1 //===- Reassociate.cpp - Reassociate binary expressions -------------------===//
3 // This pass reassociates commutative expressions in an order that is designed
4 // to promote better constant propogation, GCSE, LICM, PRE...
6 // For example: 4 + (x + 5) -> x + (4 + 5)
8 // Note that this pass works best if left shifts have been promoted to explicit
9 // multiplies before this pass executes.
11 // In the implementation of this algorithm, constants are assigned rank = 0,
12 // function arguments are rank = 1, and other values are assigned ranks
13 // corresponding to the reverse post order traversal of current function
14 // (starting at 2), which effectively gives values in deep loops higher rank
15 // than values not in loops.
17 // This code was originally written by Chris Lattner, and was then cleaned up
18 // and perfected by Casey Carter.
20 //===----------------------------------------------------------------------===//
22 #include "llvm/Transforms/Scalar.h"
23 #include "llvm/Function.h"
24 #include "llvm/iOperators.h"
25 #include "llvm/Type.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Constant.h"
28 #include "llvm/Support/CFG.h"
29 #include "Support/PostOrderIterator.h"
30 #include "Support/Statistic.h"
33 Statistic<> NumLinear ("reassociate","Number of insts linearized");
34 Statistic<> NumChanged("reassociate","Number of insts reassociated");
35 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");
37 class Reassociate : public FunctionPass {
38 std::map<BasicBlock*, unsigned> RankMap;
40 bool runOnFunction(Function &F);
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
46 void BuildRankMap(Function &F);
47 unsigned getRank(Value *V);
48 bool ReassociateExpr(BinaryOperator *I);
49 bool ReassociateBB(BasicBlock *BB);
52 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
55 Pass *createReassociatePass() { return new Reassociate(); }
57 void Reassociate::BuildRankMap(Function &F) {
59 ReversePostOrderTraversal<Function*> RPOT(&F);
60 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
61 E = RPOT.end(); I != E; ++I)
65 unsigned Reassociate::getRank(Value *V) {
66 if (isa<Argument>(V)) return 1; // Function argument...
67 if (Instruction *I = dyn_cast<Instruction>(V)) {
68 // If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we
69 // can reassociate expressions for code motion! Since we do not recurse for
70 // PHI nodes, we cannot have infinite recursion here, because there cannot
71 // be loops in the value graph (except for PHI nodes).
73 if (I->getOpcode() == Instruction::PHINode ||
74 I->getOpcode() == Instruction::Alloca ||
75 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
77 return RankMap[I->getParent()];
79 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
80 for (unsigned i = 0, e = I->getNumOperands();
81 i != e && Rank != MaxRank; ++i)
82 Rank = std::max(Rank, getRank(I->getOperand(i)));
87 // Otherwise it's a global or constant, rank 0.
92 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
93 Value *LHS = I->getOperand(0);
94 Value *RHS = I->getOperand(1);
95 unsigned LHSRank = getRank(LHS);
96 unsigned RHSRank = getRank(RHS);
100 // Make sure the LHS of the operand always has the greater rank...
101 if (LHSRank < RHSRank) {
102 bool Success = !I->swapOperands();
103 assert(Success && "swapOperands failed");
106 std::swap(LHSRank, RHSRank);
109 DEBUG(std::cerr << "Transposed: " << I << " Result BB: " << I->getParent());
112 // If the LHS is the same operator as the current one is, and if we are the
113 // only expression using it...
115 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
116 if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) {
117 // If the rank of our current RHS is less than the rank of the LHS's LHS,
118 // then we reassociate the two instructions...
