1 //===-- GCSE.cpp - SSA based Global Common Subexpr Elimination ------------===//
3 // This pass is designed to be a very quick global transformation that
4 // eliminates global common subexpressions from a function. It does this by
5 // using an existing value numbering implementation to identify the common
6 // subexpressions, eliminating them when possible.
8 //===----------------------------------------------------------------------===//
10 #include "llvm/Transforms/Scalar.h"
11 #include "llvm/iMemory.h"
12 #include "llvm/Type.h"
13 #include "llvm/Analysis/Dominators.h"
14 #include "llvm/Analysis/ValueNumbering.h"
15 #include "llvm/Support/InstIterator.h"
16 #include "Support/Statistic.h"
20 Statistic<> NumInstRemoved("gcse", "Number of instructions removed");
21 Statistic<> NumLoadRemoved("gcse", "Number of loads removed");
22 Statistic<> NumNonInsts ("gcse", "Number of instructions removed due "
23 "to non-instruction values");
25 class GCSE : public FunctionPass {
26 std::set<Instruction*> WorkList;
27 DominatorSet *DomSetInfo;
30 virtual bool runOnFunction(Function &F);
33 bool EliminateRedundancies(Instruction *I,std::vector<Value*> &EqualValues);
34 Instruction *EliminateCSE(Instruction *I, Instruction *Other);
35 void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
37 // This transformation requires dominator and immediate dominator info
38 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.addRequired<DominatorSet>();
41 AU.addRequired<ImmediateDominators>();
42 AU.addRequired<ValueNumbering>();
46 RegisterOpt<GCSE> X("gcse", "Global Common Subexpression Elimination");
49 // createGCSEPass - The public interface to this file...
50 Pass *createGCSEPass() { return new GCSE(); }
53 // GCSE::runOnFunction - This is the main transformation entry point for a
56 bool GCSE::runOnFunction(Function &F) {
59 // Get pointers to the analysis results that we will be using...
60 DomSetInfo = &getAnalysis<DominatorSet>();
61 VN = &getAnalysis<ValueNumbering>();
63 // Step #1: Add all instructions in the function to the worklist for
64 // processing. All of the instructions are considered to be our
65 // subexpressions to eliminate if possible.
67 WorkList.insert(inst_begin(F), inst_end(F));
69 // Step #2: WorkList processing. Iterate through all of the instructions,
70 // checking to see if there are any additionally defined subexpressions in the
71 // program. If so, eliminate them!
73 while (!WorkList.empty()) {
74 Instruction &I = **WorkList.begin(); // Get an instruction from the worklist
75 WorkList.erase(WorkList.begin());
77 // If this instruction computes a value, try to fold together common
78 // instructions that compute it.
80 if (I.getType() != Type::VoidTy) {
81 std::vector<Value*> EqualValues;
82 VN->getEqualNumberNodes(&I, EqualValues);
84 if (!EqualValues.empty())
85 Changed |= EliminateRedundancies(&I, EqualValues);
89 // When the worklist is empty, return whether or not we changed anything...
93 bool GCSE::EliminateRedundancies(Instruction *I,
94 std::vector<Value*> &EqualValues) {
95 // If the EqualValues set contains any non-instruction values, then we know
96 // that all of the instructions can be replaced with the non-instruction value
97 // because it is guaranteed to dominate all of the instructions in the
98 // function. We only have to do hard work if all we have are instructions.
100 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
101 if (!isa<Instruction>(EqualValues[i])) {
102 // Found a non-instruction. Replace all instructions with the
105 Value *Replacement = EqualValues[i];
107 // Make sure we get I as well...
110 // Replace all instructions with the Replacement value.
111 for (i = 0; i != e; ++i)
112 if (Instruction *I = dyn_cast<Instruction>(EqualValues[i])) {
113 // Change all users of I to use Replacement.
114 I->replaceAllUsesWith(Replacement);
116 if (isa<LoadInst>(I))
117 ++NumLoadRemoved; // Keep track of loads eliminated
118 ++NumInstRemoved; // Keep track of number of instructions eliminated
119 ++NumNonInsts; // Keep track of number of insts repl with values
121 // Erase the instruction from the program.
