1 //===- Reassociate.cpp - Reassociate binary expressions -------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass reassociates commutative expressions in an order that is designed
11 // to promote better constant propagation, GCSE, LICM, PRE...
13 // For example: 4 + (x + 5) -> x + (4 + 5)
15 // Note that this pass works best if left shifts have been promoted to explicit
16 // multiplies before this pass executes.
18 // In the implementation of this algorithm, constants are assigned rank = 0,
19 // function arguments are rank = 1, and other values are assigned ranks
20 // corresponding to the reverse post order traversal of current function
21 // (starting at 2), which effectively gives values in deep loops higher rank
22 // than values not in loops.
24 //===----------------------------------------------------------------------===//
26 #include "llvm/Transforms/Scalar.h"
27 #include "llvm/Function.h"
28 #include "llvm/iOperators.h"
29 #include "llvm/Type.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Constant.h"
32 #include "llvm/Support/CFG.h"
33 #include "Support/Debug.h"
34 #include "Support/PostOrderIterator.h"
35 #include "Support/Statistic.h"
38 Statistic<> NumLinear ("reassociate","Number of insts linearized");
39 Statistic<> NumChanged("reassociate","Number of insts reassociated");
40 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");
42 class Reassociate : public FunctionPass {
43 std::map<BasicBlock*, unsigned> RankMap;
44 std::map<Value*, unsigned> ValueRankMap;
46 bool runOnFunction(Function &F);
48 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
52 void BuildRankMap(Function &F);
53 unsigned getRank(Value *V);
54 bool ReassociateExpr(BinaryOperator *I);
55 bool ReassociateBB(BasicBlock *BB);
58 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
61 Pass *createReassociatePass() { return new Reassociate(); }
63 void Reassociate::BuildRankMap(Function &F) {
66 // Assign distinct ranks to function arguments
67 for (Function::aiterator I = F.abegin(), E = F.aend(); I != E; ++I)
68 ValueRankMap[I] = ++i;
70 ReversePostOrderTraversal<Function*> RPOT(&F);
71 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
72 E = RPOT.end(); I != E; ++I)
73 RankMap[*I] = ++i << 16;
76 unsigned Reassociate::getRank(Value *V) {
77 if (isa<Argument>(V)) return ValueRankMap[V]; // Function argument...
79 if (Instruction *I = dyn_cast<Instruction>(V)) {
80 // If this is an expression, return the 1+MAX(rank(LHS), rank(RHS)) so that
81 // we can reassociate expressions for code motion! Since we do not recurse
82 // for PHI nodes, we cannot have infinite recursion here, because there
83 // cannot be loops in the value graph that do not go through PHI nodes.
85 if (I->getOpcode() == Instruction::PHI ||
86 I->getOpcode() == Instruction::Alloca ||
87 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
88 I->mayWriteToMemory()) // Cannot move inst if it writes to memory!
89 return RankMap[I->getParent()];
91 unsigned &CachedRank = ValueRankMap[I];
92 if (CachedRank) return CachedRank; // Rank already known?
94 // If not, compute it!
95 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
96 for (unsigned i = 0, e = I->getNumOperands();
97 i != e && Rank != MaxRank; ++i)
98 Rank = std::max(Rank, getRank(I->getOperand(i)));
100 DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = "
103 return CachedRank = Rank+1;
106 // Otherwise it's a global or constant, rank 0.
111 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
112 Value *LHS = I->getOperand(0);
113 Value *RHS = I->getOperand(1);
114 unsigned LHSRank = getRank(LHS);
115 unsigned RHSRank = getRank(RHS);
117 bool Changed = false;
119 // Make sure the LHS of the operand always has the greater rank...
120 if (LHSRank < RHSRank) {
121 bool Success = !I->swapOperands();
122 assert(Success && "swapOperands failed");
125 std::swap(LHSRank, RHSRank);
128 DEBUG(std::cerr << "Transposed: " << I
129 /* << " Result BB: " << I->getParent()*/);
132 // If the LHS is the same operator as the current one is, and if we are the
133 // only expression using it...
135 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
136 if (LHSI->getOpcode() == I->getOpcode() && LHSI->hasOneUse()) {
137 // If the rank of our current RHS is less than the rank of the LHS's LHS,
138 // then we reassociate the two instructions...
