MipsMulMulBugPass.cpp

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//===- MipsMulMulBugPass.cpp - Mips VR4300 mulmul bugfix pass -------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Early revisions of the VR4300 have a hardware bug where two consecutive
// multiplications can produce an incorrect result in the second multiply.
//
// This pass scans for mul instructions in each basic block and inserts
// a nop whenever the following conditions are met:
//
// - The current instruction is a single or double-precision floating-point
//   mul instruction.
// - The next instruction is either a mul instruction (any kind)
//   or a branch instruction.
//===----------------------------------------------------------------------===//
 
#include "Mips.h"
#include "MipsInstrInfo.h"
#include "MipsSubtarget.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetMachine.h"
 
#define DEBUG_TYPE "mips-vr4300-mulmul-fix"
 
using namespace llvm;
 
namespace {
 
class MipsMulMulBugFix : public MachineFunctionPass {
public:
  MipsMulMulBugFix() : MachineFunctionPass(ID) {
    initializeMipsMulMulBugFixPass(*PassRegistry::getPassRegistry());
  }
 
  StringRef getPassName() const override { return "Mips VR4300 mulmul bugfix"; }
 
  MachineFunctionProperties getRequiredProperties() const override {
    return MachineFunctionProperties().set(
        MachineFunctionProperties::Property::NoVRegs);
  }
 
  bool runOnMachineFunction(MachineFunction &MF) override;
 
  static char ID;
 
private:
  bool fixMulMulBB(MachineBasicBlock &MBB, const MipsInstrInfo &MipsII);
};
 
} // namespace
 
INITIALIZE_PASS(MipsMulMulBugFix, "mips-vr4300-mulmul-fix",
                "Mips VR4300 mulmul bugfix", false, false)
 
char MipsMulMulBugFix::ID = 0;
 
bool MipsMulMulBugFix::runOnMachineFunction(MachineFunction &MF) {
  const MipsInstrInfo &MipsII =
      *static_cast<const MipsInstrInfo *>(MF.getSubtarget().getInstrInfo());
 
  bool Modified = false;
 
  for (auto &MBB : MF)
    Modified |= fixMulMulBB(MBB, MipsII);
 
  return Modified;
}
 
static bool isFirstMul(const MachineInstr &MI) {
  switch (MI.getOpcode()) {
  case Mips::FMUL_S:
  case Mips::FMUL_D:
  case Mips::FMUL_D32:
  case Mips::FMUL_D64:
    return true;
  default:
    return false;
  }
}
 
static bool isSecondMulOrBranch(const MachineInstr &MI) {
  if (MI.isBranch() || MI.isIndirectBranch() || MI.isCall())
    return true;
 
  switch (MI.getOpcode()) {
  case Mips::MUL:
  case Mips::FMUL_S:
  case Mips::FMUL_D:
  case Mips::FMUL_D32:
  case Mips::FMUL_D64:
  case Mips::MULT:
  case Mips::MULTu:
  case Mips::DMULT:
  case Mips::DMULTu:
    return true;
  default:
    return false;
  }
}
 
bool MipsMulMulBugFix::fixMulMulBB(MachineBasicBlock &MBB,
                                   const MipsInstrInfo &MipsII) {
  bool Modified = false;
 
  MachineBasicBlock::instr_iterator NextMII;
 
  // Iterate through the instructions in the basic block
  for (MachineBasicBlock::instr_iterator MII = MBB.instr_begin(),
                                         E = MBB.instr_end();
       MII != E; MII = NextMII) {
 
    NextMII = next_nodbg(MII, E);
 
    // Trigger when the current instruction is a mul and the next instruction
    // is either a mul or a branch in case the branch target start with a mul
    if (NextMII != E && isFirstMul(*MII) && isSecondMulOrBranch(*NextMII)) {
      LLVM_DEBUG(dbgs() << "Found mulmul!\n");
 
      const MCInstrDesc &NewMCID = MipsII.get(Mips::NOP);
      BuildMI(MBB, NextMII, DebugLoc(), NewMCID);
      Modified = true;
    }
  }
 
  return Modified;
}
 
FunctionPass *llvm::createMipsMulMulBugPass() { return new MipsMulMulBugFix(); }