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llvm-project/llvm/lib/CodeGen/MIRCanonicalizerPass.cpp

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//===-------------- MIRCanonicalizer.cpp - MIR Canonicalizer --------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The purpose of this pass is to employ a canonical code transformation so
// that code compiled with slightly different IR passes can be diffed more
// effectively than otherwise. This is done by renaming vregs in a given
// LiveRange in a canonical way. This pass also does a pseudo-scheduling to
// move defs closer to their use inorder to reduce diffs caused by slightly
// different schedules.
//
// Basic Usage:
//
// llc -o - -run-pass mir-canonicalizer example.mir
//
// Reorders instructions canonically.
// Renames virtual register operands canonically.
// Strips certain MIR artifacts (optionally).
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/raw_ostream.h"
#include <queue>
using namespace llvm;
namespace llvm {
extern char &MIRCanonicalizerID;
} // namespace llvm
#define DEBUG_TYPE "mir-canonicalizer"
static cl::opt<unsigned>
CanonicalizeFunctionNumber("canon-nth-function", cl::Hidden, cl::init(~0u),
cl::value_desc("N"),
cl::desc("Function number to canonicalize."));
static cl::opt<unsigned>
CanonicalizeBasicBlockNumber("canon-nth-basicblock", cl::Hidden, cl::init(~0u),
cl::value_desc("N"),
cl::desc("BasicBlock number to canonicalize."));
namespace {
class MIRCanonicalizer : public MachineFunctionPass {
public:
static char ID;
MIRCanonicalizer() : MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "Rename register operands in a canonical ordering.";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
};
} // end anonymous namespace
enum VRType { RSE_Reg = 0, RSE_FrameIndex, RSE_NewCandidate };
class TypedVReg {
VRType type;
unsigned reg;
public:
TypedVReg(unsigned reg) : type(RSE_Reg), reg(reg) {}
TypedVReg(VRType type) : type(type), reg(~0U) {
assert(type != RSE_Reg && "Expected a non-register type.");
}
bool isReg() const { return type == RSE_Reg; }
bool isFrameIndex() const { return type == RSE_FrameIndex; }
bool isCandidate() const { return type == RSE_NewCandidate; }
VRType getType() const { return type; }
unsigned getReg() const {
assert(this->isReg() && "Expected a virtual or physical register.");
return reg;
}
};
char MIRCanonicalizer::ID;
char &llvm::MIRCanonicalizerID = MIRCanonicalizer::ID;
INITIALIZE_PASS_BEGIN(MIRCanonicalizer, "mir-canonicalizer",
"Rename Register Operands Canonically", false, false)
INITIALIZE_PASS_END(MIRCanonicalizer, "mir-canonicalizer",
"Rename Register Operands Canonically", false, false)
static std::vector<MachineBasicBlock *> GetRPOList(MachineFunction &MF) {
ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
std::vector<MachineBasicBlock *> RPOList;
for (auto MBB : RPOT) {
RPOList.push_back(MBB);
}
return RPOList;
}
// Set a dummy vreg. We use this vregs register class to generate throw-away
// vregs that are used to skip vreg numbers so that vreg numbers line up.
static unsigned GetDummyVReg(const MachineFunction &MF) {
for (auto &MBB : MF) {
for (auto &MI : MBB) {
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
continue;
return MO.getReg();
}
}
}
return ~0U;
}
static bool rescheduleCanonically(MachineBasicBlock *MBB) {
bool Changed = false;
// Calculates the distance of MI from the begining of its parent BB.
auto getInstrIdx = [](const MachineInstr &MI) {
unsigned i = 0;
for (auto &CurMI : *MI.getParent()) {
if (&CurMI == &MI)
return i;
i++;
}
return ~0U;
};
// Pre-Populate vector of instructions to reschedule so that we don't
// clobber the iterator.
