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//===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This pass assigns local frame indices to stack slots relative to one another
// and allocates additional base registers to access them when the target
// estimates they are likely to be out of range of stack pointer and frame
// pointer relative addressing.
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <tuple>
using namespace llvm;
#define DEBUG_TYPE "localstackalloc"
STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
STATISTIC(NumReplacements, "Number of frame indices references replaced");
namespace {
class FrameRef {
MachineBasicBlock::iterator MI; // Instr referencing the frame
int64_t LocalOffset; // Local offset of the frame idx referenced
int FrameIdx; // The frame index
// Order reference instruction appears in program. Used to ensure
// deterministic order when multiple instructions may reference the same
// location.
unsigned Order;
FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) :
MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {}
bool operator<(const FrameRef &RHS) const {
return std::tie(LocalOffset, FrameIdx, Order) <
std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order);
MachineBasicBlock::iterator getMachineInstr() const { return MI; }
int64_t getLocalOffset() const { return LocalOffset; }
int getFrameIndex() const { return FrameIdx; }
class LocalStackSlotPass: public MachineFunctionPass {
SmallVector<int64_t, 16> LocalOffsets;
/// StackObjSet - A set of stack object indexes
using StackObjSet = SmallSetVector<int, 8>;
void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset,
bool StackGrowsDown, unsigned &MaxAlign);
void AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
SmallSet<int, 16> &ProtectedObjs,
MachineFrameInfo &MFI, bool StackGrowsDown,
int64_t &Offset, unsigned &MaxAlign);
void calculateFrameObjectOffsets(MachineFunction &Fn);
bool insertFrameReferenceRegisters(MachineFunction &Fn);
static char ID; // Pass identification, replacement for typeid
explicit LocalStackSlotPass() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
char LocalStackSlotPass::ID = 0;
char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
"Local Stack Slot Allocation", false, false)
bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
MachineFrameInfo &MFI = MF.getFrameInfo();
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
unsigned LocalObjectCount = MFI.getObjectIndexEnd();
// If the target doesn't want/need this pass, or if there are no locals
// to consider, early exit.
if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
return true;
// Make sure we have enough space to store the local offsets.
// Lay out the local blob.
// Insert virtual base registers to resolve frame index references.
bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
// Tell MFI whether any base registers were allocated. PEI will only
// want to use the local block allocations from this pass if there were any.
// Otherwise, PEI can do a bit better job of getting the alignment right
// without a hole at the start since it knows the alignment of the stack
// at the start of local allocation, and this pass doesn't.
return true;
/// AdjustStackOffset - Helper function used to adjust the stack frame offset.
void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI,
int FrameIdx, int64_t &Offset,
bool StackGrowsDown,
unsigned &MaxAlign) {
// If the stack grows down, add the object size to find the lowest address.
if (StackGrowsDown)
Offset += MFI.getObjectSize(FrameIdx);
unsigned Align = MFI.getObjectAlignment(FrameIdx);
// If the alignment of this object is greater than that of the stack, then
// increase the stack alignment to match.
MaxAlign = std::max(MaxAlign, Align);
// Adjust to alignment boundary.
Offset = (Offset + Align - 1) / Align * Align;
int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
<< LocalOffset << "\n");
// Keep the offset available for base register allocation
LocalOffsets[FrameIdx] = LocalOffset;
// And tell MFI about it for PEI to use later
MFI.mapLocalFrameObject(FrameIdx, LocalOffset);
if (!StackGrowsDown)
Offset += MFI.getObjectSize(FrameIdx);
/// AssignProtectedObjSet - Helper function to assign large stack objects (i.e.,
/// those required to be close to the Stack Protector) to stack offsets.
void LocalStackSlotPass::AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
SmallSet<int, 16> &ProtectedObjs,
MachineFrameInfo &MFI,
bool StackGrowsDown, int64_t &Offset,
unsigned &MaxAlign) {
for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
E = UnassignedObjs.end(); I != E; ++I) {
int i = *I;
AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
/// abstract stack objects.
void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
// Loop over all of the stack objects, assigning sequential addresses...
MachineFrameInfo &MFI = Fn.getFrameInfo();
const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
bool StackGrowsDown =
TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
int64_t Offset = 0;
unsigned MaxAlign = 0;
// Make sure that the stack protector comes before the local variables on the
// stack.
SmallSet<int, 16> ProtectedObjs;
if (MFI.getStackProtectorIndex() >= 0) {
StackObjSet LargeArrayObjs;
StackObjSet SmallArrayObjs;
StackObjSet AddrOfObjs;
AdjustStackOffset(MFI, MFI.getStackProtectorIndex(), Offset,
StackGrowsDown, MaxAlign);
// Assign large stack objects first.
for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
if (MFI.isDeadObjectIndex(i))
if (MFI.getStackProtectorIndex() == (int)i)
switch (MFI.getObjectSSPLayout(i)) {
case MachineFrameInfo::SSPLK_None:
case MachineFrameInfo::SSPLK_SmallArray:
case MachineFrameInfo::SSPLK_AddrOf:
case MachineFrameInfo::SSPLK_LargeArray:
llvm_unreachable("Unexpected SSPLayoutKind.");
AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
Offset, MaxAlign);
AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
Offset, MaxAlign);
AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
Offset, MaxAlign);
// Then assign frame offsets to stack objects that are not used to spill
// callee saved registers.
for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
if (MFI.isDeadObjectIndex(i))
if (MFI.getStackProtectorIndex() == (int)i)
if (ProtectedObjs.count(i))
AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
// Remember how big this blob of stack space is
static inline bool
lookupCandidateBaseReg(unsigned BaseReg,
int64_t BaseOffset,
int64_t FrameSizeAdjust,
int64_t LocalFrameOffset,
const MachineInstr &MI,
const TargetRegisterInfo *TRI) {
// Check if the relative offset from the where the base register references
// to the target address is in range for the instruction.
int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset;
return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset);
bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
// Scan the function's instructions looking for frame index references.
