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//===-- GCRootLowering.cpp - Garbage collection infrastructure ------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements the lowering for the gc.root mechanism.
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/CodeGen/GCStrategy.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
namespace {
/// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
/// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
/// directed by the GCStrategy. It also performs automatic root initialization
/// and custom intrinsic lowering.
class LowerIntrinsics : public FunctionPass {
bool PerformDefaultLowering(Function &F, GCStrategy &S);
static char ID;
StringRef getPassName() const override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool doInitialization(Module &M) override;
bool runOnFunction(Function &F) override;
/// GCMachineCodeAnalysis - This is a target-independent pass over the machine
/// function representation to identify safe points for the garbage collector
/// in the machine code. It inserts labels at safe points and populates a
/// GCMetadata record for each function.
class GCMachineCodeAnalysis : public MachineFunctionPass {
GCFunctionInfo *FI;
MachineModuleInfo *MMI;
const TargetInstrInfo *TII;
void FindSafePoints(MachineFunction &MF);
void VisitCallPoint(MachineBasicBlock::iterator CI);
MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
const DebugLoc &DL) const;
void FindStackOffsets(MachineFunction &MF);
static char ID;
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction &MF) override;
// -----------------------------------------------------------------------------
INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
char LowerIntrinsics::ID = 0;
LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
StringRef LowerIntrinsics::getPassName() const {
return "Lower Garbage Collection Instructions";
void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
static bool NeedsDefaultLoweringPass(const GCStrategy &C) {
// Default lowering is necessary only if read or write barriers have a default
// action. The default for roots is no action.
return !C.customWriteBarrier() || !C.customReadBarrier() ||
/// doInitialization - If this module uses the GC intrinsics, find them now.
bool LowerIntrinsics::doInitialization(Module &M) {
GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
if (!I->isDeclaration() && I->hasGC())
MI->getFunctionInfo(*I); // Instantiate the GC strategy.
return false;
/// CouldBecomeSafePoint - Predicate to conservatively determine whether the
/// instruction could introduce a safe point.
static bool CouldBecomeSafePoint(Instruction *I) {
// The natural definition of instructions which could introduce safe points
// are:
// - call, invoke (AfterCall, BeforeCall)
// - phis (Loops)
// - invoke, ret, unwind (Exit)
// However, instructions as seemingly inoccuous as arithmetic can become
// libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
// it is necessary to take a conservative approach.
if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
return false;
// llvm.gcroot is safe because it doesn't do anything at runtime.
if (CallInst *CI = dyn_cast<CallInst>(I))
if (Function *F = CI->getCalledFunction())
if (Intrinsic::ID IID = F->getIntrinsicID())
if (IID == Intrinsic::gcroot)
return false;
return true;
static bool InsertRootInitializers(Function &F, AllocaInst **Roots,
unsigned Count) {
// Scroll past alloca instructions.
BasicBlock::iterator IP = F.getEntryBlock().begin();
while (isa<AllocaInst>(IP))
// Search for initializers in the initial BB.
SmallPtrSet<AllocaInst *, 16> InitedRoots;
for (; !CouldBecomeSafePoint(&*IP); ++IP)
if (StoreInst *SI = dyn_cast<StoreInst>(IP))
if (AllocaInst *AI =
// Add root initializers.
bool MadeChange = false;
for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
if (!InitedRoots.count(*I)) {
StoreInst *SI = new StoreInst(
MadeChange = true;
return MadeChange;
/// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
/// Leave gcroot intrinsics; the code generator needs to see those.
bool LowerIntrinsics::runOnFunction(Function &F) {
// Quick exit for functions that do not use GC.
if (!F.hasGC())
return false;
GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
GCStrategy &S = FI.getStrategy();
bool MadeChange = false;
if (NeedsDefaultLoweringPass(S))
MadeChange |= PerformDefaultLowering(F, S);
return MadeChange;
bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
bool LowerWr = !S.customWriteBarrier();
bool LowerRd = !S.customReadBarrier();
bool InitRoots = S.initializeRoots();
SmallVector<AllocaInst *, 32> Roots;
bool MadeChange = false;
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
Function *F = CI->getCalledFunction();
switch (F->getIntrinsicID()) {
case Intrinsic::gcwrite:
if (LowerWr) {
// Replace a write barrier with a simple store.
Value *St =
new StoreInst(CI->getArgOperand(0), CI->getArgOperand(2), CI);
case Intrinsic::gcread:
if (LowerRd) {
// Replace a read barrier with a simple load.
Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
case Intrinsic::gcroot:
if (InitRoots) {
// Initialize the GC root, but do not delete the intrinsic. The
// backend needs the intrinsic to flag the stack slot.
MadeChange = true;
if (Roots.size())
MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
return MadeChange;
// -----------------------------------------------------------------------------
char GCMachineCodeAnalysis::ID = 0;
char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
"Analyze Machine Code For Garbage Collection", false, false)
GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
MachineBasicBlock::iterator MI,
const DebugLoc &DL) const {
MCSymbol *Label = MBB.getParent()->getContext().createTempSymbol();
BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
return Label;
void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
// Find the return address (next instruction), too, so as to bracket the call
// instruction.
MachineBasicBlock::iterator RAI = CI;
if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
MCSymbol *Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
for (MachineFunction::iterator BBI = MF.begin(), BBE = MF.end(); BBI != BBE;
for (MachineBasicBlock::iterator MI = BBI->begin(), ME = BBI->end();
MI != ME; ++MI)
if (MI->isCall()) {
// Do not treat tail or sibling call sites as safe points. This is
// legal since any arguments passed to the callee which live in the
// remnants of the callers frame will be owned and updated by the
// callee if required.
if (MI->isTerminator())
void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
assert(TFI && "TargetRegisterInfo not available!");
for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
RI != FI->roots_end();) {
// If the root references a dead object, no need to keep it.
if (MF.getFrameInfo().isDeadObjectIndex(RI->Num)) {
RI = FI->removeStackRoot(RI);
} else {
unsigned FrameReg; // FIXME: surely GCRoot ought to store the
// register that the offset is from?
RI->StackOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
// Quick exit for functions that do not use GC.
if (!MF.getFunction().hasGC())
return false;
FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(MF.getFunction());
MMI = &getAnalysis<MachineModuleInfo>();
TII = MF.getSubtarget().getInstrInfo();
// Find the size of the stack frame. There may be no correct static frame
// size, we use UINT64_MAX to represent this.
const MachineFrameInfo &MFI = MF.getFrameInfo();
const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
const bool DynamicFrameSize = MFI.hasVarSizedObjects() ||
FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
// Find all safe points.
if (FI->getStrategy().needsSafePoints())
// Find the concrete stack offsets for all roots (stack slots)
return false;