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//===-- GCRootLowering.cpp - Garbage collection infrastructure ------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements the lowering for the gc.root mechanism.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.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/InitializePasses.h"
#include "llvm/MC/MCContext.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 DoLowering(Function &F, GCStrategy &S);
public:
static char ID;
LowerIntrinsics();
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;
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);
public:
static char ID;
GCMachineCodeAnalysis();
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction &MF) override;
};
}
// -----------------------------------------------------------------------------
INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
false)
INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
char LowerIntrinsics::ID = 0;
char &llvm::GCLoweringID = LowerIntrinsics::ID;
LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
}
StringRef LowerIntrinsics::getPassName() const {
return "Lower Garbage Collection Instructions";
}
void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
FunctionPass::getAnalysisUsage(AU);
AU.addRequired<GCModuleInfo>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
/// 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 (Function &F : M)
if (!F.isDeclaration() && F.hasGC())
MI->getFunctionInfo(F); // 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) ||
isa<LoadInst>(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, ArrayRef<AllocaInst *> Roots) {
// Scroll past alloca instructions.
BasicBlock::iterator IP = F.getEntryBlock().begin();
while (isa<AllocaInst>(IP))
++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 =
dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
InitedRoots.insert(AI);
// Add root initializers.
bool MadeChange = false;
for (AllocaInst *Root : Roots)
if (!InitedRoots.count(Root)) {
new StoreInst(
ConstantPointerNull::get(cast<PointerType>(Root->getAllocatedType())),
Root, Root->getNextNode());
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();
return DoLowering(F, S);
}
/// Lower barriers out of existance (if the associated GCStrategy hasn't
/// already done so...), and insert initializing stores to roots as a defensive
/// measure. Given we're going to report all roots live at all safepoints, we
/// need to be able to ensure each root has been initialized by the point the
/// first safepoint is reached. This really should have been done by the
/// frontend, but the old API made this non-obvious, so we do a potentially
/// redundant store just in case.
bool LowerIntrinsics::DoLowering(Function &F, GCStrategy &S) {
SmallVector<AllocaInst *, 32> Roots;
bool MadeChange = false;
for (BasicBlock &BB : F)
for (Instruction &I : llvm::make_early_inc_range(BB)) {
IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I);
if (!CI)
continue;
Function *F = CI->getCalledFunction();
switch (F->getIntrinsicID()) {
default: break;
case Intrinsic::gcwrite: {
// Replace a write barrier with a simple store.
Value *St = new StoreInst(CI->getArgOperand(0),
CI->getArgOperand(2), CI);
CI->replaceAllUsesWith(St);
CI->eraseFromParent();
MadeChange = true;
break;
}
case Intrinsic::gcread: {
// Replace a read barrier with a simple load.
Value *Ld = new LoadInst(CI->getType(), CI->getArgOperand(1), "", CI);
Ld->takeName(CI);
CI->replaceAllUsesWith(Ld);
CI->eraseFromParent();
MadeChange = true;
break;
}
case Intrinsic::gcroot: {
// Initialize the GC root, but do not delete the intrinsic. The
// backend needs the intrinsic to flag the stack slot.
Roots.push_back(
cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
break;
}
}
}
if (Roots.size())
MadeChange |= InsertRootInitializers(F, Roots);
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 {
MachineFunctionPass::getAnalysisUsage(AU);
AU.setPreservesAll();
AU.addRequired<GCModuleInfo>();
}
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), since that's what will be on
// the stack when the call is suspended and we need to inspect the stack.
MachineBasicBlock::iterator RAI = CI;
++RAI;
MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
FI->addSafePoint(Label, CI->getDebugLoc());
}
void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
for (MachineBasicBlock &MBB : MF)
for (MachineInstr &MI : MBB)
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())
continue;
VisitCallPoint(&MI);
}
}
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 {
Register FrameReg; // FIXME: surely GCRoot ought to store the
// register that the offset is from?
auto FrameOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
assert(!FrameOffset.getScalable() &&
"Frame offsets with a scalable component are not supported");
RI->StackOffset = FrameOffset.getFixed();
++RI;
}
}
}
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());
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() || RegInfo->hasStackRealignment(MF);
FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
// Find all safe points.
if (FI->getStrategy().needsSafePoints())
FindSafePoints(MF);
// Find the concrete stack offsets for all roots (stack slots)
FindStackOffsets(MF);
return false;
}