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"
40 Statistic<> NumLinear ("reassociate","Number of insts linearized");
41 Statistic<> NumChanged("reassociate","Number of insts reassociated");
42 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");
44 class Reassociate : public FunctionPass {
45 std::map<BasicBlock*, unsigned> RankMap;
46 std::map<Value*, unsigned> ValueRankMap;
48 bool runOnFunction(Function &F);
50 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
54 void BuildRankMap(Function &F);
55 unsigned getRank(Value *V);
56 bool ReassociateExpr(BinaryOperator *I);
57 bool ReassociateBB(BasicBlock *BB);
60 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
63 // Public interface to the Reassociate pass
64 FunctionPass *createReassociatePass() { return new Reassociate(); }
66 void Reassociate::BuildRankMap(Function &F) {
69 // Assign distinct ranks to function arguments
70 for (Function::aiterator I = F.abegin(), E = F.aend(); I != E; ++I)
71 ValueRankMap[I] = ++i;
73 ReversePostOrderTraversal<Function*> RPOT(&F);
74 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
75 E = RPOT.end(); I != E; ++I)
76 RankMap[*I] = ++i << 16;
79 unsigned Reassociate::getRank(Value *V) {
80 if (isa<Argument>(V)) return ValueRankMap[V]; // Function argument...
82 if (Instruction *I = dyn_cast<Instruction>(V)) {
83 // If this is an expression, return the 1+MAX(rank(LHS), rank(RHS)) so that
84 // we can reassociate expressions for code motion! Since we do not recurse
85 // for PHI nodes, we cannot have infinite recursion here, because there
86 // cannot be loops in the value graph that do not go through PHI nodes.
88 if (I->getOpcode() == Instruction::PHI ||
89 I->getOpcode() == Instruction::Alloca ||
90 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
91 I->mayWriteToMemory()) // Cannot move inst if it writes to memory!
92 return RankMap[I->getParent()];
94 unsigned &CachedRank = ValueRankMap[I];
95 if (CachedRank) return CachedRank; // Rank already known?
97 // If not, compute it!
98 unsigned Rank = 0, MaxRank = RankMap[I->getParent()];
99 for (unsigned i = 0, e = I->getNumOperands();
100 i != e && Rank != MaxRank; ++i)
101 Rank = std::max(Rank, getRank(I->getOperand(i)));
103 DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = "
106 return CachedRank = Rank+1;
109 // Otherwise it's a global or constant, rank 0.
114 bool Reassociate::ReassociateExpr(BinaryOperator *I) {
115 Value *LHS = I->getOperand(0);
116 Value *RHS = I->getOperand(1);
117 unsigned LHSRank = getRank(LHS);
118 unsigned RHSRank = getRank(RHS);
120 bool Changed = false;
122 // Make sure the LHS of the operand always has the greater rank...
123 if (LHSRank < RHSRank) {
124 bool Success = !I->swapOperands();
125 assert(Success && "swapOperands failed");
128 std::swap(LHSRank, RHSRank);
131 DEBUG(std::cerr << "Transposed: " << I
132 /* << " Result BB: " << I->getParent()*/);
135 // If the LHS is the same operator as the current one is, and if we are the
136 // only expression using it...
138 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
139 if (LHSI->getOpcode() == I->getOpcode() && LHSI->hasOneUse()) {
140 // If the rank of our current RHS is less than the rank of the LHS's LHS,
141 // then we reassociate the two instructions...
144 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
145 if (IOp->getOpcode() == LHSI->getOpcode())
146 TakeOp = 1; // Hoist out non-tree portion
148 if (RHSRank < getRank(LHSI->getOperand(TakeOp))) {
149 // Convert ((a + 12) + 10) into (a + (12 + 10))
150 I->setOperand(0, LHSI->getOperand(TakeOp));
151 LHSI->setOperand(TakeOp, RHS);
152 I->setOperand(1, LHSI);
154 // Move the LHS expression forward, to ensure that it is dominated by
156 LHSI->getParent()->getInstList().remove(LHSI);
157 I->getParent()->getInstList().insert(I, LHSI);
160 DEBUG(std::cerr << "Reassociated: " << I/* << " Result BB: "
161 << I->getParent()*/);
163 // Since we modified the RHS instruction, make sure that we recheck it.
