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;
39 std::map<Instruction*, unsigned> InstRankMap;
41 bool runOnFunction(Function &F);
43 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 void BuildRankMap(Function &F);
48 unsigned getRank(Value *V);
49 bool ReassociateExpr(BinaryOperator *I);
50 bool ReassociateBB(BasicBlock *BB);
53 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
56 Pass *createReassociatePass() { return new Reassociate(); }
58 void Reassociate::BuildRankMap(Function &F) {
60 ReversePostOrderTraversal<Function*> RPOT(&F);
61 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
62 E = RPOT.end(); I != E; ++I)
66 unsigned Reassociate::getRank(Value *V) {
67 if (isa<Argument>(V)) return 1; // Function argument...
68 if (Instruction *I = dyn_cast<Instruction>(V)) {
69 // If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we
70 // can reassociate expressions for code motion! Since we do not recurse for
71 // PHI nodes, we cannot have infinite recursion here, because there cannot
72 // be loops in the value graph that do not go through PHI nodes.
74 if (I->getOpcode() == Instruction::PHINode ||
75 I->getOpcode() == Instruction::Alloca ||
76 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
78 return RankMap[I->getParent()];
80 unsigned &CachedRank = InstRankMap[I];
81 if (CachedRank) return CachedRank; // Rank already known?
83 // If not, compute it!
84 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
85 for (unsigned i = 0, e = I->getNumOperands();
86 i != e && Rank != MaxRank; ++i)
87 Rank = std::max(Rank, getRank(I->getOperand(i)));
89 return CachedRank = Rank;
92 // Otherwise it's a global or constant, rank 0.
97 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
98 Value *LHS = I->getOperand(0);
99 Value *RHS = I->getOperand(1);
100 unsigned LHSRank = getRank(LHS);
101 unsigned RHSRank = getRank(RHS);
103 bool Changed = false;
105 // Make sure the LHS of the operand always has the greater rank...
106 if (LHSRank < RHSRank) {
107 bool Success = !I->swapOperands();
108 assert(Success && "swapOperands failed");
111 std::swap(LHSRank, RHSRank);
114 DEBUG(std::cerr << "Transposed: " << I
115 /* << " Result BB: " << I->getParent()*/);
118 // If the LHS is the same operator as the current one is, and if we are the
119 // only expression using it...
121 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
122 if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) {
123 // If the rank of our current RHS is less than the rank of the LHS's LHS,
124 // then we reassociate the two instructions...
125 if (RHSRank < getRank(LHSI->getOperand(0))) {
127 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
128 if (IOp->getOpcode() == LHSI->getOpcode())
129 TakeOp = 1; // Hoist out non-tree portion
131 // Convert ((a + 12) + 10) into (a + (12 + 10))
132 I->setOperand(0, LHSI->getOperand(TakeOp));
133 LHSI->setOperand(TakeOp, RHS);
134 I->setOperand(1, LHSI);
136 // Move the LHS expression forward, to ensure that it is dominated by
138 LHSI->getParent()->getInstList().remove(LHSI);
139 I->getParent()->getInstList().insert(I, LHSI);
142 DEBUG(std::cerr << "Reassociated: " << I/* << " Result BB: "
143 << I->getParent()*/);
145 // Since we modified the RHS instruction, make sure that we recheck it.
146 ReassociateExpr(LHSI);
155 // NegateValue - Insert instructions before the instruction pointed to by BI,
156 // that computes the negative version of the value specified. The negative
157 // version of the value is returned, and BI is left pointing at the instruction
158 // that should be processed next by the reassociation pass.
160 static Value *NegateValue(Value *V, BasicBlock::iterator &BI) {
161 // We are trying to expose opportunity for reassociation. One of the things
162 // that we want to do to achieve this is to push a negation as deep into an
163 // expression chain as possible, to expose the add instructions. In practice,
164 // this means that we turn this:
165 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
166 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
167 // the constants. We assume that instcombine will clean up the mess later if
168 // we introduce tons of unneccesary negation instructions...
170 if (Instruction *I = dyn_cast<Instruction>(V))
171 if (I->getOpcode() == Instruction::Add && I->use_size() == 1) {
172 Value *RHS = NegateValue(I->getOperand(1), BI);
173 Value *LHS = NegateValue(I->getOperand(0), BI);
175 // We must actually insert a new add instruction here, because the neg
176 // instructions do not dominate the old add instruction in general. By
177 // adding it now, we are assured that the neg instructions we just
178 // inserted dominate the instruction we are about to insert after them.
180 return BinaryOperator::create(Instruction::Add, LHS, RHS,
182 cast<Instruction>(RHS)->getNext());
185 // Insert a 'neg' instruction that subtracts the value from zero to get the
188 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
192 bool Reassociate::ReassociateBB(BasicBlock *BB) {
193 bool Changed = false;
194 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
196 DEBUG(std::cerr << "Processing: " << *BI);
197 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) {
198 // Convert a subtract into an add and a neg instruction... so that sub
199 // instructions can be commuted with other add instructions...
201 // Calculate the negative value of Operand 1 of the sub instruction...
202 // and set it as the RHS of the add instruction we just made...
204 std::string Name = BI->getName();
207 BinaryOperator::create(Instruction::Add, BI->getOperand(0),
208 BI->getOperand(1), Name, BI);
210 // Everyone now refers to the add instruction...
211 BI->replaceAllUsesWith(New);
213 // Put the new add in the place of the subtract... deleting the subtract
214 BB->getInstList().erase(BI);
217 New->setOperand(1, NegateValue(New->getOperand(1), BI));
220 DEBUG(std::cerr << "Negated: " << New /*<< " Result BB: " << BB*/);
223 // If this instruction is a commutative binary operator, and the ranks of
224 // the two operands are sorted incorrectly, fix it now.
226 if (BI->isAssociative()) {
227 BinaryOperator *I = cast<BinaryOperator>(&*BI);
228 if (!I->use_empty()) {
229 // Make sure that we don't have a tree-shaped computation. If we do,
230 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
232 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
233 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
234 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
235 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
236 RHSI->use_size() == 1) {
237 // Insert a new temporary instruction... (A+B)+C
238 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
240 RHSI->getName()+".ra",
243 I->setOperand(0, Tmp);
244 I->setOperand(1, RHSI->getOperand(1));
246 // Process the temporary instruction for reassociation now.
250 DEBUG(std::cerr << "Linearized: " << I/* << " Result BB: " << BB*/);
253 // Make sure that this expression is correctly reassociated with respect
254 // to it's used values...
256 Changed |= ReassociateExpr(I);
265 bool Reassociate::runOnFunction(Function &F) {
266 // Recalculate the rank map for F
269 bool Changed = false;
270 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
271 Changed |= ReassociateBB(FI);
273 // We are done with the rank map...