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Re: [PATCH] Add named address support to GCC 4.5


On Sun, 19 Apr 2009, Chris Lattner wrote:

> 
> On Apr 19, 2009, at 9:16 PM, Ben Elliston wrote:
> 
> > On Fri, 2009-04-17 at 09:58 -0700, Chris Lattner wrote:
> > 
> > > Out of curiosity, what is the motivation for making these actual
> > > keywords?  Clang has support for address spaces as well, and clients
> > > that use them just install a predefine that expands to an attribute.
> > > For example, cellspu could install:
> > > 
> > > #define __ea __attribute__((address_space(1)))
> > > Which enables, stuff like:
> > > 
> > > int __ea *P;
> > 
> > Won't the attribute apply to the whole type?  We want to be able to have
> > a multiplicity of types like:
> > 
> > 	int __ea *p;
> > 	int * __ea p;
> 
> No, attribute address_space is parsed as part of type-qualifier-list (and
> apply to types, not decls), which means that things like:
> 
> int __attribute__((address_space(1))) * __attribute__((address_space(2)))
> *__attribute__((address_space(3))) P;
> 
> Do what you'd expect.

The attribute syntax rules for GNU C ("Attribute Syntax" in the manual) 
mean that an attribute among the initial declaration specifiers generally 
applies to the declaration as a whole rather than acting like a type 
qualifier.  If you want a particular attribute to act like a type 
qualifier there you have to do some elaborate type reconstruction, as done 
for vector_size attributes.  No doubt that reconstruction code could be 
reused, but it's still a significant complication.

In general, experience shows that having the implementation not follow the 
terminology and concepts of the specification is liable to cause trouble 
and lead to an inaccurate implementation.  In this case, TR 18037 
describes address spaces as type qualifiers, so the implementation is most 
likely to be accurate if they go through the code paths for qualifiers 
instead of those for attributes.

-- 
Joseph S. Myers
joseph@codesourcery.com


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