[LLVMdev] metadata syntax

Chris Lattner clattner at apple.com
Sat Apr 25 13:49:05 PDT 2009

On Apr 22, 2009, at 12:14 AM, Nick Lewycky wrote:

> 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.

I don't see a great need to support references to values in different  

> 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?

We can't break this.  I'd suggest putting void-value'd stuff in a  
separate namespace, perhaps !123 ?

That way they'd be numbered independently of the %'s.


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