[LLVMdev] RFC: New Linkage Type linker_private_weak

[Chris Lattner]

On Jun 30, 2010, at 1:36 PM, Bill Wendling wrote:

> This patch introduces a the new "linker_private_weak" linkage type.
> 
> Why a new linkage type? The idea behind the "linker" linkage types is that they are used by the linker and then discarded. I.e., the symbols won't show up in the final linked image. In that regard, all "linker" linkage types are inherently "private". The "linker" linkages are currently used only by Objective-C metadata, and only Darwin's linker has the facility to handle them.

Right.

> However, they aren't restricted to Objective-C metadata or Darwin.

I'd say that the semantics aren't darwin-specific, but it is very unlikely that any other target would actually implement them.  In this way, they are similar to dllimport/dllexport which really are windows specific even though they could be described in a target independent way.

> The "linker_private_weak" linkage has similar semantics to the "linker_private" linkage: it's private to the linkage unit and doesn't appear in the final linked image. Unlike the linker_private linkage, it has a "weak" definition.

In other words, "redefinitions are merged, not reported as errors".

> I implemented the new linkage to have its own prefix from linker_private. As it currently stands, the symbol's prefix ("l") is the same for "linker_private" and "linker_private_weak". If this will always be the case, then I can remove the code in my patch that allows for a different prefix.

I'm quite certain that it will always be the case :), please remove MAI::LinkerPrivateWeakGlobalPrefix and Mangler::LinkerPrivateWeak.

Otherwise, the patch looks good.  Please commit it with 1) a testcase showing the next linkage type round trips through llvm-as/llvm-dis (in llvm/test/Feature/)   2) a codegen test for at least one darwin target that filechecks that the right directives are being generated.   3) a LangRef.html patch.

Thanks again for working on this!

-Chris