164 ReassociateExpr(LHSI);
174 // NegateValue - Insert instructions before the instruction pointed to by BI,
175 // that computes the negative version of the value specified. The negative
176 // version of the value is returned, and BI is left pointing at the instruction
177 // that should be processed next by the reassociation pass.
179 static Value *NegateValue(Value *V, BasicBlock::iterator &BI) {
180 // We are trying to expose opportunity for reassociation. One of the things
181 // that we want to do to achieve this is to push a negation as deep into an
182 // expression chain as possible, to expose the add instructions. In practice,
183 // this means that we turn this:
184 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
185 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
186 // the constants. We assume that instcombine will clean up the mess later if
187 // we introduce tons of unnecessary negation instructions...
189 if (Instruction *I = dyn_cast<Instruction>(V))
190 if (I->getOpcode() == Instruction::Add && I->hasOneUse()) {
191 Value *RHS = NegateValue(I->getOperand(1), BI);
192 Value *LHS = NegateValue(I->getOperand(0), BI);
194 // We must actually insert a new add instruction here, because the neg
195 // instructions do not dominate the old add instruction in general. By
196 // adding it now, we are assured that the neg instructions we just
197 // inserted dominate the instruction we are about to insert after them.
199 return BinaryOperator::create(Instruction::Add, LHS, RHS,
201 cast<Instruction>(RHS)->getNext());
204 // Insert a 'neg' instruction that subtracts the value from zero to get the
207 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
211 bool Reassociate::ReassociateBB(BasicBlock *BB) {
212 bool Changed = false;
213 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
215 DEBUG(std::cerr << "Processing: " << *BI);
216 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) {
217 // Convert a subtract into an add and a neg instruction... so that sub
218 // instructions can be commuted with other add instructions...
220 // Calculate the negative value of Operand 1 of the sub instruction...
221 // and set it as the RHS of the add instruction we just made...
223 std::string Name = BI->getName();
226 BinaryOperator::create(Instruction::Add, BI->getOperand(0),
227 BI->getOperand(1), Name, BI);
229 // Everyone now refers to the add instruction...
230 BI->replaceAllUsesWith(New);
232 // Put the new add in the place of the subtract... deleting the subtract
233 BB->getInstList().erase(BI);
236 New->setOperand(1, NegateValue(New->getOperand(1), BI));
239 DEBUG(std::cerr << "Negated: " << New /*<< " Result BB: " << BB*/);
242 // If this instruction is a commutative binary operator, and the ranks of
243 // the two operands are sorted incorrectly, fix it now.
245 if (BI->isAssociative()) {
246 BinaryOperator *I = cast<BinaryOperator>(BI);
247 if (!I->use_empty()) {
248 // Make sure that we don't have a tree-shaped computation. If we do,
249 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
251 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
252 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
253 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
254 RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
256 // Insert a new temporary instruction... (A+B)+C
257 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
259 RHSI->getName()+".ra",
262 I->setOperand(0, Tmp);
263 I->setOperand(1, RHSI->getOperand(1));
265 // Process the temporary instruction for reassociation now.
269 DEBUG(std::cerr << "Linearized: " << I/* << " Result BB: " << BB*/);
272 // Make sure that this expression is correctly reassociated with respect
273 // to it's used values...
275 Changed |= ReassociateExpr(I);
284 bool Reassociate::runOnFunction(Function &F) {
285 // Recalculate the rank map for F
288 bool Changed = false;
289 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
290 Changed |= ReassociateBB(FI);
292 // We are done with the rank map...
294 ValueRankMap.clear();
298 } // End llvm namespace