122 I->getParent()->getInstList().erase(I);
128 // Remove duplicate entries from EqualValues...
129 std::sort(EqualValues.begin(), EqualValues.end());
130 EqualValues.erase(std::unique(EqualValues.begin(), EqualValues.end()),
133 // From this point on, EqualValues is logically a vector of instructions.
135 bool Changed = false;
136 EqualValues.push_back(I); // Make sure I is included...
137 while (EqualValues.size() > 1) {
138 // FIXME, this could be done better than simple iteration!
139 Instruction *Test = cast<Instruction>(EqualValues.back());
140 EqualValues.pop_back();
142 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
143 if (Instruction *Ret = EliminateCSE(Test,
144 cast<Instruction>(EqualValues[i]))) {
145 if (Ret == Test) // Eliminated EqualValues[i]
146 EqualValues[i] = Test; // Make sure that we reprocess I at some point
155 // ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
156 // uses of the instruction use First now instead.
158 void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
159 Instruction &Second = *SI;
161 //cerr << "DEL " << (void*)Second << Second;
163 // Add the first instruction back to the worklist
164 WorkList.insert(First);
166 // Add all uses of the second instruction to the worklist
167 for (Value::use_iterator UI = Second.use_begin(), UE = Second.use_end();
169 WorkList.insert(cast<Instruction>(*UI));
171 // Make all users of 'Second' now use 'First'
172 Second.replaceAllUsesWith(First);
174 // Erase the second instruction from the program
175 Second.getParent()->getInstList().erase(SI);
178 // EliminateCSE - The two instruction I & Other have been found to be common
179 // subexpressions. This function is responsible for eliminating one of them,
180 // and for fixing the worklist to be correct. The instruction that is preserved
181 // is returned from the function if the other is eliminated, otherwise null is
184 Instruction *GCSE::EliminateCSE(Instruction *I, Instruction *Other) {
188 WorkList.erase(Other); // Other may not actually be on the worklist anymore...
190 // Handle the easy case, where both instructions are in the same basic block
191 BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
192 Instruction *Ret = 0;
195 // Eliminate the second occuring instruction. Add all uses of the second
196 // instruction to the worklist.
198 // Scan the basic block looking for the "first" instruction
199 BasicBlock::iterator BI = BB1->begin();
200 while (&*BI != I && &*BI != Other) {
202 assert(BI != BB1->end() && "Instructions not found in parent BB!");
205 // Keep track of which instructions occurred first & second
206 Instruction *First = BI;
207 Instruction *Second = I != First ? I : Other; // Get iterator to second inst
210 // Destroy Second, using First instead.
211 ReplaceInstWithInst(First, BI);
214 // Otherwise, the two instructions are in different basic blocks. If one
215 // dominates the other instruction, we can simply use it
217 } else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
218 ReplaceInstWithInst(I, Other);
220 } else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
221 ReplaceInstWithInst(Other, I);
224 // This code is disabled because it has several problems:
225 // One, the actual assumption is wrong, as shown by this code:
226 // int "test"(int %X, int %Y) {
227 // %Z = add int %X, %Y
230 // %Q = add int %X, %Y
234 // Here there are no shared dominators. Additionally, this had the habit of
235 // moving computations where they were not always computed. For example, in
244 // In thiscase, the expression would be hoisted to outside the 'if' stmt,
245 // causing the expression to be evaluated, even for the if (d) path, which
246 // could cause problems, if, for example, it caused a divide by zero. In
247 // general the problem this case is trying to solve is better addressed with
253 if (isa<LoadInst>(Ret))
254 ++NumLoadRemoved; // Keep track of loads eliminated
255 ++NumInstRemoved; // Keep track of number of instructions eliminated
257 // Add all users of Ret to the worklist...
258 for (Value::use_iterator I = Ret->use_begin(), E = Ret->use_end(); I != E;++I)
259 if (Instruction *Inst = dyn_cast<Instruction>(*I))
260 WorkList.insert(Inst);