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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.
#define DEBUG_TYPE "localstackalloc"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/LLVMContext.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetFrameLowering.h"
using namespace llvm;
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
FrameRef(MachineBasicBlock::iterator I, int64_t Offset) :
MI(I), LocalOffset(Offset) {}
bool operator<(const FrameRef &RHS) const {
return LocalOffset < RHS.LocalOffset;
MachineBasicBlock::iterator getMachineInstr() { return MI; }
class LocalStackSlotPass: public MachineFunctionPass {
SmallVector<int64_t,16> LocalOffsets;
void AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
bool StackGrowsDown, 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);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
} // end anonymous namespace
char LocalStackSlotPass::ID = 0;
char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
INITIALIZE_PASS(LocalStackSlotPass, "localstackalloc",
"Local Stack Slot Allocation", false, false)
bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
MachineFrameInfo *MFI = MF.getFrameInfo();
const TargetRegisterInfo *TRI = MF.getTarget().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;
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);
/// 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.getTarget().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> LargeStackObjs;
if (MFI->getStackProtectorIndex() >= 0) {
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)
if (!MFI->MayNeedStackProtector(i))
AdjustStackOffset(MFI, i, Offset, StackGrowsDown, 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 (LargeStackObjs.count(i))
AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
// Remember how big this blob of stack space is
static inline bool
lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
std::pair<unsigned, int64_t> &RegOffset,
int64_t FrameSizeAdjust,
int64_t LocalFrameOffset,
const MachineInstr *MI,
const TargetRegisterInfo *TRI) {
unsigned e = Regs.size();
for (unsigned i = 0; i < e; ++i) {
RegOffset = Regs[i];
// 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 - RegOffset.second;
if (TRI->isFrameOffsetLegal(MI, Offset))
return true;
return false;
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.getTarget().getRegisterInfo();
const TargetFrameLowering &TFI = *Fn.getTarget().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;
// A base register definition is a register + offset pair.
SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
for (MachineFunction::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
MachineInstr *MI = I;
// Debug value instructions can't be out of range, so they don't need
// any updates.
if (MI->isDebugValue())
// 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()))
push_back(FrameRef(MI, LocalOffsets[MI->getOperand(i).getIndex()]));
// Sort the frame references by local offset
array_pod_sort(FrameReferenceInsns.begin(), FrameReferenceInsns.end());
MachineBasicBlock *Entry = Fn.begin();
// Loop through the frame references and allocate for them as necessary.
for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
MachineBasicBlock::iterator I =
MachineInstr *MI = I;
for (unsigned idx = 0, e = MI->getNumOperands(); idx != e; ++idx) {
// Consider replacing all frame index operands that reference
// an object allocated in the local block.
if (MI->getOperand(idx).isFI()) {
int FrameIdx = MI->getOperand(idx).getIndex();
assert(MFI->isObjectPreAllocated(FrameIdx) &&
"Only pre-allocated locals expected!");
DEBUG(dbgs() << "Considering: " << *MI);
if (TRI->needsFrameBaseReg(MI, LocalOffsets[FrameIdx])) {
unsigned BaseReg = 0;
int64_t Offset = 0;
int64_t FrameSizeAdjust =
StackGrowsDown ? MFI->getLocalFrameSize() : 0;
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.
std::pair<unsigned, int64_t> RegOffset;
if (lookupCandidateBaseReg(BaseRegisters, RegOffset,
MI, TRI)) {
DEBUG(dbgs() << " Reusing base register " <<
RegOffset.first << "\n");
// We found a register to reuse.
BaseReg = RegOffset.first;
Offset = FrameSizeAdjust + LocalOffsets[FrameIdx] -
} else {
// No previously defined register was in range, so create a
// new one.
int64_t InstrOffset = TRI->getFrameIndexInstrOffset(MI, idx);
const TargetRegisterClass *RC = TRI->getPointerRegClass();
BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
DEBUG(dbgs() << " Materializing base register " << BaseReg <<
" at frame local offset " <<
LocalOffsets[FrameIdx] + 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;
int64_t BaseOffset = FrameSizeAdjust + LocalOffsets[FrameIdx] +
std::pair<unsigned, int64_t>(BaseReg, BaseOffset));
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(I, BaseReg, Offset);
DEBUG(dbgs() << "Resolved: " << *MI);
return UsedBaseReg;