std::vector<MachineInstr *> Instructions;
for (auto &MI : *MBB) {
Instructions.push_back(&MI);
}
for (auto *II : Instructions) {
if (II->getNumOperands() == 0)
continue;
MachineOperand &MO = II->getOperand(0);
if (!MO.isReg() || !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
continue;
DEBUG(dbgs() << "Operand " << 0 << " of "; II->dump(); MO.dump(););
MachineInstr *Def = II;
unsigned Distance = ~0U;
MachineInstr *UseToBringDefCloserTo = nullptr;
MachineRegisterInfo *MRI = &MBB->getParent()->getRegInfo();
for (auto &UO : MRI->use_nodbg_operands(MO.getReg())) {
MachineInstr *UseInst = UO.getParent();
const unsigned DefLoc = getInstrIdx(*Def);
const unsigned UseLoc = getInstrIdx(*UseInst);
const unsigned Delta = (UseLoc - DefLoc);
if (UseInst->getParent() != Def->getParent())
continue;
if (DefLoc >= UseLoc)
continue;
if (Delta < Distance) {
Distance = Delta;
UseToBringDefCloserTo = UseInst;
}
}
const auto BBE = MBB->instr_end();
MachineBasicBlock::iterator DefI = BBE;
MachineBasicBlock::iterator UseI = BBE;
for (auto BBI = MBB->instr_begin(); BBI != BBE; ++BBI) {
if (DefI != BBE && UseI != BBE)
break;
if ((&*BBI != Def) && (&*BBI != UseToBringDefCloserTo))
continue;
if (&*BBI == Def) {
DefI = BBI;
continue;
}
if (&*BBI == UseToBringDefCloserTo) {
UseI = BBI;
continue;
}
}
if (DefI == BBE || UseI == BBE)
continue;
DEBUG({
dbgs() << "Splicing ";
DefI->dump();
dbgs() << " right before: ";
UseI->dump();
});
Changed = true;
MBB->splice(UseI, MBB, DefI);
}
return Changed;
}
/// Here we find our candidates. What makes an interesting candidate?
/// An candidate for a canonicalization tree root is normally any kind of
/// instruction that causes side effects such as a store to memory or a copy to
/// a physical register or a return instruction. We use these as an expression
/// tree root that we walk inorder to build a canonical walk which should result
/// in canoncal vreg renaming.
static std::vector<MachineInstr *> populateCandidates(MachineBasicBlock *MBB) {
std::vector<MachineInstr *> Candidates;
MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
for (auto II = MBB->begin(), IE = MBB->end(); II != IE; ++II) {
MachineInstr *MI = &*II;
bool DoesMISideEffect = false;
if (MI->getNumOperands() > 0 && MI->getOperand(0).isReg()) {
const unsigned Dst = MI->getOperand(0).getReg();
DoesMISideEffect |= !TargetRegisterInfo::isVirtualRegister(Dst);
for (auto UI = MRI.use_begin(Dst); UI != MRI.use_end(); ++UI) {
if (DoesMISideEffect) break;
DoesMISideEffect |= (UI->getParent()->getParent() != MI->getParent());
}
}
if (!MI->mayStore() && !MI->isBranch() && !DoesMISideEffect)
continue;
DEBUG(dbgs() << "Found Candidate: "; MI->dump(););
Candidates.push_back(MI);
}
return Candidates;
}
static void doCandidateWalk(std::vector<TypedVReg> &VRegs,
std::queue<TypedVReg> &RegQueue,
std::vector<MachineInstr *> &VisitedMIs,
const MachineBasicBlock *MBB) {
const MachineFunction &MF = *MBB->getParent();
const MachineRegisterInfo &MRI = MF.getRegInfo();
while (!RegQueue.empty()) {
auto TReg = RegQueue.front();
RegQueue.pop();
if (TReg.isFrameIndex()) {
DEBUG(dbgs() << "Popping frame index.\n";);