// For each, ask the target if it wants a virtual base register for it
// based on what we can tell it about where the local will end up in the
// stack frame. If it wants one, re-use a suitable one we've previously
// allocated, or if there isn't one that fits the bill, allocate a new one
// and ask the target to create a defining instruction for it.
bool UsedBaseReg = false;
MachineFrameInfo &MFI = Fn.getFrameInfo();
const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
bool StackGrowsDown =
TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
// Collect all of the instructions in the block that reference
// a frame index. Also store the frame index referenced to ease later
// lookup. (For any insn that has more than one FI reference, we arbitrarily
// choose the first one).
SmallVector<FrameRef, 64> FrameReferenceInsns;
unsigned Order = 0;
for (MachineBasicBlock &BB : Fn) {
for (MachineInstr &MI : BB) {
// Debug value, stackmap and patchpoint instructions can't be out of
// range, so they don't need any updates.
if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
MI.getOpcode() == TargetOpcode::STACKMAP ||
MI.getOpcode() == TargetOpcode::PATCHPOINT)
// For now, allocate the base register(s) within the basic block
// where they're used, and don't try to keep them around outside
// of that. It may be beneficial to try sharing them more broadly
// than that, but the increased register pressure makes that a
// tricky thing to balance. Investigate if re-materializing these
// becomes an issue.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
// Consider replacing all frame index operands that reference
// an object allocated in the local block.
if (MI.getOperand(i).isFI()) {
// Don't try this with values not in the local block.
if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex()))
int Idx = MI.getOperand(i).getIndex();
int64_t LocalOffset = LocalOffsets[Idx];
if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
// Sort the frame references by local offset.
// Use frame index as a tie-breaker in case MI's have the same offset.
llvm::sort(FrameReferenceInsns.begin(), FrameReferenceInsns.end());
MachineBasicBlock *Entry = &Fn.front();
unsigned BaseReg = 0;
int64_t BaseOffset = 0;
// Loop through the frame references and allocate for them as necessary.
for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
FrameRef &FR = FrameReferenceInsns[ref];
MachineInstr &MI = *FR.getMachineInstr();
int64_t LocalOffset = FR.getLocalOffset();
int FrameIdx = FR.getFrameIndex();
assert(MFI.isObjectPreAllocated(FrameIdx) &&
"Only pre-allocated locals expected!");
LLVM_DEBUG(dbgs() << "Considering: " << MI);
unsigned idx = 0;
for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
if (!MI.getOperand(idx).isFI())
if (FrameIdx == MI.getOperand(idx).getIndex())
assert(idx < MI.getNumOperands() && "Cannot find FI operand");
int64_t Offset = 0;
int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
LLVM_DEBUG(dbgs() << " Replacing FI in: " << MI);
// If we have a suitable base register available, use it; otherwise
// create a new one. Note that any offset encoded in the
// instruction itself will be taken into account by the target,
// so we don't have to adjust for it here when reusing a base
// register.
if (UsedBaseReg &&
lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
LocalOffset, MI, TRI)) {
LLVM_DEBUG(dbgs() << " Reusing base register " << BaseReg << "\n");
// We found a register to reuse.
Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
} else {
// No previously defined register was in range, so create a new one.
int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
int64_t PrevBaseOffset = BaseOffset;
BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
// We'd like to avoid creating single-use virtual base registers.
// Because the FrameRefs are in sorted order, and we've already
// processed all FrameRefs before this one, just check whether or not
// the next FrameRef will be able to reuse this new register. If not,
// then don't bother creating it.
if (ref + 1 >= e ||
BaseReg, BaseOffset, FrameSizeAdjust,
FrameReferenceInsns[ref + 1].getLocalOffset(),
*FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
BaseOffset = PrevBaseOffset;
const MachineFunction *MF = MI.getMF();
const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
LLVM_DEBUG(dbgs() << " Materializing base register " << BaseReg
<< " at frame local offset "
<< LocalOffset + InstrOffset << "\n");
// Tell the target to insert the instruction to initialize
// the base register.
// MachineBasicBlock::iterator InsertionPt = Entry->begin();
TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
// The base register already includes any offset specified
// by the instruction, so account for that so it doesn't get
// applied twice.
Offset = -InstrOffset;
UsedBaseReg = true;
assert(BaseReg != 0 && "Unable to allocate virtual base register!");
// Modify the instruction to use the new base register rather
// than the frame index operand.
TRI->resolveFrameIndex(MI, BaseReg, Offset);
LLVM_DEBUG(dbgs() << "Resolved: " << MI);
return UsedBaseReg;