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//===- ShrinkWrap.cpp - Compute safe point for prolog/epilog insertion ----===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass looks for safe point where the prologue and epilogue can be
// inserted.
// The safe point for the prologue (resp. epilogue) is called Save
// (resp. Restore).
// A point is safe for prologue (resp. epilogue) if and only if
// it 1) dominates (resp. post-dominates) all the frame related operations and
// between 2) two executions of the Save (resp. Restore) point there is an
// execution of the Restore (resp. Save) point.
// For instance, the following points are safe:
// for (int i = 0; i < 10; ++i) {
// Save
// ...
// Restore
// }
// Indeed, the execution looks like Save -> Restore -> Save -> Restore ...
// And the following points are not:
// for (int i = 0; i < 10; ++i) {
// Save
// ...
// }
// for (int i = 0; i < 10; ++i) {
// ...
// Restore
// }
// Indeed, the execution looks like Save -> Save -> ... -> Restore -> Restore.
// This pass also ensures that the safe points are 3) cheaper than the regular
// entry and exits blocks.
// Property #1 is ensured via the use of MachineDominatorTree and
// MachinePostDominatorTree.
// Property #2 is ensured via property #1 and MachineLoopInfo, i.e., both
// points must be in the same loop.
// Property #3 is ensured via the MachineBlockFrequencyInfo.
// If this pass found points matching all these properties, then
// MachineFrameInfo is updated with this information.
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachinePostDominators.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include <cassert>
#include <cstdint>
#include <memory>
using namespace llvm;
#define DEBUG_TYPE "shrink-wrap"
STATISTIC(NumFunc, "Number of functions");
STATISTIC(NumCandidates, "Number of shrink-wrapping candidates");
"Number of shrink-wrapping candidates dropped because of frequency");
static cl::opt<cl::boolOrDefault>
EnableShrinkWrapOpt("enable-shrink-wrap", cl::Hidden,
cl::desc("enable the shrink-wrapping pass"));
namespace {
/// Class to determine where the safe point to insert the
/// prologue and epilogue are.
/// Unlike the paper from Fred C. Chow, PLDI'88, that introduces the
/// shrink-wrapping term for prologue/epilogue placement, this pass
/// does not rely on expensive data-flow analysis. Instead we use the
/// dominance properties and loop information to decide which point
/// are safe for such insertion.
class ShrinkWrap : public MachineFunctionPass {
/// Hold callee-saved information.
RegisterClassInfo RCI;
MachineDominatorTree *MDT;
MachinePostDominatorTree *MPDT;
/// Current safe point found for the prologue.
/// The prologue will be inserted before the first instruction
/// in this basic block.
MachineBasicBlock *Save;
/// Current safe point found for the epilogue.
/// The epilogue will be inserted before the first terminator instruction
/// in this basic block.
MachineBasicBlock *Restore;
/// Hold the information of the basic block frequency.
/// Use to check the profitability of the new points.
MachineBlockFrequencyInfo *MBFI;
/// Hold the loop information. Used to determine if Save and Restore
/// are in the same loop.
MachineLoopInfo *MLI;
// Emit remarks.
MachineOptimizationRemarkEmitter *ORE = nullptr;
/// Frequency of the Entry block.
uint64_t EntryFreq;
/// Current opcode for frame setup.
unsigned FrameSetupOpcode;
/// Current opcode for frame destroy.
unsigned FrameDestroyOpcode;
/// Stack pointer register, used by llvm.{savestack,restorestack}
unsigned SP;
/// Entry block.
const MachineBasicBlock *Entry;
using SetOfRegs = SmallSetVector<unsigned, 16>;
/// Registers that need to be saved for the current function.
mutable SetOfRegs CurrentCSRs;
/// Current MachineFunction.
MachineFunction *MachineFunc;
/// Check if \p MI uses or defines a callee-saved register or
/// a frame index. If this is the case, this means \p MI must happen
/// after Save and before Restore.
bool useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS) const;
const SetOfRegs &getCurrentCSRs(RegScavenger *RS) const {
if (CurrentCSRs.empty()) {
BitVector SavedRegs;
const TargetFrameLowering *TFI =
TFI->determineCalleeSaves(*MachineFunc, SavedRegs, RS);
for (int Reg = SavedRegs.find_first(); Reg != -1;
Reg = SavedRegs.find_next(Reg))
return CurrentCSRs;
/// Update the Save and Restore points such that \p MBB is in
/// the region that is dominated by Save and post-dominated by Restore
/// and Save and Restore still match the safe point definition.
