LLVM  9.0.0svn
UnreachableBlockElim.cpp
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1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This pass is an extremely simple version of the SimplifyCFG pass. Its sole
10 // job is to delete LLVM basic blocks that are not reachable from the entry
11 // node. To do this, it performs a simple depth first traversal of the CFG,
12 // then deletes any unvisited nodes.
13 //
14 // Note that this pass is really a hack. In particular, the instruction
15 // selectors for various targets should just not generate code for unreachable
16 // blocks. Until LLVM has a more systematic way of defining instruction
17 // selectors, however, we cannot really expect them to handle additional
18 // complexity.
19 //
20 //===----------------------------------------------------------------------===//
21 
24 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/CodeGen/Passes.h"
33 #include "llvm/IR/CFG.h"
34 #include "llvm/IR/Constant.h"
35 #include "llvm/IR/Dominators.h"
36 #include "llvm/IR/Function.h"
37 #include "llvm/IR/Instructions.h"
38 #include "llvm/IR/Type.h"
39 #include "llvm/Pass.h"
40 using namespace llvm;
41 
44 
45  // Mark all reachable blocks.
46  for (BasicBlock *BB : depth_first_ext(&F, Reachable))
47  (void)BB/* Mark all reachable blocks */;
48 
49  // Loop over all dead blocks, remembering them and deleting all instructions
50  // in them.
51  std::vector<BasicBlock*> DeadBlocks;
52  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
53  if (!Reachable.count(&*I)) {
54  BasicBlock *BB = &*I;
55  DeadBlocks.push_back(BB);
56  while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
57  PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
58  BB->getInstList().pop_front();
59  }
60  for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
61  (*SI)->removePredecessor(BB);
62  BB->dropAllReferences();
63  }
64 
65  // Actually remove the blocks now.
66  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
67  DeadBlocks[i]->eraseFromParent();
68  }
69 
70  return !DeadBlocks.empty();
71 }
72 
73 namespace {
74 class UnreachableBlockElimLegacyPass : public FunctionPass {
75  bool runOnFunction(Function &F) override {
76  return eliminateUnreachableBlock(F);
77  }
78 
79 public:
80  static char ID; // Pass identification, replacement for typeid
81  UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
84  }
85 
86  void getAnalysisUsage(AnalysisUsage &AU) const override {
88  }
89 };
90 }
92 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
93  "Remove unreachable blocks from the CFG", false, false)
94 
96  return new UnreachableBlockElimLegacyPass();
97 }
98 
101  bool Changed = eliminateUnreachableBlock(F);
102  if (!Changed)
103  return PreservedAnalyses::all();
106  return PA;
107 }
108 
109 namespace {
110  class UnreachableMachineBlockElim : public MachineFunctionPass {
111  bool runOnMachineFunction(MachineFunction &F) override;
112  void getAnalysisUsage(AnalysisUsage &AU) const override;
113  MachineModuleInfo *MMI;
114  public:
115  static char ID; // Pass identification, replacement for typeid
116  UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
117  };
118 }
120 
121 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
122  "Remove unreachable machine basic blocks", false, false)
123 
125 
126 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
130 }
131 
132 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
134  bool ModifiedPHI = false;
135 
136  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
137  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
138  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
139 
140  // Mark all reachable blocks.
141  for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
142  (void)BB/* Mark all reachable blocks */;
143 
144  // Loop over all dead blocks, remembering them and deleting all instructions
145  // in them.
146  std::vector<MachineBasicBlock*> DeadBlocks;
147  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
148  MachineBasicBlock *BB = &*I;
149 
150  // Test for deadness.
151  if (!Reachable.count(BB)) {
152  DeadBlocks.push_back(BB);
153 
154  // Update dominator and loop info.
155  if (MLI) MLI->removeBlock(BB);
156  if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
157 
158  while (BB->succ_begin() != BB->succ_end()) {
159  MachineBasicBlock* succ = *BB->succ_begin();
160 
161  MachineBasicBlock::iterator start = succ->begin();
162  while (start != succ->end() && start->isPHI()) {
163  for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
164  if (start->getOperand(i).isMBB() &&
165  start->getOperand(i).getMBB() == BB) {
166  start->RemoveOperand(i);
167  start->RemoveOperand(i-1);
168  }
169 
170  start++;
171  }
172 
173  BB->removeSuccessor(BB->succ_begin());
174  }
175  }
176  }
177 
178  // Actually remove the blocks now.
179  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
180  DeadBlocks[i]->eraseFromParent();
181 
182  // Cleanup PHI nodes.
183  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
184  MachineBasicBlock *BB = &*I;
185  // Prune unneeded PHI entries.
