[clang] [clang][DebugInfo] Attach DW_AT_const_value to static data-member definitions if available (PR #72730)

[David Blaikie via cfe-commits]

dwblaikie wrote:

> My understanding was that the DIExpression parameter to DIGlobalVariableExpression was empty for global variables with locations. So the patch just encodes the constant into that expression if it's otherwise empty.

I think in theory it can be non-empty (see what happens under a merge globals optimization... I'm not sure of an example of when this optimization fires, or what the debug info we emit looks like - in theory it should look like a global with a non-empty expression that describes offsetting the pointer forward to reach the global inside the merged global) & so then you'd have a hard time telling whether the expression is meant to be used in addition to the location, or as part of evaluating the location.

We don't really have a mechanism for encoding a variable in two locations (or a constant and a location) at the same time, I think. We could invent a special opcode to use in the expression to communicate this, or define some special handling if there's two separate expressions providing a location (or a location and a constant in this case) for the same variable (say that they're both valid, and emit them both if we can). 

@adrian-prantl thoughts?

https://github.com/llvm/llvm-project/pull/72730