[cfe-dev] Where do we really need mangled names

[Greg Clayton]

On Feb 15, 2014, at 2:57 PM, David Blaikie <dblaikie at gmail.com> wrote:

> So when comparing Clang's debug info strings to GCC's I came across a couple of disparities hinging on the inclusion of linkage names on certain functions. Here are a few differences:
> 
> * Clang includes linkage names on file-local (static or anon namespace) functions. GCC does not.
> * Clang does not include the linkage name of member functions of function-local classes. GCC does, if the function is non-static/non-anonymous namespace and inline (ie: the member function has linkonce-odr linkage, not internal linkage)
> * Clang does not include the linkage name for constructors and destructors - this may be necessary due to the difference (GCC duplicates, Clang has one version call the other) in implementations, but I doubt it. I assume we still emit multiple member functions, one to describe each version of the ctor/dtor we're emitting.
> 
> It looks like at least for the first case, this may've been deliberate ( http://llvm.org/viewvc/llvm-project?view=revision&revision=154570 which doesn't explain why and points to rdar://11079003 which Jim Grosbach told didn't have a great deal more context) but I don't have enough context to understand why and whether it's just a GCC bug that they don't emit it, or something tools should handle better, etc.
> 
> So - any thoughts on the disparity and if/why it's necessary?

It is necessary if you want your debugger to be able to evaluate an expression that fully qualifies the name of the static:

(lldb) expr a::b::g_foo

If you don't have the mangled name, you can only do:

(lldb) expr g_foo

Then you might get many many many different versions if more than one file contains a static named "g_foo".

Also, users never tend to fully qualify names and might try to execute:

(lldb) expr b::g_foo

expecting to see "a::b::g_foo". LLDB is able to find these variables by first looking for all the variable base names that match "g_foo", then removing any whose demangled named doesn't contain "b::g_foo".

So the mangled names on statics allows debuggers to do the right thing and be able to correctly display qualified static variables and is very important for good debugging.

Greg