/// Such point may not exist and Save and/or Restore may be null after
/// this call.
void updateSaveRestorePoints(MachineBasicBlock &MBB, RegScavenger *RS);
/// Initialize the pass for \p MF.
void init(MachineFunction &MF) {
MDT = &getAnalysis<MachineDominatorTree>();
MPDT = &getAnalysis<MachinePostDominatorTree>();
Save = nullptr;
Restore = nullptr;
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
ORE = &getAnalysis<MachineOptimizationRemarkEmitterPass>().getORE();
EntryFreq = MBFI->getEntryFreq();
const TargetSubtargetInfo &Subtarget = MF.getSubtarget();
const TargetInstrInfo &TII = *Subtarget.getInstrInfo();
FrameSetupOpcode = TII.getCallFrameSetupOpcode();
FrameDestroyOpcode = TII.getCallFrameDestroyOpcode();
SP = Subtarget.getTargetLowering()->getStackPointerRegisterToSaveRestore();
Entry = &MF.front();
MachineFunc = &MF;
/// Check whether or not Save and Restore points are still interesting for
/// shrink-wrapping.
bool ArePointsInteresting() const { return Save != Entry && Save && Restore; }
/// Check if shrink wrapping is enabled for this target and function.
static bool isShrinkWrapEnabled(const MachineFunction &MF);
static char ID;
ShrinkWrap() : MachineFunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
StringRef getPassName() const override { return "Shrink Wrapping analysis"; }
/// Perform the shrink-wrapping analysis and update
/// the MachineFrameInfo attached to \p MF with the results.
bool runOnMachineFunction(MachineFunction &MF) override;
} // end anonymous namespace
char ShrinkWrap::ID = 0;
char &llvm::ShrinkWrapID = ShrinkWrap::ID;
INITIALIZE_PASS_BEGIN(ShrinkWrap, DEBUG_TYPE, "Shrink Wrap Pass", false, false)
INITIALIZE_PASS_END(ShrinkWrap, DEBUG_TYPE, "Shrink Wrap Pass", false, false)
bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI,
RegScavenger *RS) const {
// This prevents premature stack popping when occurs a indirect stack
// access. It is overly aggressive for the moment.
// TODO: - Obvious non-stack loads and store, such as global values,
// are known to not access the stack.
// - Further, data dependency and alias analysis can validate
// that load and stores never derive from the stack pointer.
if (MI.mayLoadOrStore())
return true;
if (MI.getOpcode() == FrameSetupOpcode ||
MI.getOpcode() == FrameDestroyOpcode) {
LLVM_DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
return true;
for (const MachineOperand &MO : MI.operands()) {
bool UseOrDefCSR = false;
if (MO.isReg()) {
// Ignore instructions like DBG_VALUE which don't read/def the register.
if (!MO.isDef() && !MO.readsReg())
Register PhysReg = MO.getReg();
if (!PhysReg)
assert(Register::isPhysicalRegister(PhysReg) && "Unallocated register?!");
// The stack pointer is not normally described as a callee-saved register
// in calling convention definitions, so we need to watch for it
// separately. An SP mentioned by a call instruction, we can ignore,
// though, as it's harmless and we do not want to effectively disable tail
// calls by forcing the restore point to post-dominate them.
UseOrDefCSR = (!MI.isCall() && PhysReg == SP) ||
} else if (MO.isRegMask()) {
// Check if this regmask clobbers any of the CSRs.
for (unsigned Reg : getCurrentCSRs(RS)) {
if (MO.clobbersPhysReg(Reg)) {
UseOrDefCSR = true;
// Skip FrameIndex operands in DBG_VALUE instructions.
if (UseOrDefCSR || (MO.isFI() && !MI.isDebugValue())) {
LLVM_DEBUG(dbgs() << "Use or define CSR(" << UseOrDefCSR << ") or FI("
<< MO.isFI() << "): " << MI << '\n');
return true;
return false;
/// Helper function to find the immediate (post) dominator.
template <typename ListOfBBs, typename DominanceAnalysis>
static MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
DominanceAnalysis &Dom) {
MachineBasicBlock *IDom = &Block;
for (MachineBasicBlock *BB : BBs) {
IDom = Dom.findNearestCommonDominator(IDom, BB);
if (!IDom)
if (IDom == &Block)
return nullptr;
return IDom;
void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
RegScavenger *RS) {
// Get rid of the easy cases first.
if (!Save)
Save = &MBB;
Save = MDT->findNearestCommonDominator(Save, &MBB);
if (!Save) {
LLVM_DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
if (!Restore)
Restore = &MBB;
else if (MPDT->getNode(&MBB)) // If the block is not in the post dom tree, it
// means the block never returns. If that's the
// case, we don't want to call
// `findNearestCommonDominator`, which will
// return `Restore`.
Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
Restore = nullptr; // Abort, we can't find a restore point in this case.
// Make sure we would be able to insert the restore code before the
// terminator.
if (Restore == &MBB) {
for (const MachineInstr &Terminator : MBB.terminators()) {
if (!useOrDefCSROrFI(Terminator, RS))
// One of the terminator needs to happen before the restore point.
if (MBB.succ_empty()) {
Restore = nullptr; // Abort, we can't find a restore point in this case.
// Look for a restore point that post-dominates all the successors.
// The immediate post-dominator is what we are looking for.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
if (!Restore) {
dbgs() << "Restore point needs to be spanned on several blocks\n");
// Make sure Save and Restore are suitable for shrink-wrapping:
// 1. all path from Save needs to lead to Restore before exiting.
// 2. all path to Restore needs to go through Save from Entry.
// We achieve that by making sure that:
// A. Save dominates Restore.
// B. Restore post-dominates Save.
// C. Save and Restore are in the same loop.
bool SaveDominatesRestore = false;
bool RestorePostDominatesSave = false;
while (Save && Restore &&
(!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
!(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
// Post-dominance is not enough in loops to ensure that all uses/defs
// are after the prologue and before the epilogue at runtime.
// E.g.,
// while(1) {
// Save
// Restore
// if (...)
// break;
// use/def CSRs
// }
// All the uses/defs of CSRs are dominated by Save and post-dominated
// by Restore. However, the CSRs uses are still reachable after
// Restore and before Save are executed.
// For now, just push the restore/save points outside of loops.
// FIXME: Refine the criteria to still find interesting cases
// for loops.
MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
// Fix (A).
if (!SaveDominatesRestore) {
Save = MDT->findNearestCommonDominator(Save, Restore);
// Fix (B).
if (!RestorePostDominatesSave)
Restore = MPDT->findNearestCommonDominator(Restore, Save);
// Fix (C).
if (Save && Restore &&
(MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore)) {
// Push Save outside of this loop if immediate dominator is different
// from save block. If immediate dominator is not different, bail out.
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
if (!Save)
} else {
// If the loop does not exit, there is no point in looking
// for a post-dominator outside the loop.
SmallVector<MachineBasicBlock*, 4> ExitBlocks;
// Push Restore outside of this loop.
// Look for the immediate post-dominator of the loop exits.
MachineBasicBlock *IPdom = Restore;
for (MachineBasicBlock *LoopExitBB: ExitBlocks) {
IPdom = FindIDom<>(*IPdom, LoopExitBB->successors(), *MPDT);
if (!IPdom)
// If the immediate post-dominator is not in a less nested loop,
// then we are stuck in a program with an infinite loop.
// In that case, we will not find a safe point, hence, bail out.
if (IPdom && MLI->getLoopDepth(IPdom) < MLI->getLoopDepth(Restore))
Restore = IPdom;
else {
Restore = nullptr;
static bool giveUpWithRemarks(MachineOptimizationRemarkEmitter *ORE,
StringRef RemarkName, StringRef RemarkMessage,
const DiagnosticLocation &Loc,
const MachineBasicBlock *MBB) {
ORE->emit([&]() {
return MachineOptimizationRemarkMissed(DEBUG_TYPE, RemarkName, Loc, MBB)
<< RemarkMessage;
LLVM_DEBUG(dbgs() << RemarkMessage << '\n');
return false;
bool ShrinkWrap::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()) || MF.empty() || !isShrinkWrapEnabled(MF))
return false;
LLVM_DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
if (containsIrreducibleCFG<MachineBasicBlock *>(RPOT, *MLI)) {
// If MF is irreducible, a block may be in a loop without
// MachineLoopInfo reporting it. I.e., we may use the
// post-dominance property in loops, which lead to incorrect
// results. Moreover, we may miss that the prologue and
// epilogue are not in the same loop, leading to unbalanced
// construction/deconstruction of the stack frame.
return giveUpWithRemarks(ORE, "UnsupportedIrreducibleCFG",
"Irreducible CFGs are not supported yet.",
MF.getFunction().getSubprogram(), &MF.front());
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
std::unique_ptr<RegScavenger> RS(
TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : nullptr);
for (MachineBasicBlock &MBB : MF) {
LLVM_DEBUG(dbgs() << "Look into: " << MBB.getNumber() << ' '
<< MBB.getName() << '\n');
if (MBB.isEHFuncletEntry())
return giveUpWithRemarks(ORE, "UnsupportedEHFunclets",
"EH Funclets are not supported yet.",
MBB.front().getDebugLoc(), &MBB);
if (MBB.isEHPad()) {
// Push the prologue and epilogue outside of
// the region that may throw by making sure
// that all the landing pads are at least at the
// boundary of the save and restore points.
