IfConversion.cpp revision c5d05ef35780bf822765c5a3e2c13201450591ba 
1//===-- IfConversion.cpp - Machine code if conversion pass. ---------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the Evan Cheng and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the machine instruction level if-conversion pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "ifconversion"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/CodeGen/MachineModuleInfo.h"
17#include "llvm/CodeGen/MachineFunctionPass.h"
18#include "llvm/Target/TargetInstrInfo.h"
19#include "llvm/Target/TargetMachine.h"
20#include "llvm/Support/Debug.h"
21#include "llvm/ADT/Statistic.h"
22using namespace llvm;
23
24STATISTIC(NumIfConvBBs, "Number of if-converted blocks");
25
26namespace {
27  class IfConverter : public MachineFunctionPass {
28    enum BBICKind {
29      ICInvalid,       // BB data invalid.
30      ICNotClassfied,  // BB data valid, but not classified.
31      ICTriangle,      // BB is part of a triangle sub-CFG.
32      ICDiamond,       // BB is part of a diamond sub-CFG.
33      ICTriangleEntry, // BB is entry of a triangle sub-CFG.
34      ICDiamondEntry   // BB is entry of a diamond sub-CFG.
35    };
36
37    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
38    /// if-conversion feasibility analysis. This includes results from
39    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
40    /// classification, and common merge block of its successors (if it's a
41    /// diamond shape).
42    struct BBInfo {
43      BBICKind Kind;
44      MachineBasicBlock *EBB;
45      MachineBasicBlock *TBB;
46      MachineBasicBlock *FBB;
47      MachineBasicBlock *CMBB;
48      std::vector<MachineOperand> Cond;
49      BBInfo() : Kind(ICInvalid), EBB(0), TBB(0), FBB(0), CMBB(0) {}
50    };
51
52    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
53    /// basic block number.
54    std::vector<BBInfo> BBAnalysis;
55
56    const TargetInstrInfo *TII;
57    bool MadeChange;
58  public:
59    static char ID;
60    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
61
62    virtual bool runOnMachineFunction(MachineFunction &MF);
63    virtual const char *getPassName() const { return "If converter"; }
64
65  private:
66    void AnalyzeBlock(MachineBasicBlock *BB);
67    void InitialFunctionAnalysis(MachineFunction &MF,
68                                 std::vector<int> &Candidates);
69    bool IfConvertDiamond(BBInfo &BBI);
70    bool IfConvertTriangle(BBInfo &BBI);
71    bool isBlockPredicatable(MachineBasicBlock *BB,
72                             bool IgnoreTerm = false) const;
73    void PredicateBlock(MachineBasicBlock *BB,
74                        std::vector<MachineOperand> &Cond,
75                        bool IgnoreTerm = false);
76    void MergeBlocks(MachineBasicBlock *TBB, MachineBasicBlock *FBB);
77  };
78  char IfConverter::ID = 0;
79}
80
81FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
82
83bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
84  TII = MF.getTarget().getInstrInfo();
85  if (!TII) return false;
86
87  MadeChange = false;
88
89  MF.RenumberBlocks();
90  unsigned NumBBs = MF.getNumBlockIDs();
91  BBAnalysis.resize(NumBBs);
92
93  std::vector<int> Candidates;
94  // Do an intial analysis for each basic block and finding all the potential
95  // candidates to perform if-convesion.
96  InitialFunctionAnalysis(MF, Candidates);
97
98  for (unsigned i = 0, e = Candidates.size(); i != e; ++i) {
99    BBInfo &BBI = BBAnalysis[i];
100    switch (BBI.Kind) {
101    default: assert(false && "Unexpected!");
102      break;
103    case ICTriangleEntry:
104      MadeChange |= IfConvertTriangle(BBI);
105      break;
106    case ICDiamondEntry:
107      MadeChange |= IfConvertDiamond(BBI);
108      break;
109    }
110  }
111  return MadeChange;
112}
113
114static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
115                                         MachineBasicBlock *TBB) {
116  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
117         E = BB->succ_end(); SI != E; ++SI) {
118    MachineBasicBlock *SuccBB = *SI;
119    if (SuccBB != TBB)
120      return SuccBB;
121  }
122  return NULL;
123}
124
125void IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
126  BBInfo &BBI = BBAnalysis[BB->getNumber()];
127
128  if (BBI.Kind != ICInvalid)
129    return;  // Always analyzed.
