[LLVMdev] MachO and ELF Writers/MachineCodeEmittersarehard-codedinto LLVMTargetMachine

[someguy]

Aaron, I mailed in the same mail twice (by mistake), you answered both
copies. Differently!

In any case, I've re-read what exists. I'm dumping what I understand
here, so that we can discuss in detail. I'm using MachO as the example
object format, as the ELF code is totally broken and outdated. Lets
use the following as the basis for our discussion?

There are 3 classes which participate in object file emission:
1. MachOWriter
    - a MachineFunctionPass, with a donothing runOnMachineFunction.

doInitialization and doFinalization are used to emit the object file
header and finalize the various object file segments, respectively.

The MachOWriter is responsible for creation of MachOCodeEmitter, via
it's getMachineCodeEmitter function.

2. MachOCodeEmitter
   - a MachineCodeEmitter. Friend class of MachOWriter (friend class
== broken encapsulation!?)

startFunction allocates storage in the text section for the current function.

finishFunction emits constant-pools, jumptables; transforms
relocations adding globals to the MachOWriter's PendingGlobals list,
and all relocations to the parent section's relocation list; adds a
symbol for the function to the MachOWriter's SymbolTable.

[In general, all the operations in finishFunction actually modify the
data of the MachOWriter. Shouldn't these be pushed into the
MachOWriter? ]


3. X86CodeEmitter
   - a MachineFunctionPass, NOT a MachineCodeEmitter (Could the naming
change perhaps?)

This class receives (during construction) a reference to a
MachineCodeEmitter (e.g. MachOCodeEmitter, which in turn stores a
reference to a MachOWritter).

The runOnMachineFunction for the X86CodeEmitter does:
- call MachOCodeEmitter::startFunction
- for each basicblock in function:
   - call MachOCodeEmitter::StartMachineBasicBlock
   - for each instruction in basicblock:
       - emit instruction, using MachineCodeEmitter::emit* functions
- call MachOCodeEmitter::finishFunction

[This runOnMachineFunction could definitely be generalized, i.e.
implemented in a base class ('EmitterMachineFunctionPass' or a better
name). This base class would then have (abstract) emitInstruction,
emitOperand, etc... methods. It should also integrate with the
*GenCodeEmitter emitted by tblgen so that you get automatic code
emission. When implementing a new target, one would simply need to
inherit the baseclass, and override the functions necessary to tweak
output.]