119 if (RHSRank < getRank(LHSI->getOperand(0))) {
121 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
122 if (IOp->getOpcode() == LHSI->getOpcode())
123 TakeOp = 1; // Hoist out non-tree portion
125 // Convert ((a + 12) + 10) into (a + (12 + 10))
126 I->setOperand(0, LHSI->getOperand(TakeOp));
128 // Move the LHS expression forward, to ensure that it is dominated by
130 std::string Name = LHSI->getName();
132 BinaryOperator *NewLHS =
133 BinaryOperator::create(LHSI->getOpcode(),
134 LHSI->getOperand(0), LHSI->getOperand(1),
137 NewLHS->setOperand(TakeOp, RHS);
138 I->setOperand(1, NewLHS);
140 assert(LHSI->use_size() == 0 && "References to LHS shouldn't exist!");
141 LHSI->getParent()->getInstList().erase(LHSI);
144 DEBUG(std::cerr << "Reassociated: " << I << " Result BB: "
147 // Since we modified the RHS instruction, make sure that we recheck it.
148 ReassociateExpr(NewLHS);
157 // NegateValue - Insert instructions before the instruction pointed to by BI,
158 // that computes the negative version of the value specified. The negative
159 // version of the value is returned, and BI is left pointing at the instruction
160 // that should be processed next by the reassociation pass.
162 static Value *NegateValue(Value *V, BasicBlock::iterator &BI) {
163 // We are trying to expose opportunity for reassociation. One of the things
164 // that we want to do to achieve this is to push a negation as deep into an
165 // expression chain as possible, to expose the add instructions. In practice,
166 // this means that we turn this:
167 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
168 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
169 // the constants. We assume that instcombine will clean up the mess later if
170 // we introduce tons of unneccesary negation instructions...
172 if (Instruction *I = dyn_cast<Instruction>(V))
173 if (I->getOpcode() == Instruction::Add && I->use_size() == 1) {
174 Value *RHS = NegateValue(I->getOperand(1), BI);
175 Value *LHS = NegateValue(I->getOperand(0), BI);
177 // We must actually insert a new add instruction here, because the neg
178 // instructions do not dominate the old add instruction in general. By
179 // adding it now, we are assured that the neg instructions we just
180 // inserted dominate the instruction we are about to insert after them.
182 return BinaryOperator::create(Instruction::Add, LHS, RHS,
184 cast<Instruction>(RHS)->getNext());
187 // Insert a 'neg' instruction that subtracts the value from zero to get the
190 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
194 bool Reassociate::ReassociateBB(BasicBlock *BB) {
195 bool Changed = false;
196 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
198 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) {
199 // Convert a subtract into an add and a neg instruction... so that sub
200 // instructions can be commuted with other add instructions...
202 // Calculate the negative value of Operand 1 of the sub instruction...
203 // and set it as the RHS of the add instruction we just made...
205 std::string Name = BI->getName();
208 BinaryOperator::create(Instruction::Add, BI->getOperand(0),
209 BI->getOperand(1), Name, BI);
211 // Everyone now refers to the add instruction...
212 BI->replaceAllUsesWith(New);
214 // Put the new add in the place of the subtract... deleting the subtract
215 BB->getInstList().erase(BI);
218 New->setOperand(1, NegateValue(New->getOperand(1), BI));
221 DEBUG(std::cerr << "Negated: " << New << " Result BB: " << BB);
224 // If this instruction is a commutative binary operator, and the ranks of
225 // the two operands are sorted incorrectly, fix it now.
227 if (BI->isAssociative()) {
228 BinaryOperator *I = cast<BinaryOperator>(&*BI);
229 if (!I->use_empty()) {
230 // Make sure that we don't have a tree-shaped computation. If we do,
231 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
233 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
234 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
235 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
236 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
237 RHSI->use_size() == 1) {
238 // Insert a new temporary instruction... (A+B)+C
239 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
241 RHSI->getName()+".ra",
244 I->setOperand(0, Tmp);
245 I->setOperand(1, RHSI->getOperand(1));
247 // Process the temporary instruction for reassociation now.
251 DEBUG(std::cerr << "Linearized: " << I << " Result BB: " << BB);
254 // Make sure that this expression is correctly reassociated with respect
255 // to it's used values...
257 Changed |= ReassociateExpr(I);
266 bool Reassociate::runOnFunction(Function &F) {
267 // Recalculate the rank map for F
270 bool Changed = false;
271 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
272 Changed |= ReassociateBB(FI);
274 // We are done with the rank map...