[LLVMdev] metadata syntax

[Nick Lewycky]

I'm looking for input into the syntax for future metadata work. The plan 
is to make MDNodes hold a list of WeakVHs which will allow us to track 
metadata associated with values even through calls to 
ReplaceAllUsesWith. It also means that you can refer to other Values in 
the program, including instructions in another Function.

That's where the problems begin. There's currently no way in llvm 
assembly to refer to instructions in another function, though that's not 
hard to solve; I propose "@func/%tmp" as the syntax. The harder problem 
is how to express references to void instructions (stores, branches, 
switches) which historically have not been allowed to have names since 
they could never be referred to.

The obvious solution is to let them have names. The trouble is that this 
breaks .ll syntax for any program that's using old-style anonymous 
instructions. For example:

define i32 @foo(i32* %ptr) {
   add i32 1, 2             ; %1
   store i32 0, i32* %ptr   ; previously ignored, now %2
   add i32 3, 4             ; previously %2, now %3
   ret i32 %2    ;; illegal! refers to the store not the add
}

Is this okay? Or do we need to come up with a different solution? Any 
suggestions?

Nick