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[RFC] Attributes on types


I am looking at extending gcc's features by adding the ability to declare attributes on types. I.e. Making persistent attributes that stick to the type decleration and will give warnings/errors if any assignments across types are made:

1) int var __attribute__((a));

2)   int *ptr = &var;     // Should give warning because ptr's
                          // type does not contain the attribute 'a'

3)   int __attribute__((a)) *ptr = &var;
                          // Valid because of the attribute decl.

The first problem that arises from this syntax the fact that some attributes will work against the object, while obviously other attributes will target the type decleration involved with the object.

I have two proposals:

1a) int var __attribute__((a, weak));

'weak' will address the object 'var', while 'a' indicates type 'int __attribute__((a))'. Based by the design of the attribute 'a', it will tag itself to the decleration not the object.

1b) int __attribute__((a)) var __attribute__((weak));

As long as 'a' is listed prior to the object name, its a part of the type. It has better readability. And it might be simpler to implement, as the two attribute categories are not mixed.

In the 3rd line, please note that the attribute must be present before the '*'. If not, intermixed pointers to pointers (and so on and so forth) with or without attribute a will not be possible.

Other changes would include:
- typedef must support attributes

I have been digging hard into tree.h and the various c-xxx files, without any immediate enlightenments. Does anyone have any suggestion on where I should look and start working on this, please? If I can get this to work it would be a part of a /major/ feature enhancement for gcc and especially the AVR port.

The main point of this is to make GCC able to use different memory access methods based on the usage of these attributes. The AVR has more than one memory space; code and data space, and one needs to use completely different opcodes to access the code space than compared to the data space.

Regards, Svein

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