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/BasicBlock.h"
22 #include "llvm/iOperators.h"
23 #include "llvm/Type.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Constant.h"
26 #include "llvm/Support/CFG.h"
27 #include "Support/PostOrderIterator.h"
28 #include "Support/StatisticReporter.h"
30 static Statistic<> NumLinear ("reassociate\t- Number of insts linearized");
31 static Statistic<> NumChanged("reassociate\t- Number of insts reassociated");
32 static Statistic<> NumSwapped("reassociate\t- Number of insts with operands swapped");
35 class Reassociate : public FunctionPass {
36 std::map<BasicBlock*, unsigned> RankMap;
38 bool runOnFunction(Function &F);
40 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44 void BuildRankMap(Function &F);
45 unsigned getRank(Value *V);
46 bool ReassociateExpr(BinaryOperator *I);
47 bool ReassociateBB(BasicBlock *BB);
50 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
53 Pass *createReassociatePass() { return new Reassociate(); }
55 void Reassociate::BuildRankMap(Function &F) {
57 ReversePostOrderTraversal<Function*> RPOT(&F);
58 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
59 E = RPOT.end(); I != E; ++I)
63 unsigned Reassociate::getRank(Value *V) {
64 if (isa<Argument>(V)) return 1; // Function argument...
65 if (Instruction *I = dyn_cast<Instruction>(V)) {
66 // If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we
67 // can reassociate expressions for code motion! Since we do not recurse for
68 // PHI nodes, we cannot have infinite recursion here, because there cannot
69 // be loops in the value graph (except for PHI nodes).
71 if (I->getOpcode() == Instruction::PHINode ||
72 I->getOpcode() == Instruction::Alloca ||
73 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
75 return RankMap[I->getParent()];
77 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
78 for (unsigned i = 0, e = I->getNumOperands();
79 i != e && Rank != MaxRank; ++i)
80 Rank = std::max(Rank, getRank(I->getOperand(i)));
85 // Otherwise it's a global or constant, rank 0.
90 // isCommutativeOperator - Return true if the specified instruction is
91 // commutative and associative. If the instruction is not commutative and
92 // associative, we can not reorder its operands!
94 static inline BinaryOperator *isCommutativeOperator(Instruction *I) {
95 // Floating point operations do not commute!
96 if (I->getType()->isFloatingPoint()) return 0;
98 if (I->getOpcode() == Instruction::Add ||
99 I->getOpcode() == Instruction::Mul ||
100 I->getOpcode() == Instruction::And ||
101 I->getOpcode() == Instruction::Or ||
102 I->getOpcode() == Instruction::Xor)
103 return cast<BinaryOperator>(I);
108 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
109 Value *LHS = I->getOperand(0);
110 Value *RHS = I->getOperand(1);
111 unsigned LHSRank = getRank(LHS);
112 unsigned RHSRank = getRank(RHS);
114 bool Changed = false;
116 // Make sure the LHS of the operand always has the greater rank...
117 if (LHSRank < RHSRank) {
120 std::swap(LHSRank, RHSRank);
123 DEBUG(std::cerr << "Transposed: " << I << " Result BB: " << I->getParent());
126 // If the LHS is the same operator as the current one is, and if we are the
127 // only expression using it...
129 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
130 if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) {
131 // If the rank of our current RHS is less than the rank of the LHS's LHS,
132 // then we reassociate the two instructions...
133 if (RHSRank < getRank(LHSI->getOperand(0))) {
135 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
136 if (IOp->getOpcode() == LHSI->getOpcode())
137 TakeOp = 1; // Hoist out non-tree portion
139 // Convert ((a + 12) + 10) into (a + (12 + 10))
140 I->setOperand(0, LHSI->getOperand(TakeOp));
141 LHSI->setOperand(TakeOp, RHS);
142 I->setOperand(1, LHSI);
145 DEBUG(std::cerr << "Reassociated: " << I << " Result BB: "
148 // Since we modified the RHS instruction, make sure that we recheck it.
