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 #define DEBUG_TYPE "gcse"
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Function.h"
21 #include "llvm/Type.h"
22 #include "llvm/Analysis/ConstantFolding.h"
23 #include "llvm/Analysis/Dominators.h"
24 #include "llvm/Analysis/ValueNumbering.h"
25 #include "llvm/ADT/DepthFirstIterator.h"
26 #include "llvm/ADT/Statistic.h"
30 STATISTIC(NumInstRemoved, "Number of instructions removed");
31 STATISTIC(NumLoadRemoved, "Number of loads removed");
32 STATISTIC(NumCallRemoved, "Number of calls removed");
33 STATISTIC(NumNonInsts , "Number of instructions removed due "
34 "to non-instruction values");
35 STATISTIC(NumArgsRepl , "Number of function arguments replaced "
36 "with constant values");
38 struct GCSE : public FunctionPass {
39 virtual bool runOnFunction(Function &F);
42 void ReplaceInstructionWith(Instruction *I, Value *V);
44 // This transformation requires dominator and immediate dominator info
45 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 AU.addRequired<ETForest>();
48 AU.addRequired<DominatorTree>();
49 AU.addRequired<ValueNumbering>();
53 RegisterPass<GCSE> X("gcse", "Global Common Subexpression Elimination");
56 // createGCSEPass - The public interface to this file...
57 FunctionPass *llvm::createGCSEPass() { return new GCSE(); }
59 // GCSE::runOnFunction - This is the main transformation entry point for a
62 bool GCSE::runOnFunction(Function &F) {
65 // Get pointers to the analysis results that we will be using...
66 ETForest &EF = getAnalysis<ETForest>();
67 ValueNumbering &VN = getAnalysis<ValueNumbering>();
68 DominatorTree &DT = getAnalysis<DominatorTree>();
70 std::vector<Value*> EqualValues;
72 // Check for value numbers of arguments. If the value numbering
73 // implementation can prove that an incoming argument is a constant or global
74 // value address, substitute it, making the argument dead.
75 for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E; ++AI)
76 if (!AI->use_empty()) {
77 VN.getEqualNumberNodes(AI, EqualValues);
78 if (!EqualValues.empty()) {
79 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
80 if (isa<Constant>(EqualValues[i])) {
81 AI->replaceAllUsesWith(EqualValues[i]);
90 // Traverse the CFG of the function in dominator order, so that we see each
91 // instruction after we see its operands.
92 for (df_iterator<DominatorTree::Node*> DI = df_begin(DT.getRootNode()),
93 E = df_end(DT.getRootNode()); DI != E; ++DI) {
94 BasicBlock *BB = DI->getBlock();
96 // Remember which instructions we've seen in this basic block as we scan.
97 std::set<Instruction*> BlockInsts;
99 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
100 Instruction *Inst = I++;
102 if (Constant *C = ConstantFoldInstruction(Inst)) {
103 ReplaceInstructionWith(Inst, C);
104 } else if (Inst->getType() != Type::VoidTy) {
105 // If this instruction computes a value, try to fold together common
106 // instructions that compute it.
108 VN.getEqualNumberNodes(Inst, EqualValues);
110 // If this instruction computes a value that is already computed
111 // elsewhere, try to recycle the old value.
112 if (!EqualValues.empty()) {
113 if (Inst == &*BB->begin())
119 // First check to see if we were able to value number this instruction
120 // to a non-instruction value. If so, prefer that value over other
121 // instructions which may compute the same thing.
122 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
123 if (!isa<Instruction>(EqualValues[i])) {
124 ++NumNonInsts; // Keep track of # of insts repl with values
126 // Change all users of Inst to use the replacement and remove it
128 ReplaceInstructionWith(Inst, EqualValues[i]);
130 EqualValues.clear(); // don't enter the next loop
134 // If there were no non-instruction values that this instruction
135 // produces, find a dominating instruction that produces the same
136 // value. If we find one, use it's value instead of ours.
137 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i) {
138 Instruction *OtherI = cast<Instruction>(EqualValues[i]);
139 bool Dominates = false;
140 if (OtherI->getParent() == BB)
141 Dominates = BlockInsts.count(OtherI);
143 Dominates = EF.dominates(OtherI->getParent(), BB);
146 // Okay, we found an instruction with the same value as this one
147 // and that dominates this one. Replace this instruction with the
149 ReplaceInstructionWith(Inst, OtherI);
158 I = Inst; ++I; // Deleted no instructions
159 } else if (I == BB->end()) { // Deleted first instruction
161 } else { // Deleted inst in middle of block.
167 BlockInsts.insert(Inst);
172 // When the worklist is empty, return whether or not we changed anything...
177 void GCSE::ReplaceInstructionWith(Instruction *I, Value *V) {
178 if (isa<LoadInst>(I))
179 ++NumLoadRemoved; // Keep track of loads eliminated
180 if (isa<CallInst>(I))
181 ++NumCallRemoved; // Keep track of calls eliminated
182 ++NumInstRemoved; // Keep track of number of insts eliminated
184 // Update value numbering
185 getAnalysis<ValueNumbering>().deleteValue(I);
187 I->replaceAllUsesWith(V);
189 if (InvokeInst *II = dyn_cast<InvokeInst>(I)) {
190 // Removing an invoke instruction requires adding a branch to the normal
191 // destination and removing PHI node entries in the exception destination.
192 new BranchInst(II->getNormalDest(), II);
193 II->getUnwindDest()->removePredecessor(II->getParent());
196 // Erase the instruction from the program.
197 I->getParent()->getInstList().erase(I);