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//===- LiveRangeShrink.cpp - Move instructions to shrink live range -------===//
//
// 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
//
///===---------------------------------------------------------------------===//
///
/// \file
/// This pass moves instructions close to the definition of its operands to
/// shrink live range of the def instruction. The code motion is limited within
/// the basic block. The moved instruction should have 1 def, and more than one
/// uses, all of which are the only use of the def.
///
///===---------------------------------------------------------------------===//
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/CodeGen/MachineBasicBlock.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/TargetRegisterInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "lrshrink"
STATISTIC(NumInstrsHoistedToShrinkLiveRange,
"Number of insructions hoisted to shrink live range.");
namespace {
class LiveRangeShrink : public MachineFunctionPass {
public:
static char ID;
LiveRangeShrink() : MachineFunctionPass(ID) {
initializeLiveRangeShrinkPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
StringRef getPassName() const override { return "Live Range Shrink"; }
bool runOnMachineFunction(MachineFunction &MF) override;
};
} // end anonymous namespace
char LiveRangeShrink::ID = 0;
char &llvm::LiveRangeShrinkID = LiveRangeShrink::ID;
INITIALIZE_PASS(LiveRangeShrink, "lrshrink", "Live Range Shrink Pass", false,
false)
using InstOrderMap = DenseMap<MachineInstr *, unsigned>;
/// Returns \p New if it's dominated by \p Old, otherwise return \p Old.
/// \p M maintains a map from instruction to its dominating order that satisfies
/// M[A] > M[B] guarantees that A is dominated by B.
/// If \p New is not in \p M, return \p Old. Otherwise if \p Old is null, return
/// \p New.
static MachineInstr *FindDominatedInstruction(MachineInstr &New,
MachineInstr *Old,
const InstOrderMap &M) {
auto NewIter = M.find(&New);
if (NewIter == M.end())
return Old;
if (Old == nullptr)
return &New;
unsigned OrderOld = M.find(Old)->second;
unsigned OrderNew = NewIter->second;
if (OrderOld != OrderNew)
return OrderOld < OrderNew ? &New : Old;
// OrderOld == OrderNew, we need to iterate down from Old to see if it
// can reach New, if yes, New is dominated by Old.
for (MachineInstr *I = Old->getNextNode(); M.find(I)->second == OrderNew;
I = I->getNextNode())
if (I == &New)
return &New;
return Old;
}
/// Builds Instruction to its dominating order number map \p M by traversing
/// from instruction \p Start.
static void BuildInstOrderMap(MachineBasicBlock::iterator Start,
InstOrderMap &M) {
M.clear();
unsigned i = 0;
for (MachineInstr &I : make_range(Start, Start->getParent()->end()))
M[&I] = i++;
}
bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
MachineRegisterInfo &MRI = MF.getRegInfo();
LLVM_DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
InstOrderMap IOM;
// Map from register to instruction order (value of IOM) where the
// register is used last. When moving instructions up, we need to
// make sure all its defs (including dead def) will not cross its
// last use when moving up.
DenseMap<unsigned, std::pair<unsigned, MachineInstr *>> UseMap;
for (MachineBasicBlock &MBB : MF) {
if (MBB.empty())
continue;
bool SawStore = false;
BuildInstOrderMap(MBB.begin(), IOM);
UseMap.clear();
for (MachineBasicBlock::iterator Next = MBB.begin(); Next != MBB.end();) {
MachineInstr &MI = *Next;
++Next;
if (MI.isPHI() || MI.isDebugInstr())
continue;
if (MI.mayStore())
SawStore = true;
unsigned CurrentOrder = IOM[&MI];
unsigned Barrier = 0;
MachineInstr *BarrierMI = nullptr;
for (const MachineOperand &MO : MI.operands()) {
if (!MO.isReg() || MO.isDebug())
continue;
if (MO.isUse())
UseMap[MO.getReg()] = std::make_pair(CurrentOrder, &MI);
else if (MO.isDead() && UseMap.count(MO.getReg()))
// Barrier is the last instruction where MO get used. MI should not
// be moved above Barrier.
if (Barrier < UseMap[MO.getReg()].first) {
Barrier = UseMap[MO.getReg()].first;
BarrierMI = UseMap[MO.getReg()].second;
}
}
if (!MI.isSafeToMove(nullptr, SawStore)) {
// If MI has side effects, it should become a barrier for code motion.
// IOM is rebuild from the next instruction to prevent later
// instructions from being moved before this MI.
if (MI.hasUnmodeledSideEffects() && Next != MBB.end()) {
BuildInstOrderMap(Next, IOM);
SawStore = false;
}
continue;
}
const MachineOperand *DefMO = nullptr;
MachineInstr *Insert = nullptr;
// Number of live-ranges that will be shortened. We do not count
// live-ranges that are defined by a COPY as it could be coalesced later.
unsigned NumEligibleUse = 0;
for (const MachineOperand &MO : MI.operands()) {
if (!MO.isReg() || MO.isDead() || MO.isDebug())
continue;
Register Reg = MO.getReg();
// Do not move the instruction if it def/uses a physical register,
// unless it is a constant physical register or a noreg.
if (!Register::isVirtualRegister(Reg)) {
if (!Reg || MRI.isConstantPhysReg(Reg))
continue;
Insert = nullptr;
break;
}
if (MO.isDef()) {
// Do not move if there is more than one def.
if (DefMO) {
Insert = nullptr;
break;
}
DefMO = &MO;
} else if (MRI.hasOneNonDBGUse(Reg) && MRI.hasOneDef(Reg) && DefMO &&
MRI.getRegClass(DefMO->getReg()) ==
MRI.getRegClass(MO.getReg())) {
// The heuristic does not handle different register classes yet
// (registers of different sizes, looser/tighter constraints). This
// is because it needs more accurate model to handle register
// pressure correctly.
MachineInstr &DefInstr = *MRI.def_instr_begin(Reg);
if (!DefInstr.isCopy())
NumEligibleUse++;
Insert = FindDominatedInstruction(DefInstr, Insert, IOM);
} else {
Insert = nullptr;
break;
}
}
// If Barrier equals IOM[I], traverse forward to find if BarrierMI is
// after Insert, if yes, then we should not hoist.
for (MachineInstr *I = Insert; I && IOM[I] == Barrier;
I = I->getNextNode())
if (I == BarrierMI) {
Insert = nullptr;
break;
}
// Move the instruction when # of shrunk live range > 1.
if (DefMO && Insert && NumEligibleUse > 1 && Barrier <= IOM[Insert]) {
MachineBasicBlock::iterator I = std::next(Insert->getIterator());
// Skip all the PHI and debug instructions.
while (I != MBB.end() && (I->isPHI() || I->isDebugInstr()))
I = std::next(I);
if (I == MI.getIterator())
continue;
// Update the dominator order to be the same as the insertion point.
// We do this to maintain a non-decreasing order without need to update
// all instruction orders after the insertion point.
unsigned NewOrder = IOM[&*I];
IOM[&MI] = NewOrder;
NumInstrsHoistedToShrinkLiveRange++;
// Find MI's debug value following MI.
MachineBasicBlock::iterator EndIter = std::next(MI.getIterator());
if (MI.getOperand(0).isReg())
for (; EndIter != MBB.end() && EndIter->isDebugValue() &&
EndIter->getOperand(0).isReg() &&
EndIter->getOperand(0).getReg() == MI.getOperand(0).getReg();
++EndIter, ++Next)
IOM[&*EndIter] = NewOrder;
MBB.splice(I, &MBB, MI.getIterator(), EndIter);
}
}
}
return false;
}