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 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/Function.h"
21 #include "llvm/iOperators.h"
22 #include "llvm/Type.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Constant.h"
25 #include "llvm/Support/CFG.h"
26 #include "Support/PostOrderIterator.h"
27 #include "Support/Statistic.h"
30 Statistic<> NumLinear ("reassociate","Number of insts linearized");
31 Statistic<> NumChanged("reassociate","Number of insts reassociated");
32 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");
34 class Reassociate : public FunctionPass {
35 std::map<BasicBlock*, unsigned> RankMap;
37 bool runOnFunction(Function &F);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 void BuildRankMap(Function &F);
44 unsigned getRank(Value *V);
45 bool ReassociateExpr(BinaryOperator *I);
46 bool ReassociateBB(BasicBlock *BB);
49 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
52 Pass *createReassociatePass() { return new Reassociate(); }
54 void Reassociate::BuildRankMap(Function &F) {
56 ReversePostOrderTraversal<Function*> RPOT(&F);
57 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
58 E = RPOT.end(); I != E; ++I)
62 unsigned Reassociate::getRank(Value *V) {
63 if (isa<Argument>(V)) return 1; // Function argument...
64 if (Instruction *I = dyn_cast<Instruction>(V)) {
65 // If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we
66 // can reassociate expressions for code motion! Since we do not recurse for
67 // PHI nodes, we cannot have infinite recursion here, because there cannot
68 // be loops in the value graph (except for PHI nodes).
70 if (I->getOpcode() == Instruction::PHINode ||
71 I->getOpcode() == Instruction::Alloca ||
72 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
74 return RankMap[I->getParent()];
76 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
77 for (unsigned i = 0, e = I->getNumOperands();
78 i != e && Rank != MaxRank; ++i)
79 Rank = std::max(Rank, getRank(I->getOperand(i)));
84 // Otherwise it's a global or constant, rank 0.
89 // isCommutativeOperator - Return true if the specified instruction is
90 // commutative and associative. If the instruction is not commutative and
91 // associative, we can not reorder its operands!
93 static inline BinaryOperator *isCommutativeOperator(Instruction *I) {
94 // Floating point operations do not commute!
95 if (I->getType()->isFloatingPoint()) return 0;
97 if (I->getOpcode() == Instruction::Add ||
98 I->getOpcode() == Instruction::Mul ||
99 I->getOpcode() == Instruction::And ||
100 I->getOpcode() == Instruction::Or ||
101 I->getOpcode() == Instruction::Xor)
102 return cast<BinaryOperator>(I);
107 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
108 Value *LHS = I->getOperand(0);
109 Value *RHS = I->getOperand(1);
110 unsigned LHSRank = getRank(LHS);
111 unsigned RHSRank = getRank(RHS);
113 bool Changed = false;
115 // Make sure the LHS of the operand always has the greater rank...
116 if (LHSRank < RHSRank) {
119 std::swap(LHSRank, RHSRank);
122 DEBUG(std::cerr << "Transposed: " << I << " Result BB: " << I->getParent());
125 // If the LHS is the same operator as the current one is, and if we are the
126 // only expression using it...
128 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
129 if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) {
130 // If the rank of our current RHS is less than the rank of the LHS's LHS,
131 // then we reassociate the two instructions...
132 if (RHSRank < getRank(LHSI->getOperand(0))) {
134 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
135 if (IOp->getOpcode() == LHSI->getOpcode())
136 TakeOp = 1; // Hoist out non-tree portion
138 // Convert ((a + 12) + 10) into (a + (12 + 10))
139 I->setOperand(0, LHSI->getOperand(TakeOp));
140 LHSI->setOperand(TakeOp, RHS);
141 I->setOperand(1, LHSI);
144 DEBUG(std::cerr << "Reassociated: " << I << " Result BB: "
147 // Since we modified the RHS instruction, make sure that we recheck it.
148 ReassociateExpr(LHSI);
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 *BB, 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), BB, BI);
175 Value *LHS = NegateValue(I->getOperand(0), BB, 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
191 BinaryOperator::create(Instruction::Sub,
192 Constant::getNullValue(V->getType()), V,
193 V->getName()+".neg", BI);
199 bool Reassociate::ReassociateBB(BasicBlock *BB) {
200 bool Changed = false;
201 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
203 // If this instruction is a commutative binary operator, and the ranks of
204 // the two operands are sorted incorrectly, fix it now.
206 if (BinaryOperator *I = isCommutativeOperator(BI)) {
207 if (!I->use_empty()) {
208 // Make sure that we don't have a tree-shaped computation. If we do,
209 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
211 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
212 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
213 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
214 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
215 RHSI->use_size() == 1) {
216 // Insert a new temporary instruction... (A+B)+C
217 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
219 RHSI->getName()+".ra",
222 I->setOperand(0, Tmp);
223 I->setOperand(1, RHSI->getOperand(1));
225 // Process the temporary instruction for reassociation now.
229 DEBUG(std::cerr << "Linearized: " << I << " Result BB: " << BB);
232 // Make sure that this expression is correctly reassociated with respect
233 // to it's used values...
235 Changed |= ReassociateExpr(I);
238 } else if (BI->getOpcode() == Instruction::Sub &&
239 BI->getOperand(0) != Constant::getNullValue(BI->getType())) {
240 // Convert a subtract into an add and a neg instruction... so that sub
241 // instructions can be commuted with other add instructions...
243 Instruction *New = BinaryOperator::create(Instruction::Add,
247 Value *NegatedValue = BI->getOperand(1);
249 // Everyone now refers to the add instruction...
250 BI->replaceAllUsesWith(New);
252 // Put the new add in the place of the subtract... deleting the subtract
253 BI = BB->getInstList().erase(BI);
254 BI = ++BB->getInstList().insert(BI, New);
256 // Calculate the negative value of Operand 1 of the sub instruction...
257 // and set it as the RHS of the add instruction we just made...
258 New->setOperand(1, NegateValue(NegatedValue, BB, BI));
261 DEBUG(std::cerr << "Negated: " << New << " Result BB: " << BB);
269 bool Reassociate::runOnFunction(Function &F) {
270 // Recalculate the rank map for F
273 bool Changed = false;
274 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
275 Changed |= ReassociateBB(FI);
277 // We are done with the rank map...