1//===- LoopInstSimplify.cpp - Loop Instruction Simplification Pass --------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass performs lightweight instruction simplification on loop bodies.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "loop-instsimplify"
15#include "llvm/Transforms/Scalar.h"
16#include "llvm/ADT/Statistic.h"
17#include "llvm/ADT/STLExtras.h"
18#include "llvm/Analysis/Dominators.h"
19#include "llvm/Analysis/InstructionSimplify.h"
20#include "llvm/Analysis/LoopInfo.h"
21#include "llvm/Analysis/LoopPass.h"
22#include "llvm/IR/DataLayout.h"
23#include "llvm/IR/Instructions.h"
24#include "llvm/Support/Debug.h"
25#include "llvm/Target/TargetLibraryInfo.h"
26#include "llvm/Transforms/Utils/Local.h"
27using namespace llvm;
28
29STATISTIC(NumSimplified, "Number of redundant instructions simplified");
30
31namespace {
32  class LoopInstSimplify : public LoopPass {
33  public:
34    static char ID; // Pass ID, replacement for typeid
35    LoopInstSimplify() : LoopPass(ID) {
36      initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
37    }
38
39    bool runOnLoop(Loop*, LPPassManager&);
40
41    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42      AU.setPreservesCFG();
43      AU.addRequired<LoopInfo>();
44      AU.addRequiredID(LoopSimplifyID);
45      AU.addPreservedID(LoopSimplifyID);
46      AU.addPreservedID(LCSSAID);
47      AU.addPreserved("scalar-evolution");
48      AU.addRequired<TargetLibraryInfo>();
49    }
50  };
51}
52
53char LoopInstSimplify::ID = 0;
54INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
55                "Simplify instructions in loops", false, false)
56INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
57INITIALIZE_PASS_DEPENDENCY(DominatorTree)
58INITIALIZE_PASS_DEPENDENCY(LoopInfo)
59INITIALIZE_PASS_DEPENDENCY(LCSSA)
60INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
61                "Simplify instructions in loops", false, false)
62
63Pass *llvm::createLoopInstSimplifyPass() {
64  return new LoopInstSimplify();
65}
66
67bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
68  DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
69  LoopInfo *LI = &getAnalysis<LoopInfo>();
70  const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
71  const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
72
73  SmallVector<BasicBlock*, 8> ExitBlocks;
74  L->getUniqueExitBlocks(ExitBlocks);
75  array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
76
77  SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
78
79  // The bit we are stealing from the pointer represents whether this basic
80  // block is the header of a subloop, in which case we only process its phis.
81  typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
82  SmallVector<WorklistItem, 16> VisitStack;
83  SmallPtrSet<BasicBlock*, 32> Visited;
84
85  bool Changed = false;
86  bool LocalChanged;
87  do {
88    LocalChanged = false;
89
90    VisitStack.clear();
91    Visited.clear();
92
93    VisitStack.push_back(WorklistItem(L->getHeader(), false));
94
95    while (!VisitStack.empty()) {
96      WorklistItem Item = VisitStack.pop_back_val();
97      BasicBlock *BB = Item.getPointer();
98      bool IsSubloopHeader = Item.getInt();
99
100      // Simplify instructions in the current basic block.
101      for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
102        Instruction *I = BI++;
103
104        // The first time through the loop ToSimplify is empty and we try to
105        // simplify all instructions. On later iterations ToSimplify is not
106        // empty and we only bother simplifying instructions that are in it.
107        if (!ToSimplify->empty() && !ToSimplify->count(I))
108          continue;
109
110        // Don't bother simplifying unused instructions.
111        if (!I->use_empty()) {
112          Value *V = SimplifyInstruction(I, TD, TLI, DT);
113          if (V && LI->replacementPreservesLCSSAForm(I, V)) {
114            // Mark all uses for resimplification next time round the loop.
115            for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
116                 UI != UE; ++UI)
117              Next->insert(cast<Instruction>(*UI));
118
119            I->replaceAllUsesWith(V);
120            LocalChanged = true;
121            ++NumSimplified;
122          }
123        }
124        LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
125
126        if (IsSubloopHeader && !isa<PHINode>(I))
127          break;
128      }
129
130      // Add all successors to the worklist, except for loop exit blocks and the
131      // bodies of subloops. We visit the headers of loops so that we can process
132      // their phis, but we contract the rest of the subloop body and only follow
133      // edges leading back to the original loop.
134      for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
135           ++SI) {
136        BasicBlock *SuccBB = *SI;
137        if (!Visited.insert(SuccBB))
138          continue;
139
140        const Loop *SuccLoop = LI->getLoopFor(SuccBB);
141        if (SuccLoop && SuccLoop->getHeader() == SuccBB
142                     && L->contains(SuccLoop)) {
143          VisitStack.push_back(WorklistItem(SuccBB, true));
144
145          SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
146          SuccLoop->getExitBlocks(SubLoopExitBlocks);
147
148          for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
149            BasicBlock *ExitBB = SubLoopExitBlocks[i];
150            if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB))
151              VisitStack.push_back(WorklistItem(ExitBB, false));
152          }
153
154          continue;
155        }
156
157        bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
158                                              ExitBlocks.end(), SuccBB);
159        if (IsExitBlock)
160          continue;
161
162        VisitStack.push_back(WorklistItem(SuccBB, false));
163      }
164    }
165
166    // Place the list of instructions to simplify on the next loop iteration
167    // into ToSimplify.
168    std::swap(ToSimplify, Next);
169    Next->clear();
170
171    Changed |= LocalChanged;
172  } while (LocalChanged);
173
174  return Changed;
175}
176