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//===- StackSlotColoring.cpp - Stack slot coloring pass. ------------------===//
//
// 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 stack slot coloring pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/LiveStacks.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "stack-slot-coloring"
static cl::opt<bool>
DisableSharing("no-stack-slot-sharing",
cl::init(false), cl::Hidden,
cl::desc("Suppress slot sharing during stack coloring"));
static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated");
namespace {
class StackSlotColoring : public MachineFunctionPass {
LiveStacks* LS;
MachineFrameInfo *MFI;
const TargetInstrInfo *TII;
const MachineBlockFrequencyInfo *MBFI;
// SSIntervals - Spill slot intervals.
std::vector<LiveInterval*> SSIntervals;
// SSRefs - Keep a list of MachineMemOperands for each spill slot.
// MachineMemOperands can be shared between instructions, so we need
// to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
// become FI0 -> FI1 -> FI2.
SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
// OrigAlignments - Alignments of stack objects before coloring.
SmallVector<Align, 16> OrigAlignments;
// OrigSizes - Sizes of stack objects before coloring.
SmallVector<unsigned, 16> OrigSizes;
// AllColors - If index is set, it's a spill slot, i.e. color.
// FIXME: This assumes PEI locate spill slot with smaller indices
// closest to stack pointer / frame pointer. Therefore, smaller
// index == better color. This is per stack ID.
SmallVector<BitVector, 2> AllColors;
// NextColor - Next "color" that's not yet used. This is per stack ID.
SmallVector<int, 2> NextColors = { -1 };
// UsedColors - "Colors" that have been assigned. This is per stack ID
SmallVector<BitVector, 2> UsedColors;
// Assignments - Color to intervals mapping.
SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
public:
static char ID; // Pass identification
StackSlotColoring() : MachineFunctionPass(ID) {
initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<SlotIndexes>();
AU.addPreserved<SlotIndexes>();
AU.addRequired<LiveStacks>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addPreserved<MachineBlockFrequencyInfo>();
AU.addPreservedID(MachineDominatorsID);
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
void InitializeSlots();
void ScanForSpillSlotRefs(MachineFunction &MF);
bool OverlapWithAssignments(LiveInterval *li, int Color) const;
int ColorSlot(LiveInterval *li);
bool ColorSlots(MachineFunction &MF);
void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
MachineFunction &MF);
bool RemoveDeadStores(MachineBasicBlock* MBB);
};
} // end anonymous namespace
char StackSlotColoring::ID = 0;
char &llvm::StackSlotColoringID = StackSlotColoring::ID;
INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
"Stack Slot Coloring", false, false)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(LiveStacks)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
"Stack Slot Coloring", false, false)
namespace {
// IntervalSorter - Comparison predicate that sort live intervals by
// their weight.
struct IntervalSorter {
bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
return LHS->weight() > RHS->weight();
}
};
} // end anonymous namespace
/// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
/// references and update spill slot weights.
void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
SSRefs.resize(MFI->getObjectIndexEnd());
// FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
for (MachineBasicBlock &MBB : MF) {
for (MachineInstr &MI : MBB) {
for (const MachineOperand &MO : MI.operands()) {
if (!MO.isFI())
continue;
int FI = MO.getIndex();
if (FI < 0)
continue;
if (!LS->hasInterval(FI))
continue;
LiveInterval &li = LS->getInterval(FI);
if (!MI.isDebugInstr())
li.incrementWeight(
LiveIntervals::getSpillWeight(false, true, MBFI, MI));
}
for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
EE = MI.memoperands_end();
MMOI != EE; ++MMOI) {
MachineMemOperand *MMO = *MMOI;
if (const FixedStackPseudoSourceValue *FSV =
dyn_cast_or_null<FixedStackPseudoSourceValue>(
MMO->getPseudoValue())) {
int FI = FSV->getFrameIndex();
if (FI >= 0)
SSRefs[FI].push_back(MMO);
}
}
}
}
}
/// InitializeSlots - Process all spill stack slot liveintervals and add them
/// to a sorted (by weight) list.
void StackSlotColoring::InitializeSlots() {
int LastFI = MFI->getObjectIndexEnd();
// There is always at least one stack ID.