130  BBI.EBB = BB;
131
132  // Look for 'root' of a simple (non-nested) triangle or diamond.
133  BBI.Kind = ICNotClassfied;
134  if (TII->AnalyzeBranch(*BB, BBI.TBB, BBI.FBB, BBI.Cond)
135      || !BBI.TBB || BBI.Cond.size() == 0)
136    return;
137  AnalyzeBlock(BBI.TBB);
138  BBInfo &TBBI = BBAnalysis[BBI.TBB->getNumber()];
139  if (TBBI.Kind != ICNotClassfied)
140    return;
141
142  if (!BBI.FBB)
143    BBI.FBB = findFalseBlock(BB, BBI.TBB);
144  assert(BBI.FBB && "Expected to find the fallthrough block!");
145
146  AnalyzeBlock(BBI.FBB);
147  BBInfo &FBBI = BBAnalysis[BBI.FBB->getNumber()];
148  if (FBBI.Kind != ICNotClassfied)
149    return;
150
151  // TODO: Only handle very simple cases for now.
152  if (TBBI.FBB || FBBI.FBB || TBBI.Cond.size() > 1 || FBBI.Cond.size() > 1)
153    return;
154
155  if (TBBI.TBB && TBBI.TBB == BBI.FBB) {
156    // Triangle:
157    //   EBB
158    //   | \_
159    //   |  |
160    //   | TBB
161    //   |  /
162    //   FBB
163    BBI.Kind = ICTriangleEntry;
164    TBBI.Kind = FBBI.Kind = ICTriangle;
165  } else if (TBBI.TBB == FBBI.TBB) {
166    // Diamond:
167    //   EBB
168    //   / \_
169    //  |   |
170    // TBB FBB
171    //   \ /
172    //   MBB
173    // Note MBB can be empty in case both TBB and FBB are return blocks.
174    BBI.Kind = ICDiamondEntry;
175    TBBI.Kind = FBBI.Kind = ICDiamond;
176    BBI.CMBB = TBBI.TBB;
177  }
178  return;
179}
180
181void IfConverter::InitialFunctionAnalysis(MachineFunction &MF,
182                                          std::vector<int> &Candidates) {
183  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
184    MachineBasicBlock *BB = I;
185    AnalyzeBlock(BB);
186    BBInfo &BBI = BBAnalysis[BB->getNumber()];
187    if (BBI.Kind == ICTriangleEntry || BBI.Kind == ICDiamondEntry)
188      Candidates.push_back(BB->getNumber());
189  }
190}
191
192bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
193  if (isBlockPredicatable(BBI.TBB, true)) {
194    // Predicate the 'true' block after removing its branch.
195    TII->RemoveBranch(*BBI.TBB);
196    PredicateBlock(BBI.TBB, BBI.Cond);
197
198    // Join the 'true' and 'false' blocks by copying the instructions
199    // from the 'false' block to the 'true' block.
200    MergeBlocks(BBI.TBB, BBI.FBB);
201
202    // Adjust entry block, it should have but a single unconditional
203    // branch.
204    BBI.EBB->removeSuccessor(BBI.FBB);
205    TII->RemoveBranch(*BBI.EBB);
206    std::vector<MachineOperand> NoCond;
207    TII->InsertBranch(*BBI.EBB, BBI.TBB, NULL, NoCond);
208
209    // FIXME: Must maintain LiveIns.