187  BB->pred_end());
189  while (phi != BB->end() && phi->isPHI()) {
190  for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
191  if (!preds.count(phi->getOperand(i).getMBB())) {
192  phi->RemoveOperand(i);
193  phi->RemoveOperand(i-1);
194  ModifiedPHI = true;
195  }
196 
197  if (phi->getNumOperands() == 3) {
198  const MachineOperand &Input = phi->getOperand(1);
199  const MachineOperand &Output = phi->getOperand(0);
200  unsigned InputReg = Input.getReg();
201  unsigned OutputReg = Output.getReg();
202  assert(Output.getSubReg() == 0 && "Cannot have output subregister");
203  ModifiedPHI = true;
204 
205  if (InputReg != OutputReg) {
207  unsigned InputSub = Input.getSubReg();
208  if (InputSub == 0 &&
209  MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
210  !Input.isUndef()) {
211  MRI.replaceRegWith(OutputReg, InputReg);
212  } else {
213  // The input register to the PHI has a subregister or it can't be
214  // constrained to the proper register class or it is undef:
215  // insert a COPY instead of simply replacing the output
216  // with the input.
218  BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
219  TII->get(TargetOpcode::COPY), OutputReg)
220  .addReg(InputReg, getRegState(Input), InputSub);
221  }
222  phi++->eraseFromParent();
223  }
224  continue;
225  }
226 
227  ++phi;
228  }
229  }
230 
231  F.RenumberBlocks();
232 
233  return (!DeadBlocks.empty() || ModifiedPHI);
234 }
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
const TargetRegisterClass * getRegClass(unsigned Reg) const
Return the register class of the specified virtual register.
unsigned getRegState(const MachineOperand &RegOp)
Get all register state flags from machine operand RegOp.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
void RenumberBlocks(MachineBasicBlock *MBBFrom=nullptr)
RenumberBlocks - This discards all of the MachineBasicBlock numbers and recomputes them...
iterator end()
Definition: Function.h:657
unsigned getReg() const
getReg - Returns the register number.
unsigned getSubReg() const
Analysis pass which computes a DominatorTree.
Definition: Dominators.h:230
F(f)
void initializeUnreachableBlockElimLegacyPassPass(PassRegistry &)
static Constant * getNullValue(Type *Ty)
Constructor to create a &#39;0&#39; constant of arbitrary type.
Definition: Constants.cpp:264
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:268
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
static bool eliminateUnreachableBlock(Function &F)
Interval::succ_iterator succ_begin(Interval *I)
succ_begin/succ_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:102
char & UnreachableMachineBlockElimID
UnreachableMachineBlockElimination - This pass removes unreachable machine basic blocks.
FunctionPass * createUnreachableBlockEliminationPass()
createUnreachableBlockEliminationPass - The LLVM code generator does not work well with unreachable b...
virtual const TargetInstrInfo * getInstrInfo() const
iterator begin()
Definition: Function.h:655
const TargetRegisterClass * constrainRegClass(unsigned Reg, const TargetRegisterClass *RC, unsigned MinNumRegs=0)
constrainRegClass - Constrain the register class of the specified virtual register to be a common sub...
Interval::succ_iterator succ_end(Interval *I)
Definition: Interval.h:105
TargetInstrInfo - Interface to description of machine instruction set.
MachineInstrBuilder BuildMI(MachineFunction &MF, const DebugLoc &DL, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
static bool runOnFunction(Function &F, bool PostInlining)
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:153
unsigned const MachineRegisterInfo * MRI
LLVM Basic Block Representation.
Definition: BasicBlock.h:57
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
void getAnalysisUsage(AnalysisUsage &AU) const override
getAnalysisUsage - Subclasses that override getAnalysisUsage must call this.
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
iterator_range< df_ext_iterator< T, SetTy > > depth_first_ext(const T &G, SetTy &S)
Represent the analysis usage information of a pass.
void pop_front()
Definition: ilist.h:312
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:284
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:381
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:159
const InstListType & getInstList() const
Return the underlying instruction list container.
Definition: BasicBlock.h:333
Iterator for intrusive lists based on ilist_node.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:417
MachineOperand class - Representation of each machine instruction operand.
void replaceRegWith(unsigned FromReg, unsigned ToReg)
replaceRegWith - Replace all instances of FromReg with ToReg in the machine function.
MachineRegisterInfo - Keep track of information for virtual and physical registers, including vreg register classes, use/def chains for registers, etc.
This file provides various utilities for inspecting and working with the control flow graph in LLVM I...
MachineRegisterInfo & getRegInfo()
getRegInfo - Return information about the registers currently in use.
const MCInstrDesc & get(unsigned Opcode) const
Return the machine instruction descriptor that corresponds to the specified instruction opcode...
Definition: MCInstrInfo.h:44
#define I(x, y, z)
Definition: MD5.cpp:58
void preserve()
Mark an analysis as preserved.
Definition: PassManager.h:174
iterator getFirstNonPHI()
Returns a pointer to the first instruction in this block that is not a PHINode instruction.
void removeSuccessor(MachineBasicBlock *Succ, bool NormalizeSuccProbs=false)
Remove successor from the successors list of this MachineBasicBlock.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
A container for analyses that lazily runs them and caches their results.
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:259
DominatorTree Class - Concrete subclass of DominatorTreeBase that is used to compute a normal dominat...
void dropAllReferences()
Cause all subinstructions to "let go" of all the references that said subinstructions are maintaining...
Definition: BasicBlock.cpp:226
INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim", "Remove unreachable blocks from the CFG", false, false) FunctionPass *llvm
This class contains meta information specific to a module.