[LLVMdev] RFC: Code Gen Change!

Bill Wendling isanbard at gmail.com
Tue May 12 17:46:17 PDT 2009

I just finished coding up a change to how code generation builds
machine instructions. The change is in
include/llvm/CodeGen/MachineInstrBuilder.h, where when you want to add
a register, you have to specify a long list of booleans indicating if
it's defined, implicit, killed, dead, or early clobbered. I don't know
about you, but it was hard for me to read the source and understand
what was going on without looking at the header file. So, I made these

Instead of all of the booleans, you pass in a flag that has bits set
to indicate what state the register is in:

namespace RegState {
  enum {
    Define         = 0x1,
    Implicit       = 0x2,
    Kill           = 0x4,
    Dead           = 0x8,
    EarlyClobber   = 0x10,
    ImplicitDefine = Implicit | Define,
    ImplicitKill   = Implicit | Kill

class MachineInstrBuilder {
  MachineInstr *MI;
  explicit MachineInstrBuilder(MachineInstr *mi) : MI(mi) {}

  /// addReg - Add a new virtual register operand...
  MachineInstrBuilder &addReg(unsigned RegNo, unsigned flags = 0,
                              unsigned SubReg = 0) const {
                                             flags & RegState::Define,
                                             flags & RegState::Implicit,
                                             flags & RegState::Kill,
                                             flags & RegState::Dead,
                                             flags & RegState::EarlyClobber));
    return *this;

To use this, you would do something like:

BuildMI(...).addReg(Reg, RegState::Kill);


In a lot of cases, an explicit true/false isn't passed into the addReg
method. I added some helper functions to help make this less of a

inline unsigned getDefRegState(bool B) {
  return B ? RegState::Define : 0;
inline unsigned getImplRegState(bool B) {
  return B ? RegState::Implicit : 0;
inline unsigned getKillRegState(bool B) {
  return B ? RegState::Kill : 0;
inline unsigned getDeadRegState(bool B) {
  return B ? RegState::Dead : 0;

So you can use them like this:

BuildMI(...).addReg(Reg, getKillRegState(isKill);

My hope is that this is a cleaner way of building these machine instructions.

Comments and suggestions are welcome.


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