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);
129 // Remove duplicate entries from EqualValues...
130 std::sort(EqualValues.begin(), EqualValues.end());
131 EqualValues.erase(std::unique(EqualValues.begin(), EqualValues.end()),
134 // From this point on, EqualValues is logically a vector of instructions.
136 bool Changed = false;
137 EqualValues.push_back(I); // Make sure I is included...
138 while (EqualValues.size() > 1) {
139 // FIXME, this could be done better than simple iteration!
140 Instruction *Test = cast<Instruction>(EqualValues.back());
141 EqualValues.pop_back();
143 for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
144 if (Instruction *Ret = EliminateCSE(Test,
145 cast<Instruction>(EqualValues[i]))) {
146 if (Ret == Test) // Eliminated EqualValues[i]
147 EqualValues[i] = Test; // Make sure that we reprocess I at some point
156 // ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
157 // uses of the instruction use First now instead.
159 void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
160 Instruction &Second = *SI;
162 //cerr << "DEL " << (void*)Second << Second;
164 // Add the first instruction back to the worklist
165 WorkList.insert(First);
167 // Add all uses of the second instruction to the worklist
168 for (Value::use_iterator UI = Second.use_begin(), UE = Second.use_end();
170 WorkList.insert(cast<Instruction>(*UI));
172 // Make all users of 'Second' now use 'First'
173 Second.replaceAllUsesWith(First);
175 // Erase the second instruction from the program
176 Second.getParent()->getInstList().erase(SI);
179 // EliminateCSE - The two instruction I & Other have been found to be common
180 // subexpressions. This function is responsible for eliminating one of them,
181 // and for fixing the worklist to be correct. The instruction that is preserved
182 // is returned from the function if the other is eliminated, otherwise null is
185 Instruction *GCSE::EliminateCSE(Instruction *I, Instruction *Other) {
189 WorkList.erase(Other); // Other may not actually be on the worklist anymore...
191 // Handle the easy case, where both instructions are in the same basic block
192 BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
193 Instruction *Ret = 0;
196 // Eliminate the second occuring instruction. Add all uses of the second
197 // instruction to the worklist.
199 // Scan the basic block looking for the "first" instruction
200 BasicBlock::iterator BI = BB1->begin();
201 while (&*BI != I && &*BI != Other) {
203 assert(BI != BB1->end() && "Instructions not found in parent BB!");
206 // Keep track of which instructions occurred first & second
207 Instruction *First = BI;
208 Instruction *Second = I != First ? I : Other; // Get iterator to second inst
211 // Destroy Second, using First instead.
212 ReplaceInstWithInst(First, BI);
215 // Otherwise, the two instructions are in different basic blocks. If one
216 // dominates the other instruction, we can simply use it
218 } else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
219 ReplaceInstWithInst(I, Other);
221 } else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
222 ReplaceInstWithInst(Other, I);
225 // This code is disabled because it has several problems:
226 // One, the actual assumption is wrong, as shown by this code:
227 // int "test"(int %X, int %Y) {
228 // %Z = add int %X, %Y
231 // %Q = add int %X, %Y
235 // Here there are no shared dominators. Additionally, this had the habit of
236 // moving computations where they were not always computed. For example, in
245 // In thiscase, the expression would be hoisted to outside the 'if' stmt,
246 // causing the expression to be evaluated, even for the if (d) path, which
247 // could cause problems, if, for example, it caused a divide by zero. In
248 // general the problem this case is trying to solve is better addressed with
254 if (isa<LoadInst>(Ret))
255 ++NumLoadRemoved; // Keep track of loads eliminated
256 ++NumInstRemoved; // Keep track of number of instructions eliminated
258 // Add all users of Ret to the worklist...
259 for (Value::use_iterator I = Ret->use_begin(), E = Ret->use_end(); I != E;++I)
260 if (Instruction *Inst = dyn_cast<Instruction>(*I))
261 WorkList.insert(Inst);