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/Instructions.h"
16 #include "llvm/Analysis/Dominators.h"
17 #include "llvm/Analysis/InstructionSimplify.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Support/Debug.h"
21 #include "llvm/Target/TargetData.h"
22 #include "llvm/Transforms/Scalar.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/ADT/Statistic.h"
25 using namespace llvm;
26
27 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
28
29 namespace {
30 class LoopInstSimplify : public LoopPass {
31 public:
32 static char ID; // Pass ID, replacement for typeid
LoopInstSimplify()33 LoopInstSimplify() : LoopPass(ID) {
34 initializeLoopInstSimplifyPass(*PassRegistry::getPassRegistry());
35 }
36
37 bool runOnLoop(Loop*, LPPassManager&);
38
getAnalysisUsage(AnalysisUsage & AU) const39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.setPreservesCFG();
41 AU.addRequired<LoopInfo>();
42 AU.addRequiredID(LoopSimplifyID);
43 AU.addPreservedID(LoopSimplifyID);
44 AU.addPreservedID(LCSSAID);
45 AU.addPreserved("scalar-evolution");
46 }
47 };
48 }
49
50 char LoopInstSimplify::ID = 0;
51 INITIALIZE_PASS_BEGIN(LoopInstSimplify, "loop-instsimplify",
52 "Simplify instructions in loops", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)53 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
54 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
55 INITIALIZE_PASS_DEPENDENCY(LCSSA)
56 INITIALIZE_PASS_END(LoopInstSimplify, "loop-instsimplify",
57 "Simplify instructions in loops", false, false)
58
59 Pass *llvm::createLoopInstSimplifyPass() {
60 return new LoopInstSimplify();
61 }
62
runOnLoop(Loop * L,LPPassManager & LPM)63 bool LoopInstSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
64 DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
65 LoopInfo *LI = &getAnalysis<LoopInfo>();
66 const TargetData *TD = getAnalysisIfAvailable<TargetData>();
67
68 SmallVector<BasicBlock*, 8> ExitBlocks;
69 L->getUniqueExitBlocks(ExitBlocks);
70 array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
71
72 SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
73
74 // The bit we are stealing from the pointer represents whether this basic
75 // block is the header of a subloop, in which case we only process its phis.
76 typedef PointerIntPair<BasicBlock*, 1> WorklistItem;
77 SmallVector<WorklistItem, 16> VisitStack;
78 SmallPtrSet<BasicBlock*, 32> Visited;
79
80 bool Changed = false;
81 bool LocalChanged;
82 do {
83 LocalChanged = false;
84
85 VisitStack.clear();
86 Visited.clear();
87
88 VisitStack.push_back(WorklistItem(L->getHeader(), false));
89
90 while (!VisitStack.empty()) {
91 WorklistItem Item = VisitStack.pop_back_val();
92 BasicBlock *BB = Item.getPointer();
93 bool IsSubloopHeader = Item.getInt();
94
95 // Simplify instructions in the current basic block.
96 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
97 Instruction *I = BI++;
98
99 // The first time through the loop ToSimplify is empty and we try to
100 // simplify all instructions. On later iterations ToSimplify is not
101 // empty and we only bother simplifying instructions that are in it.
102 if (!ToSimplify->empty() && !ToSimplify->count(I))
103 continue;
104
105 // Don't bother simplifying unused instructions.
106 if (!I->use_empty()) {
107 Value *V = SimplifyInstruction(I, TD, DT);
108 if (V && LI->replacementPreservesLCSSAForm(I, V)) {
109 // Mark all uses for resimplification next time round the loop.
110 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
111 UI != UE; ++UI)
112 Next->insert(cast<Instruction>(*UI));
113
114 I->replaceAllUsesWith(V);
115 LocalChanged = true;
116 ++NumSimplified;
117 }
118 }
119 LocalChanged |= RecursivelyDeleteTriviallyDeadInstructions(I);
120
121 if (IsSubloopHeader && !isa<PHINode>(I))
122 break;
123 }
124
125 // Add all successors to the worklist, except for loop exit blocks and the
126 // bodies of subloops. We visit the headers of loops so that we can process
127 // their phis, but we contract the rest of the subloop body and only follow
128 // edges leading back to the original loop.
129 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
130 ++SI) {
131 BasicBlock *SuccBB = *SI;
132 if (!Visited.insert(SuccBB))
133 continue;
134
135 const Loop *SuccLoop = LI->getLoopFor(SuccBB);
136 if (SuccLoop && SuccLoop->getHeader() == SuccBB
137 && L->contains(SuccLoop)) {
138 VisitStack.push_back(WorklistItem(SuccBB, true));
139
140 SmallVector<BasicBlock*, 8> SubLoopExitBlocks;
141 SuccLoop->getExitBlocks(SubLoopExitBlocks);
142
143 for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
144 BasicBlock *ExitBB = SubLoopExitBlocks[i];
145 if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB))
146 VisitStack.push_back(WorklistItem(ExitBB, false));
147 }
148
149 continue;
150 }
151
152 bool IsExitBlock = std::binary_search(ExitBlocks.begin(),
153 ExitBlocks.end(), SuccBB);
154 if (IsExitBlock)
155 continue;
156
157 VisitStack.push_back(WorklistItem(SuccBB, false));
158 }
159 }
160
161 // Place the list of instructions to simplify on the next loop iteration
162 // into ToSimplify.
163 std::swap(ToSimplify, Next);
164 Next->clear();
165
166 Changed |= LocalChanged;
167 } while (LocalChanged);
168
169 return Changed;
170 }
171