VRegs.push_back(TypedVReg(RSE_FrameIndex));
continue;
}
assert(TReg.isReg() && "Expected vreg or physreg.");
unsigned Reg = TReg.getReg();
if (TargetRegisterInfo::isVirtualRegister(Reg)) {
DEBUG({
dbgs() << "Popping vreg ";
MRI.def_begin(Reg)->dump();
dbgs() << "\n";
});
if (!llvm::any_of(VRegs, [&](const TypedVReg &TR) {
return TR.isReg() && TR.getReg() == Reg;
})) {
VRegs.push_back(TypedVReg(Reg));
}
} else {
DEBUG(dbgs() << "Popping physreg.\n";);
VRegs.push_back(TypedVReg(Reg));
continue;
}
for (auto RI = MRI.def_begin(Reg), RE = MRI.def_end(); RI != RE; ++RI) {
MachineInstr *Def = RI->getParent();
if (Def->getParent() != MBB)
continue;
if (llvm::any_of(VisitedMIs,
[&](const MachineInstr *VMI) { return Def == VMI; })) {
break;
}
DEBUG({
dbgs() << "\n========================\n";
dbgs() << "Visited MI: ";
Def->dump();
dbgs() << "BB Name: " << Def->getParent()->getName() << "\n";
dbgs() << "\n========================\n";
});
VisitedMIs.push_back(Def);
for (unsigned I = 1, E = Def->getNumOperands(); I != E; ++I) {
MachineOperand &MO = Def->getOperand(I);
if (MO.isFI()) {
DEBUG(dbgs() << "Pushing frame index.\n";);
RegQueue.push(TypedVReg(RSE_FrameIndex));
}
if (!MO.isReg())
continue;
RegQueue.push(TypedVReg(MO.getReg()));
}
}
}
}
// TODO: Work to remove this in the future. One day when we have named vregs
// we should be able to form the canonical name based on some characteristic
// we see in that point of the expression tree (like if we were to name based
// on some sort of value numbering scheme).
static void SkipVRegs(unsigned &VRegGapIndex, MachineRegisterInfo &MRI,
const TargetRegisterClass *RC) {
const unsigned VR_GAP = (++VRegGapIndex * 1000);
DEBUG({
dbgs() << "Adjusting per-BB VR_GAP for BB" << VRegGapIndex << " to "
<< VR_GAP << "\n";
});
unsigned I = MRI.createVirtualRegister(RC);
const unsigned E = (((I + VR_GAP) / VR_GAP) + 1) * VR_GAP;
while (I != E) {
I = MRI.createVirtualRegister(RC);
}
}
static std::map<unsigned, unsigned>
GetVRegRenameMap(const std::vector<TypedVReg> &VRegs,
const std::vector<unsigned> &renamedInOtherBB,
MachineRegisterInfo &MRI,
const TargetRegisterClass *RC) {
std::map<unsigned, unsigned> VRegRenameMap;
unsigned LastRenameReg = MRI.createVirtualRegister(RC);
bool FirstCandidate = true;
for (auto &vreg : VRegs) {
if (vreg.isFrameIndex()) {
// We skip one vreg for any frame index because there is a good chance
// (especially when comparing SelectionDAG to GlobalISel generated MIR)
// that in the other file we are just getting an incoming vreg that comes
// from a copy from a frame index. So it's safe to skip by one.
LastRenameReg = MRI.createVirtualRegister(RC);
DEBUG(dbgs() << "Skipping rename for FI " << LastRenameReg << "\n";);
continue;
} else if (vreg.isCandidate()) {
// After the first candidate, for every subsequent candidate, we skip mod
// 10 registers so that the candidates are more likely to start at the
// same vreg number making it more likely that the canonical walk from the
// candidate insruction. We don't need to skip from the first candidate of
// the BasicBlock because we already skip ahead several vregs for each BB.