210    NumIfConvBBs++;
211    return true;
212  }
213  return false;
214}
215
216bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
217  if (isBlockPredicatable(BBI.TBB, true) &&
218      isBlockPredicatable(BBI.FBB, true)) {
219    std::vector<MachineInstr*> Dups;
220    if (!BBI.CMBB) {
221      // No common merge block. Check if the terminators (e.g. return) are
222      // the same or predicatable.
223      MachineBasicBlock::iterator TT = BBI.TBB->getFirstTerminator();
224      MachineBasicBlock::iterator FT = BBI.FBB->getFirstTerminator();
225      while (TT != BBI.TBB->end() && FT != BBI.FBB->end()) {
226        if (TT->isIdenticalTo(FT))
227          Dups.push_back(TT);  // Will erase these later.
228        else if (!TII->isPredicatable(TT) && !TII->isPredicatable(FT))
229          return false; // Can't if-convert. Abort!
230        ++TT;
231        ++FT;
232      }
233
234      while (TT != BBI.TBB->end())
235        if (!TII->isPredicatable(TT))
236          return false; // Can't if-convert. Abort!
237      while (FT != BBI.FBB->end())
238        if (!TII->isPredicatable(FT))
239          return false; // Can't if-convert. Abort!
240    }
241
242    // Remove the duplicated instructions from the 'true' block.
243    for (unsigned i = 0, e = Dups.size(); i != e; ++i)
244      Dups[i]->eraseFromParent();
245
246    // Predicate the 'true' block after removing its branch.
247    TII->RemoveBranch(*BBI.TBB);
248    PredicateBlock(BBI.TBB, BBI.Cond);
249
250    // Predicate the 'false' block.
251    std::vector<MachineOperand> NewCond(BBI.Cond);
252    TII->ReverseBranchCondition(NewCond);
253    PredicateBlock(BBI.FBB, NewCond, true);
254
255    // Join the 'true' and 'false' blocks by copying the instructions
256    // from the 'false' block to the 'true' block.
257    MergeBlocks(BBI.TBB, BBI.FBB);
258
259    // Adjust entry block, it should have but a single unconditional
260    // branch .
261    BBI.EBB->removeSuccessor(BBI.FBB);
262    TII->RemoveBranch(*BBI.EBB);
263    std::vector<MachineOperand> NoCond;
264    TII->InsertBranch(*BBI.EBB, BBI.TBB, NULL, NoCond);
265
266    // FIXME: Must maintain LiveIns.
267    NumIfConvBBs += 2;
268    return true;
269  }
270  return false;
271}
272
273/// isBlockPredicatable - Returns true if the block is predicatable. In most
274/// cases, that means all the instructions in the block has M_PREDICATED flag.
275/// If IgnoreTerm is true, assume all the terminator instructions can be
276/// converted or deleted.
277bool IfConverter::isBlockPredicatable(MachineBasicBlock *BB,
278                                      bool IgnoreTerm) const {
279  for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
280       I != E; ++I) {
281    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
282      continue;
283    if (!TII->isPredicatable(I))
284      return false;
285  }
286  return true;
287}
288
289/// PredicateBlock - Predicate every instruction in the block with the specified
290/// condition. If IgnoreTerm is true, skip over all terminator instructions.
291void IfConverter::PredicateBlock(MachineBasicBlock *BB,
292                                 std::vector<MachineOperand> &Cond,
293                                 bool IgnoreTerm) {
294  for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
295       I != E; ++I) {
296    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
297      continue;
298    TII->PredicateInstruction(&*I, Cond);
299  }
300}
301
302/// MergeBlocks - Move all instructions from FBB to the end of TBB.
303///
304void IfConverter::MergeBlocks(MachineBasicBlock *TBB, MachineBasicBlock *FBB) {
305  TBB->splice(TBB->end(), FBB, FBB->begin(), FBB->end());
306}
307