llvm-6502/lib/Transforms/Scalar/Reassociate.cpp
Chris Lattner e4b730441d Fixes to the reassociate pass to make it respect dominance properties
Huge thanks go to Casey Carter for writing this fix, reassociate is now
reoperational!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4471 91177308-0d34-0410-b5e6-96231b3b80d8
2002-10-31 17:12:59 +00:00

278 lines
10 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.
//
// This code was originally written by Chris Lattner, and was then cleaned up
// and perfected by Casey Carter.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.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/Statistic.h"
namespace {
Statistic<> NumLinear ("reassociate","Number of insts linearized");
Statistic<> NumChanged("reassociate","Number of insts reassociated");
Statistic<> NumSwapped("reassociate","Number of insts with operands swapped");
class Reassociate : public FunctionPass {
std::map<BasicBlock*, unsigned> RankMap;
public:
bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
private:
void BuildRankMap(Function &F);
unsigned getRank(Value *V);
bool ReassociateExpr(BinaryOperator *I);
bool ReassociateBB(BasicBlock *BB);
};
RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions");
}
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, MaxRank = RankMap[I->getParent()];
for (unsigned i = 0, e = I->getNumOperands();
i != e && Rank != MaxRank; ++i)
Rank = std::max(Rank, getRank(I->getOperand(i)));
return Rank;
}
// Otherwise it's a global or constant, rank 0.
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) {
bool Success = !I->swapOperands();
assert(Success && "swapOperands failed");
std::swap(LHS, RHS);
std::swap(LHSRank, RHSRank);
Changed = true;
++NumSwapped;
DEBUG(std::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));
// Move the LHS expression forward, to ensure that it is dominated by
// its operands.
std::string Name = LHSI->getName();
LHSI->setName("");
BinaryOperator *NewLHS =
BinaryOperator::create(LHSI->getOpcode(),
LHSI->getOperand(0), LHSI->getOperand(1),
Name, I);
NewLHS->setOperand(TakeOp, RHS);
I->setOperand(1, NewLHS);
assert(LHSI->use_size() == 0 && "References to LHS shouldn't exist!");
LHSI->getParent()->getInstList().erase(LHSI);
++NumChanged;
DEBUG(std::cerr << "Reassociated: " << I << " Result BB: "
<< I->getParent());
// Since we modified the RHS instruction, make sure that we recheck it.
ReassociateExpr(NewLHS);
return true;
}
}
return Changed;
}
// NegateValue - Insert instructions before the instruction pointed to by BI,
// that computes the negative version of the value specified. The negative
// version of the value is returned, and BI is left pointing at the instruction
// that should be processed next by the reassociation pass.
//
static Value *NegateValue(Value *V, BasicBlock::iterator &BI) {
// We are trying to expose opportunity for reassociation. One of the things
// that we want to do to achieve this is to push a negation as deep into an
// expression chain as possible, to expose the add instructions. In practice,
// this means that we turn this:
// X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D
// so that later, a: Y = 12+X could get reassociated with the -12 to eliminate
// the constants. We assume that instcombine will clean up the mess later if
// we introduce tons of unneccesary negation instructions...
//
if (Instruction *I = dyn_cast<Instruction>(V))
if (I->getOpcode() == Instruction::Add && I->use_size() == 1) {
Value *RHS = NegateValue(I->getOperand(1), BI);
Value *LHS = NegateValue(I->getOperand(0), BI);
// We must actually insert a new add instruction here, because the neg
// instructions do not dominate the old add instruction in general. By
// adding it now, we are assured that the neg instructions we just
// inserted dominate the instruction we are about to insert after them.
//
return BinaryOperator::create(Instruction::Add, LHS, RHS,
I->getName()+".neg",
cast<Instruction>(RHS)->getNext());
}
// Insert a 'neg' instruction that subtracts the value from zero to get the
// negation.
//
return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
}
bool Reassociate::ReassociateBB(BasicBlock *BB) {
bool Changed = false;
for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) {
// Convert a subtract into an add and a neg instruction... so that sub
// instructions can be commuted with other add instructions...
//
// Calculate the negative value of Operand 1 of the sub instruction...
// and set it as the RHS of the add instruction we just made...
//
std::string Name = BI->getName();
BI->setName("");
Instruction *New =
BinaryOperator::create(Instruction::Add, BI->getOperand(0),
BI->getOperand(1), Name, BI);
// Everyone now refers to the add instruction...
BI->replaceAllUsesWith(New);
// Put the new add in the place of the subtract... deleting the subtract
BB->getInstList().erase(BI);
BI = New;
New->setOperand(1, NegateValue(New->getOperand(1), BI));
Changed = true;
DEBUG(std::cerr << "Negated: " << New << " Result BB: " << BB);
}
// If this instruction is a commutative binary operator, and the ranks of
// the two operands are sorted incorrectly, fix it now.
//
if (BI->isAssociative()) {
BinaryOperator *I = cast<BinaryOperator>(&*BI);
if (!I->use_empty()) {
// Make sure that we don't have a tree-shaped computation. If we do,
// linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D
//
Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0));
Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1));
if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() &&
RHSI && (int)RHSI->getOpcode() == I->getOpcode() &&
RHSI->use_size() == 1) {
// Insert a new temporary instruction... (A+B)+C
BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI,
RHSI->getOperand(0),
RHSI->getName()+".ra",
BI);
BI = Tmp;
I->setOperand(0, Tmp);
I->setOperand(1, RHSI->getOperand(1));
// Process the temporary instruction for reassociation now.
I = Tmp;
++NumLinear;
Changed = true;
DEBUG(std::cerr << "Linearized: " << I << " Result BB: " << BB);
}
// Make sure that this expression is correctly reassociated with respect
// to it's used values...
//
Changed |= ReassociateExpr(I);
}
}
}
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;
}