141 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
142 if (IOp->getOpcode() == LHSI->getOpcode())
143 TakeOp = 1; // Hoist out non-tree portion
145 if (RHSRank < getRank(LHSI->getOperand(TakeOp))) {
146 // Convert ((a + 12) + 10) into (a + (12 + 10))
147 I->setOperand(0, LHSI->getOperand(TakeOp));
148 LHSI->setOperand(TakeOp, RHS);
149 I->setOperand(1, LHSI);
151 // Move the LHS expression forward, to ensure that it is dominated by
153 LHSI->getParent()->getInstList().remove(LHSI);
154 I->getParent()->getInstList().insert(I, LHSI);
157 DEBUG(std::cerr << "Reassociated: " << I/* << " Result BB: "
158 << I->getParent()*/);
160 // Since we modified the RHS instruction, make sure that we recheck it.
161 ReassociateExpr(LHSI);
171 // NegateValue - Insert instructions before the instruction pointed to by BI,
172 // that computes the negative version of the value specified. The negative
173 // version of the value is returned, and BI is left pointing at the instruction
174 // that should be processed next by the reassociation pass.
176 static Value *NegateValue(Value *V, BasicBlock::iterator &BI) {
177 // We are trying to expose opportunity for reassociation. One of the things
178 // that we want to do to achieve this is to push a negation as deep into an
179 // expression chain as possible, to expose the add instructions. In practice,
180 // this means that we turn this:
181 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
182 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
183 // the constants. We assume that instcombine will clean up the mess later if
184 // we introduce tons of unnecessary negation instructions...
186 if (Instruction *I = dyn_cast<Instruction>(V))
187 if (I->getOpcode() == Instruction::Add && I->hasOneUse()) {
188 Value *RHS = NegateValue(I->getOperand(1), BI);
189 Value *LHS = NegateValue(I->getOperand(0), BI);
191 // We must actually insert a new add instruction here, because the neg
192 // instructions do not dominate the old add instruction in general. By
193 // adding it now, we are assured that the neg instructions we just
194 // inserted dominate the instruction we are about to insert after them.
196 return BinaryOperator::create(Instruction::Add, LHS, RHS,
198 cast<Instruction>(RHS)->getNext());
201 // Insert a 'neg' instruction that subtracts the value from zero to get the
204 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
208 bool Reassociate::ReassociateBB(BasicBlock *BB) {
209 bool Changed = false;
210 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
212 DEBUG(std::cerr << "Processing: " << *BI);
213 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) {
214 // Convert a subtract into an add and a neg instruction... so that sub
215 // instructions can be commuted with other add instructions...
217 // Calculate the negative value of Operand 1 of the sub instruction...
218 // and set it as the RHS of the add instruction we just made...
220 std::string Name = BI->getName();
223 BinaryOperator::create(Instruction::Add, BI->getOperand(0),
224 BI->getOperand(1), Name, BI);
226 // Everyone now refers to the add instruction...
227 BI->replaceAllUsesWith(New);
229 // Put the new add in the place of the subtract... deleting the subtract
230 BB->getInstList().erase(BI);
233 New->setOperand(1, NegateValue(New->getOperand(1), BI));
236 DEBUG(std::cerr << "Negated: " << New /*<< " Result BB: " << BB*/);
239 // If this instruction is a commutative binary operator, and the ranks of
240 // the two operands are sorted incorrectly, fix it now.
242 if (BI->isAssociative()) {
243 BinaryOperator *I = cast<BinaryOperator>(BI);
244 if (!I->use_empty()) {
245 // Make sure that we don't have a tree-shaped computation. If we do,
246 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
248 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
249 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
250 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
251 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
253 // Insert a new temporary instruction... (A+B)+C
254 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
256 RHSI->getName()+".ra",
259 I->setOperand(0, Tmp);
260 I->setOperand(1, RHSI->getOperand(1));
262 // Process the temporary instruction for reassociation now.
266 DEBUG(std::cerr << "Linearized: " << I/* << " Result BB: " << BB*/);
269 // Make sure that this expression is correctly reassociated with respect
270 // to it's used values...
272 Changed |= ReassociateExpr(I);
281 bool Reassociate::runOnFunction(Function &F) {
282 // Recalculate the rank map for F
285 bool Changed = false;
286 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
287 Changed |= ReassociateBB(FI);
289 // We are done with the rank map...
291 ValueRankMap.clear();