149 ReassociateExpr(LHSI);
158 // NegateValue - Insert instructions before the instruction pointed to by BI,
159 // that computes the negative version of the value specified. The negative
160 // version of the value is returned, and BI is left pointing at the instruction
161 // that should be processed next by the reassociation pass.
163 static Value *NegateValue(Value *V, BasicBlock *BB, BasicBlock::iterator &BI) {
164 // We are trying to expose opportunity for reassociation. One of the things
165 // that we want to do to achieve this is to push a negation as deep into an
166 // expression chain as possible, to expose the add instructions. In practice,
167 // this means that we turn this:
168 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
169 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
170 // the constants. We assume that instcombine will clean up the mess later if
171 // we introduce tons of unneccesary negation instructions...
173 if (Instruction *I = dyn_cast<Instruction>(V))
174 if (I->getOpcode() == Instruction::Add && I->use_size() == 1) {
175 Value *RHS = NegateValue(I->getOperand(1), BB, BI);
176 Value *LHS = NegateValue(I->getOperand(0), BB, BI);
178 // We must actually insert a new add instruction here, because the neg
179 // instructions do not dominate the old add instruction in general. By
180 // adding it now, we are assured that the neg instructions we just
181 // inserted dominate the instruction we are about to insert after them.
183 BasicBlock::iterator NBI = cast<Instruction>(RHS);
186 BinaryOperator::create(Instruction::Add, LHS, RHS, I->getName()+".neg");
187 BB->getInstList().insert(++NBI, Add); // Add to the basic block...
191 // Insert a 'neg' instruction that subtracts the value from zero to get the
195 BinaryOperator::create(Instruction::Sub,
196 Constant::getNullValue(V->getType()), V,
197 V->getName()+".neg");
198 BI = BB->getInstList().insert(BI, Neg); // Add to the basic block...
203 bool Reassociate::ReassociateBB(BasicBlock *BB) {
204 bool Changed = false;
205 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
207 // If this instruction is a commutative binary operator, and the ranks of
208 // the two operands are sorted incorrectly, fix it now.
210 if (BinaryOperator *I = isCommutativeOperator(BI)) {
211 if (!I->use_empty()) {
212 // Make sure that we don't have a tree-shaped computation. If we do,
213 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
215 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
216 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
217 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
218 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
219 RHSI->use_size() == 1) {
220 // Insert a new temporary instruction... (A+B)+C
221 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
223 RHSI->getName()+".ra");
224 BI = BB->getInstList().insert(BI, Tmp); // Add to the basic block...
225 I->setOperand(0, Tmp);
226 I->setOperand(1, RHSI->getOperand(1));
228 // Process the temporary instruction for reassociation now.
232 DEBUG(std::cerr << "Linearized: " << I << " Result BB: " << BB);
235 // Make sure that this expression is correctly reassociated with respect
236 // to it's used values...
238 Changed |= ReassociateExpr(I);
241 } else if (BI->getOpcode() == Instruction::Sub &&
242 BI->getOperand(0) != Constant::getNullValue(BI->getType())) {
243 // Convert a subtract into an add and a neg instruction... so that sub
244 // instructions can be commuted with other add instructions...
246 Instruction *New = BinaryOperator::create(Instruction::Add,
250 Value *NegatedValue = BI->getOperand(1);
252 // Everyone now refers to the add instruction...
253 BI->replaceAllUsesWith(New);
255 // Put the new add in the place of the subtract... deleting the subtract
256 BI = BB->getInstList().erase(BI);
257 BI = ++BB->getInstList().insert(BI, New);
259 // Calculate the negative value of Operand 1 of the sub instruction...
260 // and set it as the RHS of the add instruction we just made...
261 New->setOperand(1, NegateValue(NegatedValue, BB, BI));
264 DEBUG(std::cerr << "Negated: " << New << " Result BB: " << BB);
272 bool Reassociate::runOnFunction(Function &F) {
273 // Recalculate the rank map for F
276 bool Changed = false;
277 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
278 Changed |= ReassociateBB(FI);
280 // We are done with the rank map...