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//==- llvm/CodeGen/BreakFalseDeps.cpp - Break False Dependency Fix -*- C++ -*==//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
/// \file Break False Dependency pass.
/// Some instructions have false dependencies which cause unnecessary stalls.
/// For exmaple, instructions that only write part of a register, and implicitly
/// need to read the other parts of the register. This may cause unwanted
/// stalls preventing otherwise unrelated instructions from executing in
/// parallel in an out-of-order CPU.
/// This pass is aimed at identifying and avoiding these depepndencies when
/// possible.
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/ReachingDefAnalysis.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
using namespace llvm;
namespace llvm {
class BreakFalseDeps : public MachineFunctionPass {
MachineFunction *MF;
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
RegisterClassInfo RegClassInfo;
/// List of undefined register reads in this block in forward order.
std::vector<std::pair<MachineInstr *, unsigned>> UndefReads;
/// Storage for register unit liveness.
LivePhysRegs LiveRegSet;
ReachingDefAnalysis *RDA;
static char ID; // Pass identification, replacement for typeid
BreakFalseDeps() : MachineFunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
/// Process he given basic block.
void processBasicBlock(MachineBasicBlock *MBB);
/// Update def-ages for registers defined by MI.
/// Also break dependencies on partial defs and undef uses.
void processDefs(MachineInstr *MI);
/// Helps avoid false dependencies on undef registers by updating the
/// machine instructions' undef operand to use a register that the instruction
/// is truly dependent on, or use a register with clearance higher than Pref.
/// Returns true if it was able to find a true dependency, thus not requiring
/// a dependency breaking instruction regardless of clearance.
bool pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
unsigned Pref);
/// Return true to if it makes sense to break dependence on a partial
/// def or undef use.
bool shouldBreakDependence(MachineInstr *, unsigned OpIdx, unsigned Pref);
/// Break false dependencies on undefined register reads.
/// Walk the block backward computing precise liveness. This is expensive, so
/// we only do it on demand. Note that the occurrence of undefined register
/// reads that should be broken is very rare, but when they occur we may have
/// many in a single block.
void processUndefReads(MachineBasicBlock *);
} // namespace llvm
#define DEBUG_TYPE "break-false-deps"
char BreakFalseDeps::ID = 0;
INITIALIZE_PASS_BEGIN(BreakFalseDeps, DEBUG_TYPE, "BreakFalseDeps", false, false)
INITIALIZE_PASS_END(BreakFalseDeps, DEBUG_TYPE, "BreakFalseDeps", false, false)
FunctionPass *llvm::createBreakFalseDeps() { return new BreakFalseDeps(); }
bool BreakFalseDeps::pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
unsigned Pref) {
MachineOperand &MO = MI->getOperand(OpIdx);
assert(MO.isUndef() && "Expected undef machine operand");
unsigned OriginalReg = MO.getReg();
// Update only undef operands that have reg units that are mapped to one root.
for (MCRegUnitIterator Unit(OriginalReg, TRI); Unit.isValid(); ++Unit) {
unsigned NumRoots = 0;
for (MCRegUnitRootIterator Root(*Unit, TRI); Root.isValid(); ++Root) {
if (NumRoots > 1)
return false;
// Get the undef operand's register class
const TargetRegisterClass *OpRC =
TII->getRegClass(MI->getDesc(), OpIdx, TRI, *MF);
// If the instruction has a true dependency, we can hide the false depdency
// behind it.
for (MachineOperand &CurrMO : MI->operands()) {
if (!CurrMO.isReg() || CurrMO.isDef() || CurrMO.isUndef() ||
// We found a true dependency - replace the undef register with the true
// dependency.
return true;
// Go over all registers in the register class and find the register with
// max clearance or clearance higher than Pref.
unsigned MaxClearance = 0;
unsigned MaxClearanceReg = OriginalReg;
ArrayRef<MCPhysReg> Order = RegClassInfo.getOrder(OpRC);
for (MCPhysReg Reg : Order) {
unsigned Clearance = RDA->getClearance(MI, Reg);
if (Clearance <= MaxClearance)
MaxClearance = Clearance;
MaxClearanceReg = Reg;
if (MaxClearance > Pref)
// Update the operand if we found a register with better clearance.
if (MaxClearanceReg != OriginalReg)
return false;
bool BreakFalseDeps::shouldBreakDependence(MachineInstr *MI, unsigned OpIdx,
unsigned Pref) {
unsigned reg = MI->getOperand(OpIdx).getReg();
unsigned Clearance = RDA->getClearance(MI, reg);
LLVM_DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
if (Pref > Clearance) {
LLVM_DEBUG(dbgs() << ": Break dependency.\n");
return true;
LLVM_DEBUG(dbgs() << ": OK .\n");
return false;
void BreakFalseDeps::processDefs(MachineInstr *MI) {
assert(!MI->isDebugInstr() && "Won't process debug values");
// Break dependence on undef uses. Do this before updating LiveRegs below.
unsigned OpNum;
unsigned Pref = TII->getUndefRegClearance(*MI, OpNum, TRI);
if (Pref) {
bool HadTrueDependency = pickBestRegisterForUndef(MI, OpNum, Pref);
// We don't need to bother trying to break a dependency if this
// instruction has a true dependency on that register through another
// operand - we'll have to wait for it to be available regardless.
if (!HadTrueDependency && shouldBreakDependence(MI, OpNum, Pref))
UndefReads.push_back(std::make_pair(MI, OpNum));
const MCInstrDesc &MCID = MI->getDesc();
for (unsigned i = 0,
e = MI->isVariadic() ? MI->getNumOperands() : MCID.getNumDefs();
i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.getReg())
if (MO.isUse())
// Check clearance before partial register updates.
unsigned Pref = TII->getPartialRegUpdateClearance(*MI, i, TRI);
if (Pref && shouldBreakDependence(MI, i, Pref))
TII->breakPartialRegDependency(*MI, i, TRI);
void BreakFalseDeps::processUndefReads(MachineBasicBlock *MBB) {
if (UndefReads.empty())
// Collect this block's live out register units.
// We do not need to care about pristine registers as they are just preserved
// but not actually used in the function.
MachineInstr *UndefMI = UndefReads.back().first;
unsigned OpIdx = UndefReads.back().second;
for (MachineInstr &I : make_range(MBB->rbegin(), MBB->rend())) {
// Update liveness, including the current instruction's defs.
if (UndefMI == &I) {
if (!LiveRegSet.contains(UndefMI->getOperand(OpIdx).getReg()))
TII->breakPartialRegDependency(*UndefMI, OpIdx, TRI);
if (UndefReads.empty())
UndefMI = UndefReads.back().first;
OpIdx = UndefReads.back().second;
void BreakFalseDeps::processBasicBlock(MachineBasicBlock *MBB) {
// If this block is not done, it makes little sense to make any decisions
// based on clearance information. We need to make a second pass anyway,
// and by then we'll have better information, so we can avoid doing the work
// to try and break dependencies now.
for (MachineInstr &MI : *MBB) {
if (!MI.isDebugInstr())
bool BreakFalseDeps::runOnMachineFunction(MachineFunction &mf) {
if (skipFunction(mf.getFunction()))
return false;
MF = &mf;
TII = MF->getSubtarget().getInstrInfo();
TRI = MF->getSubtarget().getRegisterInfo();
RDA = &getAnalysis<ReachingDefAnalysis>();
LLVM_DEBUG(dbgs() << "********** BREAK FALSE DEPENDENCIES **********\n");
// Traverse the basic blocks.
for (MachineBasicBlock &MBB : mf) {
return false;