while (LastRenameReg % 10) {
if (!FirstCandidate) break;
LastRenameReg = MRI.createVirtualRegister(RC);
DEBUG({
dbgs() << "Skipping rename for new candidate " << LastRenameReg
<< "\n";
});
}
FirstCandidate = false;
continue;
} else if (!TargetRegisterInfo::isVirtualRegister(vreg.getReg())) {
LastRenameReg = MRI.createVirtualRegister(RC);
DEBUG({
dbgs() << "Skipping rename for Phys Reg " << LastRenameReg << "\n";
});
continue;
}
auto Reg = vreg.getReg();
if (llvm::find(renamedInOtherBB, Reg) != renamedInOtherBB.end()) {
DEBUG(dbgs() << "Vreg " << Reg << " already renamed in other BB.\n";);
continue;
}
auto Rename = MRI.createVirtualRegister(MRI.getRegClass(Reg));
LastRenameReg = Rename;
if (VRegRenameMap.find(Reg) == VRegRenameMap.end()) {
DEBUG(dbgs() << "Mapping vreg ";);
if (MRI.reg_begin(Reg) != MRI.reg_end()) {
DEBUG(auto foo = &*MRI.reg_begin(Reg); foo->dump(););
} else {
DEBUG(dbgs() << Reg;);
}
DEBUG(dbgs() << " to ";);
if (MRI.reg_begin(Rename) != MRI.reg_end()) {
DEBUG(auto foo = &*MRI.reg_begin(Rename); foo->dump(););
} else {
DEBUG(dbgs() << Rename;);
}
DEBUG(dbgs() << "\n";);
VRegRenameMap.insert(std::pair<unsigned, unsigned>(Reg, Rename));
}
}
return VRegRenameMap;
}
static bool doVRegRenaming(std::vector<unsigned> &RenamedInOtherBB,
const std::map<unsigned, unsigned> &VRegRenameMap,
MachineRegisterInfo &MRI) {
bool Changed = false;
for (auto I = VRegRenameMap.begin(), E = VRegRenameMap.end(); I != E; ++I) {
auto VReg = I->first;
auto Rename = I->second;
RenamedInOtherBB.push_back(Rename);
std::vector<MachineOperand *> RenameMOs;
for (auto &MO : MRI.reg_operands(VReg)) {
RenameMOs.push_back(&MO);
}
for (auto *MO : RenameMOs) {
Changed = true;
MO->setReg(Rename);
if (!MO->isDef())
MO->setIsKill(false);
}
}
return Changed;
}
static bool doDefKillClear(MachineBasicBlock *MBB) {
bool Changed = false;
for (auto &MI : *MBB) {
for (auto &MO : MI.operands()) {
if (!MO.isReg())
continue;
if (!MO.isDef() && MO.isKill()) {
Changed = true;
MO.setIsKill(false);
}
if (MO.isDef() && MO.isDead()) {
Changed = true;
MO.setIsDead(false);
}
}
}
return Changed;
}
static bool runOnBasicBlock(MachineBasicBlock *MBB,
std::vector<StringRef> &bbNames,
std::vector<unsigned> &renamedInOtherBB,
unsigned &basicBlockNum, unsigned &VRegGapIndex) {
if (CanonicalizeBasicBlockNumber != ~0U) {
if (CanonicalizeBasicBlockNumber != basicBlockNum++)
return false;
DEBUG(dbgs() << "\n Canonicalizing BasicBlock " << MBB->getName() << "\n";);
}
if (llvm::find(bbNames, MBB->getName()) != bbNames.end()) {
DEBUG({
dbgs() << "Found potentially duplicate BasicBlocks: " << MBB->getName()
<< "\n";
});
return false;
}
DEBUG({
dbgs() << "\n\n NEW BASIC BLOCK: " << MBB->getName() << " \n\n";
dbgs() << "\n\n================================================\n\n";
});
bool Changed = false;
MachineFunction &MF = *MBB->getParent();
MachineRegisterInfo &MRI = MF.getRegInfo();
const unsigned DummyVReg = GetDummyVReg(MF);
const TargetRegisterClass *DummyRC =
(DummyVReg == ~0U) ? nullptr : MRI.getRegClass(DummyVReg);
if (!DummyRC) return false;
bbNames.push_back(MBB->getName());
DEBUG(dbgs() << "\n\n NEW BASIC BLOCK: " << MBB->getName() << "\n\n";);
DEBUG(dbgs() << "MBB Before Scheduling:\n"; MBB->dump(););
Changed |= rescheduleCanonically(MBB);
DEBUG(dbgs() << "MBB After Scheduling:\n"; MBB->dump(););
std::vector<MachineInstr *> Candidates = populateCandidates(MBB);
std::vector<MachineInstr *> VisitedMIs;
std::copy(Candidates.begin(), Candidates.end(),
std::back_inserter(VisitedMIs));
std::vector<TypedVReg> VRegs;
for (auto candidate : Candidates) {
VRegs.push_back(TypedVReg(RSE_NewCandidate));
std::queue<TypedVReg> RegQueue;
// Here we walk the vreg operands of a non-root node along our walk.