AllColors.resize(1);
UsedColors.resize(1);
OrigAlignments.resize(LastFI);
OrigSizes.resize(LastFI);
AllColors[0].resize(LastFI);
UsedColors[0].resize(LastFI);
Assignments.resize(LastFI);
using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
SmallVector<Pair *, 16> Intervals;
Intervals.reserve(LS->getNumIntervals());
for (auto &I : *LS)
Intervals.push_back(&I);
llvm::sort(Intervals,
[](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
// Gather all spill slots into a list.
LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
for (auto *I : Intervals) {
LiveInterval &li = I->second;
LLVM_DEBUG(li.dump());
int FI = Register::stackSlot2Index(li.reg());
if (MFI->isDeadObjectIndex(FI))
continue;
SSIntervals.push_back(&li);
OrigAlignments[FI] = MFI->getObjectAlign(FI);
OrigSizes[FI] = MFI->getObjectSize(FI);
auto StackID = MFI->getStackID(FI);
if (StackID != 0) {
AllColors.resize(StackID + 1);
UsedColors.resize(StackID + 1);
AllColors[StackID].resize(LastFI);
UsedColors[StackID].resize(LastFI);
}
AllColors[StackID].set(FI);
}
LLVM_DEBUG(dbgs() << '\n');
// Sort them by weight.
llvm::stable_sort(SSIntervals, IntervalSorter());
NextColors.resize(AllColors.size());
// Get first "color".
for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
NextColors[I] = AllColors[I].find_first();
}
/// OverlapWithAssignments - Return true if LiveInterval overlaps with any
/// LiveIntervals that have already been assigned to the specified color.
bool
StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
LiveInterval *OtherLI = OtherLIs[i];
if (OtherLI->overlaps(*li))
return true;
}
return false;
}
/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
int StackSlotColoring::ColorSlot(LiveInterval *li) {
int Color = -1;
bool Share = false;
int FI = Register::stackSlot2Index(li->reg());
uint8_t StackID = MFI->getStackID(FI);
if (!DisableSharing) {
// Check if it's possible to reuse any of the used colors.
Color = UsedColors[StackID].find_first();
while (Color != -1) {
if (!OverlapWithAssignments(li, Color)) {
Share = true;
++NumEliminated;
break;
}
Color = UsedColors[StackID].find_next(Color);
}
}
if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
Share = false;
}
// Assign it to the first available color (assumed to be the best) if it's
// not possible to share a used color with other objects.
if (!Share) {
assert(NextColors[StackID] != -1 && "No more spill slots?");
Color = NextColors[StackID];
UsedColors[StackID].set(Color);
NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
}
assert(MFI->getStackID(Color) == MFI->getStackID(FI));
// Record the assignment.
Assignments[Color].push_back(li);
LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
// Change size and alignment of the allocated slot. If there are multiple
// objects sharing the same slot, then make sure the size and alignment
// are large enough for all.
Align Alignment = OrigAlignments[FI];
if (!Share || Alignment > MFI->getObjectAlign(Color))
MFI->setObjectAlignment(Color, Alignment);
int64_t Size = OrigSizes[FI];
if (!Share || Size > MFI->getObjectSize(Color))
MFI->setObjectSize(Color, Size);
return Color;
}
/// Colorslots - Color all spill stack slots and rewrite all frameindex machine
/// operands in the function.
bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
unsigned NumObjs = MFI->getObjectIndexEnd();
SmallVector<int, 16> SlotMapping(NumObjs, -1);
SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
BitVector UsedColors(NumObjs);
LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
bool Changed = false;
for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
LiveInterval *li = SSIntervals[i];
int SS = Register::stackSlot2Index(li->reg());
int NewSS = ColorSlot(li);
assert(NewSS >= 0 && "Stack coloring failed?");
SlotMapping[SS] = NewSS;
RevMap[NewSS].push_back(SS);
SlotWeights[NewSS] += li->weight();
UsedColors.set(NewSS);
Changed |= (SS != NewSS);
}
LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
LiveInterval *li = SSIntervals[i];
int SS = Register::stackSlot2Index(li->reg());
li->setWeight(SlotWeights[SS]);
}
// Sort them by new weight.
