[LLVMdev] MachO and ELF Writers/MachineCodeEmitters are hard-coded into LLVMTargetMachine

[someguy]

Currently, the MachO and ELF Writers and MachineCodeEmitters are
hard-coded into LLVMTargetMachine and llc.

In other words, the 'object file generation' capabilities of the
Common Code Generator are not generic.

LLVMTargetMachine::addPassesToEmitFile explicitly checks whether the
derived backend TargetMachine implements one of getMachOWriterInfo or
getELFWriterInfo, and returns a corresponding FileModel enum value.

llc's main function uses the resulting FileModel value to determine
which of the {AddMachOWriter,AddELFWriter} functions to call.

This is limiting for a number of reasons:
1. If a given platform (e.g. x86) may support both MachO and ELF,
MachO will be selected, as it is checked first. This is bad behaviour,
it should be up to the user to decide which object format he wants.
2. Extension of the object file generation capabilities to include new
object file formats is difficult, and requires modifications to LLVM
code (not just a plugin).

I suggest transforming the {getMachOWriterInfo, getELFWriterInfo}
functions (on TargetMachine) into a single (templated?)
getObjectFileWriterInfo function. Additionally a addObjectFileWriter
member should be added  to TargetMachine, taking the place of the
static {AddMachOWriter, AddELFWriter} functions.

As I need this functionality (custom object file generation) for my
current target, I'd be happy to make the modifications to the LLVM
core. Before I do so, I'd like to get feedback on my proposed
solution.

I've added a bug for this issue: http://llvm.org/bugs/show_bug.cgi?id=3813