1 //===-- GCSE.cpp - SSA-based Global Common Subexpression Elimination ------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass is designed to be a very quick global transformation that
11 // eliminates global common subexpressions from a function. It does this by
12 // using an existing value numbering implementation to identify the common
13 // subexpressions, eliminating them when possible.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/BasicBlock.h"
19 #include "llvm/Constant.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Type.h"
22 #include "llvm/Analysis/Dominators.h"
23 #include "llvm/Analysis/ValueNumbering.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 #include "llvm/ADT/DepthFirstIterator.h"
26 #include "llvm/ADT/Statistic.h"
31 Statistic<> NumInstRemoved("gcse", "Number of instructions removed");
32 Statistic<> NumLoadRemoved("gcse", "Number of loads removed");
33 Statistic<> NumCallRemoved("gcse", "Number of calls removed");
34 Statistic<> NumNonInsts ("gcse", "Number of instructions removed due "
35 "to non-instruction values");
36 Statistic<> NumArgsRepl ("gcse", "Number of function arguments replaced "
37 "with constant values");
39 struct GCSE : public FunctionPass {
40 virtual bool runOnFunction(Function &F);
43 void ReplaceInstructionWith(Instruction *I, Value *V);
45 // This transformation requires dominator and immediate dominator info
46 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
48 AU.addRequired<ETForest>();
49 AU.addRequired<DominatorTree>();
50 AU.addRequired<ValueNumbering>();
54 RegisterPass<GCSE> X("gcse", "Global Common Subexpression Elimination");
57 // createGCSEPass - The public interface to this file...
58 FunctionPass *llvm::createGCSEPass() { return new GCSE(); }
60 // GCSE::runOnFunction - This is the main transformation entry point for a
63 bool GCSE::runOnFunction(Function &F) {
66 // Get pointers to the analysis results that we will be using...
67 ETForest &EF = getAnalysis<ETForest>();
68 ValueNumbering &VN = getAnalysis<ValueNumbering>();
69 DominatorTree &DT = getAnalysis<DominatorTree>();
71 std::vector<Value*> EqualValues;
73 // Check for value numbers of arguments. If the value numbering
74 // implementation can prove that an incoming argument is a constant or global
75 // value address, substitute it, making the argument dead.
76 for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E; ++AI)
77 if (!AI->use_empty()) {
78 VN.getEqualNumberNodes(AI, EqualValues);
79 if (!EqualValues.empty()) {
80 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
81 if (isa<Constant>(EqualValues[i])) {
82 AI->replaceAllUsesWith(EqualValues[i]);
91 // Traverse the CFG of the function in dominator order, so that we see each
92 // instruction after we see its operands.
93 for (df_iterator<DominatorTree::Node*> DI = df_begin(DT.getRootNode()),
94 E = df_end(DT.getRootNode()); DI != E; ++DI) {
95 BasicBlock *BB = DI->getBlock();
97 // Remember which instructions we've seen in this basic block as we scan.
98 std::set<Instruction*> BlockInsts;
100 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
101 Instruction *Inst = I++;
103 if (Constant *C = ConstantFoldInstruction(Inst)) {
104 ReplaceInstructionWith(Inst, C);
105 } else if (Inst->getType() != Type::VoidTy) {
106 // If this instruction computes a value, try to fold together common
107 // instructions that compute it.
109 VN.getEqualNumberNodes(Inst, EqualValues);
111 // If this instruction computes a value that is already computed
112 // elsewhere, try to recycle the old value.
113 if (!EqualValues.empty()) {
114 if (Inst == &*BB->begin())
120 // First check to see if we were able to value number this instruction
121 // to a non-instruction value. If so, prefer that value over other
122 // instructions which may compute the same thing.
123 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
124 if (!isa<Instruction>(EqualValues[i])) {
125 ++NumNonInsts; // Keep track of # of insts repl with values
127 // Change all users of Inst to use the replacement and remove it
129 ReplaceInstructionWith(Inst, EqualValues[i]);
131 EqualValues.clear(); // don't enter the next loop
135 // If there were no non-instruction values that this instruction
136 // produces, find a dominating instruction that produces the same
137 // value. If we find one, use it's value instead of ours.
138 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i) {
139 Instruction *OtherI = cast<Instruction>(EqualValues[i]);
140 bool Dominates = false;
141 if (OtherI->getParent() == BB)
142 Dominates = BlockInsts.count(OtherI);
144 Dominates = EF.dominates(OtherI->getParent(), BB);
147 // Okay, we found an instruction with the same value as this one
148 // and that dominates this one. Replace this instruction with the
150 ReplaceInstructionWith(Inst, OtherI);
159 I = Inst; ++I; // Deleted no instructions
160 } else if (I == BB->end()) { // Deleted first instruction
162 } else { // Deleted inst in middle of block.
168 BlockInsts.insert(Inst);
173 // When the worklist is empty, return whether or not we changed anything...
178 void GCSE::ReplaceInstructionWith(Instruction *I, Value *V) {
179 if (isa<LoadInst>(I))
180 ++NumLoadRemoved; // Keep track of loads eliminated
181 if (isa<CallInst>(I))
182 ++NumCallRemoved; // Keep track of calls eliminated
183 ++NumInstRemoved; // Keep track of number of insts eliminated
185 // Update value numbering
186 getAnalysis<ValueNumbering>().deleteValue(I);
188 I->replaceAllUsesWith(V);
190 if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
191 // Removing an invoke instruction requires adding a branch to the normal
192 // destination and removing PHI node entries in the exception destination.
193 new BranchInst(II->getNormalDest(), II);
194 II->getUnwindDest()->removePredecessor(II->getParent());
197 // Erase the instruction from the program.
198 I->getParent()->getInstList().erase(I);