llvm::stable_sort(SSIntervals, IntervalSorter());
#ifndef NDEBUG
for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
LLVM_DEBUG(SSIntervals[i]->dump());
LLVM_DEBUG(dbgs() << '\n');
#endif
if (!Changed)
return false;
// Rewrite all MachineMemOperands.
for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
int NewFI = SlotMapping[SS];
if (NewFI == -1 || (NewFI == (int)SS))
continue;
const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
RefMMOs[i]->setValue(NewSV);
}
// Rewrite all MO_FrameIndex operands. Look for dead stores.
for (MachineBasicBlock &MBB : MF) {
for (MachineInstr &MI : MBB)
RewriteInstruction(MI, SlotMapping, MF);
RemoveDeadStores(&MBB);
}
// Delete unused stack slots.
for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
int NextColor = NextColors[StackID];
while (NextColor != -1) {
LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
MFI->RemoveStackObject(NextColor);
NextColor = AllColors[StackID].find_next(NextColor);
}
}
return true;
}
/// RewriteInstruction - Rewrite specified instruction by replacing references
/// to old frame index with new one.
void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
SmallVectorImpl<int> &SlotMapping,
MachineFunction &MF) {
// Update the operands.
for (MachineOperand &MO : MI.operands()) {
if (!MO.isFI())
continue;
int OldFI = MO.getIndex();
if (OldFI < 0)
continue;
int NewFI = SlotMapping[OldFI];
if (NewFI == -1 || NewFI == OldFI)
continue;
assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
MO.setIndex(NewFI);
}
// The MachineMemOperands have already been updated.
}
/// RemoveDeadStores - Scan through a basic block and look for loads followed
/// by stores. If they're both using the same stack slot, then the store is
/// definitely dead. This could obviously be much more aggressive (consider
/// pairs with instructions between them), but such extensions might have a
/// considerable compile time impact.
bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
// FIXME: This could be much more aggressive, but we need to investigate
// the compile time impact of doing so.
bool changed = false;
SmallVector<MachineInstr*, 4> toErase;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I) {
if (DCELimit != -1 && (int)NumDead >= DCELimit)
break;
int FirstSS, SecondSS;
if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
FirstSS != -1) {
++NumDead;
changed = true;
toErase.push_back(&*I);
continue;
}
MachineBasicBlock::iterator NextMI = std::next(I);
MachineBasicBlock::iterator ProbableLoadMI = I;
unsigned LoadReg = 0;
unsigned StoreReg = 0;
unsigned LoadSize = 0;
unsigned StoreSize = 0;
if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
continue;
// Skip the ...pseudo debugging... instructions between a load and store.
while ((NextMI != E) && NextMI->isDebugInstr()) {
++NextMI;
++I;
}
if (NextMI == E) continue;
if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
continue;
if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
LoadSize != StoreSize)
continue;
++NumDead;
changed = true;
if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
++NumDead;
toErase.push_back(&*ProbableLoadMI);
}
toErase.push_back(&*NextMI);
++I;
}
for (MachineInstr *MI : toErase)
MI->eraseFromParent();
return changed;
}
bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG({
dbgs() << "********** Stack Slot Coloring **********\n"
<< "********** Function: " << MF.getName() << '\n';
});
if (skipFunction(MF.getFunction()))
return false;
MFI = &MF.getFrameInfo();
TII = MF.getSubtarget().getInstrInfo();
LS = &getAnalysis<LiveStacks>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
bool Changed = false;
unsigned NumSlots = LS->getNumIntervals();
if (NumSlots == 0)
// Nothing to do!
return false;
// If there are calls to setjmp or sigsetjmp, don't perform stack slot
// coloring. The stack could be modified before the longjmp is executed,
// resulting in the wrong value being used afterwards. (See
// <rdar://problem/8007500>.)
if (MF.exposesReturnsTwice())
return false;
// Gather spill slot references
ScanForSpillSlotRefs(MF);
InitializeSlots();
Changed = ColorSlots(MF);
for (int &Next : NextColors)
Next = -1;
SSIntervals.clear();
for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
SSRefs[i].clear();
SSRefs.clear();
OrigAlignments.clear();
OrigSizes.clear();
AllColors.clear();
UsedColors.clear();
for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
Assignments[i].clear();
Assignments.clear();
return Changed;
}