// The root nodes are the original candidates (stores normally).
// These are normally not the root nodes (except for the case of copies to
// physical registers).
for (unsigned i = 1; i < candidate->getNumOperands(); i++) {
if (candidate->mayStore() || candidate->isBranch())
break;
MachineOperand &MO = candidate->getOperand(i);
if (!(MO.isReg() && TargetRegisterInfo::isVirtualRegister(MO.getReg())))
continue;
DEBUG(dbgs() << "Enqueue register"; MO.dump(); dbgs() << "\n";);
RegQueue.push(TypedVReg(MO.getReg()));
}
// Here we walk the root candidates. We start from the 0th operand because
// the root is normally a store to a vreg.
for (unsigned i = 0; i < candidate->getNumOperands(); i++) {
if (!candidate->mayStore() && !candidate->isBranch())
break;
MachineOperand &MO = candidate->getOperand(i);
// TODO: Do we want to only add vregs here?
if (!MO.isReg() && !MO.isFI())
continue;
DEBUG(dbgs() << "Enqueue Reg/FI"; MO.dump(); dbgs() << "\n";);
RegQueue.push(MO.isReg() ? TypedVReg(MO.getReg()) :
TypedVReg(RSE_FrameIndex));
}
doCandidateWalk(VRegs, RegQueue, VisitedMIs, MBB);
}
// If we have populated no vregs to rename then bail.
// The rest of this function does the vreg remaping.
if (VRegs.size() == 0)
return Changed;
// Skip some vregs, so we can recon where we'll land next.
SkipVRegs(VRegGapIndex, MRI, DummyRC);
auto VRegRenameMap = GetVRegRenameMap(VRegs, renamedInOtherBB, MRI, DummyRC);
Changed |= doVRegRenaming(renamedInOtherBB, VRegRenameMap, MRI);
Changed |= doDefKillClear(MBB);
DEBUG(dbgs() << "Updated MachineBasicBlock:\n"; MBB->dump(); dbgs() << "\n";);
DEBUG(dbgs() << "\n\n================================================\n\n");
return Changed;
}
bool MIRCanonicalizer::runOnMachineFunction(MachineFunction &MF) {
static unsigned functionNum = 0;
if (CanonicalizeFunctionNumber != ~0U) {
if (CanonicalizeFunctionNumber != functionNum++)
return false;
DEBUG(dbgs() << "\n Canonicalizing Function " << MF.getName() << "\n";);
}
// we need a valid vreg to create a vreg type for skipping all those
// stray vreg numbers so reach alignment/canonical vreg values.
std::vector<MachineBasicBlock*> RPOList = GetRPOList(MF);
DEBUG(
dbgs() << "\n\n NEW MACHINE FUNCTION: " << MF.getName() << " \n\n";
dbgs() << "\n\n================================================\n\n";
dbgs() << "Total Basic Blocks: " << RPOList.size() << "\n";
for (auto MBB : RPOList) {
dbgs() << MBB->getName() << "\n";
}
dbgs() << "\n\n================================================\n\n";
);
std::vector<StringRef> BBNames;
std::vector<unsigned> RenamedInOtherBB;
unsigned GapIdx = 0;
unsigned BBNum = 0;
bool Changed = false;
for (auto MBB : RPOList)
Changed |= runOnBasicBlock(MBB, BBNames, RenamedInOtherBB, BBNum, GapIdx);
return Changed;
}
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