Here's a wild idea:
Currently, DIVariable is a variable-length field that has an optional reference to a Metadata array consisting of a variable number of complex address expressions. In the case of OpPiece expressions this is wasting a lot of storage in IR, because when an aggregate type is, e.g., SROA'd into all of its n individual members, the IR will contain n copies of the DIVariable, all alike, only differing in the complex address reference at the end.
By making the complex address into an extra argument of the dbg.value/dbg.declare intrinsics, all of the pieces can reference the same variable and the complex address expressions can be uniqued across the CU, too.
Down the road, this will allow us to move other flags, such as "indirection" out of the DIVariable, too.
The new intrinsics look like this:
declare void @llvm.dbg.declare(metadata %storage, metadata %expr, metadata %var)
declare void @llvm.dbg.value(metadata %storage, i64 %offset, metadata %expr, metadata %var)
This patch adds a unique offset to the values of ComplexAddrKind that doubles as a tag, so we can detect and pretty-print DIExpression metadata nodes.
What this patch doesn't do:
- This patch does not touch the "Indirect" field in DIVariable; but moving that into the expression would be a natural next step.