but it's
> meant to follow how attributes are applied to DECLs.  Sometimes
> they get applied to the DECL itself and other times to its type.
> This has been causing confusion to both users and GCC devs such
> as myself so at some point I'd like to make this clearer somehow.
> Not sure exactly how yet.

But some users of the attribute look for the attribute on TYPE_ATTRIBUTES,
other on DECL_ATTRIBUTES, and the attributes have bools which say to what
they apply.  So, I think the API need to be very clear whether the attribute
is on a decl or on a type.

If it is similar to say sizeof/__alignof__ etc., where it accepts either
a type (for which one can use __typeof/decltype etc.), in which case it
would check the type attributes, or an expression and in that case require
it to be a decl and use decl attributes, one API is fine, but supporting
arbitrary expressions and sometimes looking at type, other times at decl
or both is not a good design.

	Jakub


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