[LLVMdev] Creating global data with [ 0 x ... ] type

[Gordon Henriksen]

On Sep 2, 2008, at 21:07, Scott Graham wrote:

> I'm trying to create some global data representing something similar
> to a vtable in C++ (that'll be pointed to by the head of objects).
> I've called this a "metadata" table below. It includes some
> miscellaneous class-level information, as well as a typical vtable
> list-of-function-pointers.
>
> I need to have the vtable "inlined" into the metadata, in that I don't
> want an extra indirection by storing a pointer to the vtable in the
> metadata table. This means that the metadata table ends in a variable
> sized array.
>
> i.e.:
>
> %VtableEntryType = type { i32, i32 }
> %MetadataTable = type { i32, [0 x %VtableEntryType] }
>
> However, I can't figure out how to initialize it:
>
> ; most entries elided for simplicity
> @MyMetadataTable = global %MetadataTable {
> 	i32 0,
> 	[1 x %VtableEntryType] [ ; <--- problem here!
> 		%VtableEntryType { i32 1234, i32 1234 }
> 	]
> }
>
>
> bitcast won't let me cast from [1 x %VtableEntryType] to [0 x
> %VtableEntryType] (I guess because the size differs). Is there another
> way to accomplish this initialization?
>
> I currently have a different metadata-type per language-type, but when
> there's 50k types in the system, it gets overly heavy to declare all
> those types when the only thing that ever happens to @MyMetadataTable
> is a store into the header of an object during allocation.


Hi Scott,

As you've already discovered, your global variable must be defined  
using its actual storage type, which includes using a concrete array  
length. Bitcast the GlobalVariable from { ..., [n x ...] }* to { ...,  
[0 x ...] }* at the use sites instead (note: casting the pointer type,  
not the struct type). You do not need to name the types, of course.

Note that the LLVM linker will introduce these bitcasts for you  
automatically (as will, in effect, the system linker).

— Gordon