overlay attributes in the compiler (overlaycall, overlaydata)

[edward-jones]

Opening a discussion from feedback on the Clang/LLVM patches.

https://reviews.llvm.org/D109372
https://reviews.llvm.org/D109371

Currently the naming of the attributes matches the spec, using separate attributes __attribute__((overlaycall)) and __attribute__((overlaydata)) for functions and global data respectively. There are a couple of possible changes that are worth discussing though:

• Renaming the attributes to overlay_call and overlay_data, or alternatively maybe something like overlay_function and overlay_data.
• Merging the attributes into a single overlay attribute.

The former change is mainly concerning readability. As far as I'm aware it's generally not consistent in GCC/Clang whether attributes include an underscore, but in generally snake case is acceptable and considered easier to read. Also I'm not sure whether overlay_function would be better than overlay_call because it's more explicit that the function exists in an overlay.

The second change is something which I implemented in a previous version of the Clang patch. It is possible to use a single overlay attribute and then differentiate entirely on what kind of Decl the attribute is applied to rather than using separate attributes. This makes the interface a bit simpler, but I can think of a couple of disadvatages:

• Diagnostics would either be less precise, or would have to differentiate functions vs data anyway
• Users might mistakenly believe that the attribute is a type attribute and then misunderstand the semantics when applying it to a global function pointer.

[edward-jones]

Updated the attribute names in #32