llvm-6502/lib/Transforms/Scalar/Reassociate.cpp
2002-05-10 15:38:35 +00:00

206 lines
6.9 KiB
C++

//===- Reassociate.cpp - Reassociate binary expressions -------------------===//
//
// This pass reassociates commutative expressions in an order that is designed
// to promote better constant propogation, GCSE, LICM, PRE...
//
// For example: 4 + (x + 5) -> x + (4 + 5)
//
// Note that this pass works best if left shifts have been promoted to explicit
// multiplies before this pass executes.
//
// In the implementation of this algorithm, constants are assigned rank = 0,
// function arguments are rank = 1, and other values are assigned ranks
// corresponding to the reverse post order traversal of current function
// (starting at 2), which effectively gives values in deep loops higher rank
// than values not in loops.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/BasicBlock.h"
#include "llvm/iOperators.h"
#include "llvm/Type.h"
#include "llvm/Pass.h"
#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
#include "Support/PostOrderIterator.h"
#include "Support/StatisticReporter.h"
static Statistic<> NumChanged("reassociate\t- Number of insts reassociated");
static Statistic<> NumSwapped("reassociate\t- Number of insts with operands swapped");
namespace {
class Reassociate : public FunctionPass {
map<BasicBlock*, unsigned> RankMap;
public:
const char *getPassName() const {
return "Expression Reassociation";
}
bool runOnFunction(Function *F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.preservesCFG();
}
private:
void BuildRankMap(Function *F);
unsigned getRank(Value *V);
bool ReassociateExpr(BinaryOperator *I);
bool ReassociateBB(BasicBlock *BB);
};
}
Pass *createReassociatePass() { return new Reassociate(); }
void Reassociate::BuildRankMap(Function *F) {
unsigned i = 1;
ReversePostOrderTraversal<Function*> RPOT(F);
for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(),
E = RPOT.end(); I != E; ++I)
RankMap[*I] = ++i;
}
unsigned Reassociate::getRank(Value *V) {
if (isa<Argument>(V)) return 1; // Function argument...
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we
// can reassociate expressions for code motion! Since we do not recurse for
// PHI nodes, we cannot have infinite recursion here, because there cannot
// be loops in the value graph (except for PHI nodes).
//
if (I->getOpcode() == Instruction::PHINode ||
I->getOpcode() == Instruction::Alloca ||
I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
I->hasSideEffects())
return RankMap[I->getParent()];
unsigned Rank = 0;
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
Rank = std::max(Rank, getRank(I->getOperand(i)));
return Rank;
}
// Otherwise it's a global or constant, rank 0.
return 0;
}
// isCommutativeOperator - Return true if the specified instruction is
// commutative and associative. If the instruction is not commutative and
// associative, we can not reorder its operands!
//
static inline BinaryOperator *isCommutativeOperator(Instruction *I) {
// Floating point operations do not commute!
if (I->getType()->isFloatingPoint()) return 0;
if (I->getOpcode() == Instruction::Add ||
I->getOpcode() == Instruction::Mul ||
I->getOpcode() == Instruction::And ||
I->getOpcode() == Instruction::Or ||
I->getOpcode() == Instruction::Xor)
return cast<BinaryOperator>(I);
return 0;
}
bool Reassociate::ReassociateExpr(BinaryOperator *I) {
Value *LHS = I->getOperand(0);
Value *RHS = I->getOperand(1);
unsigned LHSRank = getRank(LHS);
unsigned RHSRank = getRank(RHS);
bool Changed = false;
// Make sure the LHS of the operand always has the greater rank...
if (LHSRank < RHSRank) {
I->swapOperands();
std::swap(LHS, RHS);
std::swap(LHSRank, RHSRank);
Changed = true;
++NumSwapped;
//cerr << "Transposed: " << I << " Result BB: " << I->getParent();
}
// If the LHS is the same operator as the current one is, and if we are the
// only expression using it...
//
if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS))
if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) {
// If the rank of our current RHS is less than the rank of the LHS's LHS,
// then we reassociate the two instructions...
if (RHSRank < getRank(LHSI->getOperand(0))) {
unsigned TakeOp = 0;
if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0)))
if (IOp->getOpcode() == LHSI->getOpcode())
TakeOp = 1; // Hoist out non-tree portion
// Convert ((a + 12) + 10) into (a + (12 + 10))
I->setOperand(0, LHSI->getOperand(TakeOp));
LHSI->setOperand(TakeOp, RHS);
I->setOperand(1, LHSI);
++NumChanged;
//cerr << "Reassociated: " << I << " Result BB: " << I->getParent();
// Since we modified the RHS instruction, make sure that we recheck it.
ReassociateExpr(LHSI);
return true;
}
}
return Changed;
}
bool Reassociate::ReassociateBB(BasicBlock *BB) {
bool Changed = false;
for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
Instruction *Inst = *BI;
// If this instruction is a commutative binary operator, and the ranks of
// the two operands are sorted incorrectly, fix it now.
//
if (BinaryOperator *I = isCommutativeOperator(Inst)) {
// Make sure that this expression is correctly reassociated with respect
// to it's used values...
//
Changed |= ReassociateExpr(I);
} else if (Inst->getOpcode() == Instruction::Sub &&
Inst->getOperand(0) != Constant::getNullValue(Inst->getType())) {
// Convert a subtract into an add and a neg instruction... so that sub
// instructions can be commuted with other add instructions...
//
Instruction *New = BinaryOperator::create(Instruction::Add,
Inst->getOperand(0), Inst,
Inst->getName());
// Everyone now refers to the add instruction...
Inst->replaceAllUsesWith(New);
Inst->setName(Inst->getOperand(1)->getName()+".neg");
New->setOperand(1, Inst); // Except for the add inst itself!
BI = BB->getInstList().insert(BI+1, New)-1; // Add to the basic block...
Inst->setOperand(0, Constant::getNullValue(Inst->getType()));
Changed = true;
}
}
return Changed;
}
bool Reassociate::runOnFunction(Function *F) {
// Recalculate the rank map for F
BuildRankMap(F);
bool Changed = false;
for (Function::iterator FI = F->begin(), FE = F->end(); FI != FE; ++FI)
Changed |= ReassociateBB(*FI);
// We are done with the rank map...
RankMap.clear();
return Changed;
}