[LLVMdev] PROPOSAL : Introduce NamedMetadata

[Nick Lewycky]

Devang Patel wrote:
> In LLVM IR metadata is used to attach auxiliary information with
> various IR constructs. Currently metadata information is represented
> using MDNode and MDString. The metadata can refer to LLVM values but
> these references are not counted as regular "uses" of these values
> because metadata is maintained 'on the side'. This ensures that the
> optimizer is not influenced by auxiliary information. For example,
> 
> !1 = { i32 42 }
> 
> define void @foo() {
>   %x = call i8 @llvm.something(metadata !1)
>  }
> 
> 
> See http://nondot.org/~sabre/LLVMNotes/EmbeddedMetadata.txt for more
> information.
> 
> This metadata support is not useful if the auxiliary information is
> not referenced by any LLVM IR entity.  This is a limiting factor. The
> proposed solution is to introduced NamedMetadata. The NamedMetadata is
> derived from GlobalValue. The NamedMetadata is an array of metadata
> nodes and metadata strings.
> 
> 	@class.42 = ![ !5, !11, !2 ] ; NamedMetadata
> 
> 	!5 = metadata !"int"
> 	!11 = metadata !"class.21 pointer"
> 	!2 = metadata ! {I32 8 } ; size
> 
> 
> The NamedMetadata element list contains MDString and MDNodes only. The
> NamedMetadata is always implicitly typed as metadata. LLVM bitcode
> reader and writer will keep track of NamedMetadata in a Module. A
> module can use metadata that is not listed in any NamedMetadata value.
>  A module can have multiple NamedMetadata values. NamedMetadata
> implicitly uses AppendingLinkage. A  NamedMetadata value has zero
> uses.
> 
> The NamedMetadata can be used to describe Front End specific types for
> the optimizer's use. Another potential use is to encode debug
> information for a global variable.  I do not intend to make @llvm.used
> a NamedMetadata value.

Why not have a named GlobalValue with an MDNode initializer? How is this 
different from what we had before?

Nick