// The problem with exceptions is that the throw
// is not properly modeled and in particular, a
// basic block can jump out from the middle.
updateSaveRestorePoints(MBB, RS.get());
if (!ArePointsInteresting()) {
LLVM_DEBUG(dbgs() << "EHPad prevents shrink-wrapping\n");
return false;
for (const MachineInstr &MI : MBB) {
if (!useOrDefCSROrFI(MI, RS.get()))
// Save (resp. restore) point must dominate (resp. post dominate)
// MI. Look for the proper basic block for those.
updateSaveRestorePoints(MBB, RS.get());
// If we are at a point where we cannot improve the placement of
// save/restore instructions, just give up.
if (!ArePointsInteresting()) {
LLVM_DEBUG(dbgs() << "No Shrink wrap candidate found\n");
return false;
// No need to look for other instructions, this basic block
// will already be part of the handled region.
if (!ArePointsInteresting()) {
// If the points are not interesting at this point, then they must be null
// because it means we did not encounter any frame/CSR related code.
// Otherwise, we would have returned from the previous loop.
assert(!Save && !Restore && "We miss a shrink-wrap opportunity?!");
LLVM_DEBUG(dbgs() << "Nothing to shrink-wrap\n");
return false;
LLVM_DEBUG(dbgs() << "\n ** Results **\nFrequency of the Entry: " << EntryFreq
<< '\n');
const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
do {
LLVM_DEBUG(dbgs() << "Shrink wrap candidates (#, Name, Freq):\nSave: "
<< Save->getNumber() << ' ' << Save->getName() << ' '
<< MBFI->getBlockFreq(Save).getFrequency()
<< "\nRestore: " << Restore->getNumber() << ' '
<< Restore->getName() << ' '
<< MBFI->getBlockFreq(Restore).getFrequency() << '\n');
bool IsSaveCheap, TargetCanUseSaveAsPrologue = false;
if (((IsSaveCheap = EntryFreq >= MBFI->getBlockFreq(Save).getFrequency()) &&
EntryFreq >= MBFI->getBlockFreq(Restore).getFrequency()) &&
((TargetCanUseSaveAsPrologue = TFI->canUseAsPrologue(*Save)) &&
dbgs() << "New points are too expensive or invalid for the target\n");
MachineBasicBlock *NewBB;
if (!IsSaveCheap || !TargetCanUseSaveAsPrologue) {
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
if (!Save)
NewBB = Save;
} else {
// Restore is expensive.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
if (!Restore)
NewBB = Restore;
updateSaveRestorePoints(*NewBB, RS.get());
} while (Save && Restore);
if (!ArePointsInteresting()) {
return false;
LLVM_DEBUG(dbgs() << "Final shrink wrap candidates:\nSave: "
<< Save->getNumber() << ' ' << Save->getName()
<< "\nRestore: " << Restore->getNumber() << ' '
<< Restore->getName() << '\n');
MachineFrameInfo &MFI = MF.getFrameInfo();
return false;
bool ShrinkWrap::isShrinkWrapEnabled(const MachineFunction &MF) {
const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
switch (EnableShrinkWrapOpt) {
case cl::BOU_UNSET:
return TFI->enableShrinkWrapping(MF) &&
// Windows with CFI has some limitations that make it impossible
// to use shrink-wrapping.
!MF.getTarget().getMCAsmInfo()->usesWindowsCFI() &&
// Sanitizers look at the value of the stack at the location
// of the crash. Since a crash can happen anywhere, the
// frame must be lowered before anything else happen for the
// sanitizers to be able to get a correct stack frame.
!(MF.getFunction().hasFnAttribute(Attribute::SanitizeAddress) ||
MF.getFunction().hasFnAttribute(Attribute::SanitizeThread) ||
MF.getFunction().hasFnAttribute(Attribute::SanitizeMemory) ||
// If EnableShrinkWrap is set, it takes precedence on whatever the
// target sets. The rational is that we assume we want to test
// something related to shrink-wrapping.
case cl::BOU_TRUE:
return true;
case cl::BOU_FALSE:
return false;
llvm_unreachable("Invalid shrink-wrapping state");