[LLVMdev] RFC: Supporting ELF symbol aliases via GlobalAlias GEPs

[David Chisnall]

Hi Everyone,

Chris suggested[1] I should ask for feedback as to whether this is a  
desired feature before I put too much effort into it, so here goes:

I would like to be able to export a symbol that is inside an LLVM  
structure.  This is possible on ELF targets[2], and the attached proof- 
of-concept patch to AsmWriter makes it work (although in a hackish way  
that I am NOT suggesting be committed as-is).

With this patch, the you can compile this:

%0 = type { i32, i32 }
@structure = global %0 { i32 0, i32 1 }
@element1 = alias getelementptr( %0* @structure, i32 0, i32 1)

To this:

     .subsections_via_symbols
     .section    __DATA,__data
     .align  3
_structure:                                                 ##  
@structure
     .space  4
     .long   1                                           ## 0x1

     .globl  _element1
     .set _element1, _structure+4
     .size   _element1, 4
     .type   _element1, at object

The element1 symbol is an i32* pointing to element 1 in the structure  
(the one emitted by .long 1).

There are really two questions here:

1) Do we want to be able to generate this kind of output at all (I do!)
2) If we do, do we want to use the global alias initialised with a  
constant GEP to do it, or provide some other mechanism?

David

[1] http://llvm.org/bugs/show_bug.cgi?id=4739#c24
[2] I know it's not possible on Mach-O (well, it is for internal  
symbols, just not for ones exported via the symbol table) - does  
anyone know if PE allows it?
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