SIFixSGPRCopies.cpp   [plain text]

//===-- SIFixSGPRCopies.cpp - Remove potential VGPR => SGPR copies --------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
/// \file
/// Copies from VGPR to SGPR registers are illegal and the register coalescer
/// will sometimes generate these illegal copies in situations like this:
///
///  Register Class <vsrc> is the union of <vgpr> and <sgpr>
///
/// BB0:
///   %vreg0 <sgpr> = SCALAR_INST
///   %vreg1 <vsrc> = COPY %vreg0 <sgpr>
///    ...
///    BRANCH %cond BB1, BB2
///  BB1:
///    %vreg2 <vgpr> = VECTOR_INST
///    %vreg3 <vsrc> = COPY %vreg2 <vgpr>
///  BB2:
///    %vreg4 <vsrc> = PHI %vreg1 <vsrc>, <BB#0>, %vreg3 <vrsc>, <BB#1>
///    %vreg5 <vgpr> = VECTOR_INST %vreg4 <vsrc> 
///
/// 
/// The coalescer will begin at BB0 and eliminate its copy, then the resulting
/// code will look like this:
///
/// BB0:
///   %vreg0 <sgpr> = SCALAR_INST
///    ...
///    BRANCH %cond BB1, BB2
/// BB1:
///   %vreg2 <vgpr> = VECTOR_INST
///   %vreg3 <vsrc> = COPY %vreg2 <vgpr>
/// BB2:
///   %vreg4 <sgpr> = PHI %vreg0 <sgpr>, <BB#0>, %vreg3 <vsrc>, <BB#1>
///   %vreg5 <vgpr> = VECTOR_INST %vreg4 <sgpr>
///
/// Now that the result of the PHI instruction is an SGPR, the register
/// allocator is now forced to constrain the register class of %vreg3 to
/// <sgpr> so we end up with final code like this:
/// 
/// BB0:
///   %vreg0 <sgpr> = SCALAR_INST
///    ...
///    BRANCH %cond BB1, BB2
/// BB1:
///   %vreg2 <vgpr> = VECTOR_INST
///   %vreg3 <sgpr> = COPY %vreg2 <vgpr>
/// BB2:
///   %vreg4 <sgpr> = PHI %vreg0 <sgpr>, <BB#0>, %vreg3 <sgpr>, <BB#1>
///   %vreg5 <vgpr> = VECTOR_INST %vreg4 <sgpr>
///
/// Now this code contains an illegal copy from a VGPR to an SGPR. 
///
/// In order to avoid this problem, this pass searches for PHI instructions
/// which define a <vsrc> register and constrains its definition class to
/// <vgpr> if the user of the PHI's definition register is a vector instruction.
/// If the PHI's definition class is constrained to <vgpr> then the coalescer
/// will be unable to perform the COPY removal from the above example  which
/// ultimately led to the creation of an illegal COPY.
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "SIInstrInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetMachine.h"

using namespace llvm;

namespace {

class SIFixSGPRCopies : public MachineFunctionPass {

private:
  static char ID;
  const TargetRegisterClass *inferRegClass(const TargetRegisterInfo *TRI,
                                           const MachineRegisterInfo &MRI,
                                           unsigned Reg) const;

public:
  SIFixSGPRCopies(TargetMachine &tm) : MachineFunctionPass(ID) { }

  virtual bool runOnMachineFunction(MachineFunction &MF);

  const char *getPassName() const {
    return "SI Fix SGPR copies";
  }

};

} // End anonymous namespace

char SIFixSGPRCopies::ID = 0;

FunctionPass *llvm::createSIFixSGPRCopiesPass(TargetMachine &tm) {
  return new SIFixSGPRCopies(tm);
}

/// This functions walks the use/def chains starting with the definition of
/// \p Reg until it finds an Instruction that isn't a COPY returns
/// the register class of that instruction.
const TargetRegisterClass *SIFixSGPRCopies::inferRegClass(
                                                 const TargetRegisterInfo *TRI,
                                                 const MachineRegisterInfo &MRI,
                                                 unsigned Reg) const {
  // The Reg parameter to the function must always be defined by either a PHI
  // or a COPY, therefore it cannot be a physical register.
  assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
         "Reg cannot be a physical register");

  const TargetRegisterClass *RC = MRI.getRegClass(Reg);
  for (MachineRegisterInfo::use_iterator I = MRI.use_begin(Reg),
                                         E = MRI.use_end(); I != E; ++I) {
    switch (I->getOpcode()) {
    case AMDGPU::COPY:
      RC = TRI->getCommonSubClass(RC, inferRegClass(TRI, MRI,
                                                    I->getOperand(0).getReg()));
      break;
    }
  }

  return RC;
}

bool SIFixSGPRCopies::runOnMachineFunction(MachineFunction &MF) {
  MachineRegisterInfo &MRI = MF.getRegInfo();
  const TargetRegisterInfo *TRI = MF.getTarget().getRegisterInfo();
  for (MachineFunction::iterator BI = MF.begin(), BE = MF.end();
                                                  BI != BE; ++BI) {

    MachineBasicBlock &MBB = *BI;
    for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
                                                      I != E; ++I) {
      MachineInstr &MI = *I;
      if (MI.getOpcode() != AMDGPU::PHI) {
        continue;
      }
      unsigned Reg = MI.getOperand(0).getReg();
      const TargetRegisterClass *RC = inferRegClass(TRI, MRI, Reg);
      if (TRI->getCommonSubClass(RC, &AMDGPU::VReg_32RegClass)) {
        MRI.constrainRegClass(Reg, &AMDGPU::VReg_32RegClass);
      }
    }
  }
  return false;
}