//
// 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
//
//===----------------------------------------------------------------------===//
+#define DEBUG_TYPE "reassociate"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
unsigned Reassociate::getRank(Value *V) {
if (isa<Argument>(V)) return ValueRankMap[V]; // Function argument...
- if (Instruction *I = dyn_cast<Instruction>(V)) {
- // If this is an expression, return the 1+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 that do not go through PHI nodes.
- //
- if (I->getOpcode() == Instruction::PHI ||
- I->getOpcode() == Instruction::Alloca ||
- I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
- I->mayWriteToMemory()) // Cannot move inst if it writes to memory!
- return RankMap[I->getParent()];
-
- unsigned &CachedRank = ValueRankMap[I];
- if (CachedRank) return CachedRank; // Rank already known?
-
- // If not, compute it!
- 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)));
-
- DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = "
- << Rank+1 << "\n");
-
- return CachedRank = Rank+1;
- }
+ Instruction *I = dyn_cast<Instruction>(V);
+ if (I == 0) return 0; // Otherwise it's a global or constant, rank 0.
- // Otherwise it's a global or constant, rank 0.
- return 0;
+ unsigned &CachedRank = ValueRankMap[I];
+ if (CachedRank) return CachedRank; // Rank already known?
+
+ // If this is an expression, return the 1+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 that do not go through PHI nodes.
+ //
+ if (I->getOpcode() == Instruction::PHI ||
+ I->getOpcode() == Instruction::Alloca ||
+ I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) ||
+ I->mayWriteToMemory()) // Cannot move inst if it writes to memory!
+ return RankMap[I->getParent()];
+
+ // If not, compute it!
+ 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)));
+
+ DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = "
+ << Rank+1 << "\n");
+
+ return CachedRank = Rank+1;
}
// 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) {
+static Value *NegateValue(Value *V, Instruction *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,
// 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());
+ I->getName()+".neg", BI);
}
// Insert a 'neg' instruction that subtracts the value from zero to get the
// negation.
//
- return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
+ return BinaryOperator::createNeg(V, V->getName() + ".neg", BI);
}
+/// isReassociableOp - Return true if V is an instruction of the specified
+/// opcode and if it only has one use.
+static bool isReassociableOp(Value *V, unsigned Opcode) {
+ return V->hasOneUse() && isa<Instruction>(V) &&
+ cast<Instruction>(V)->getOpcode() == Opcode;
+}
+/// BreakUpSubtract - If we have (X-Y), and if either X is an add, or if this is
+/// only used by an add, transform this into (X+(0-Y)) to promote better
+/// reassociation.
+static Instruction *BreakUpSubtract(Instruction *Sub) {
+ // Reject cases where it is pointless to do this.
+ if (Sub->getType()->isFloatingPoint())
+ return 0; // Floating point adds are not associative.
+
+ // Don't bother to break this up unless either the LHS is an associable add or
+ // if this is only used by one.
+ if (!isReassociableOp(Sub->getOperand(0), Instruction::Add) &&
+ !isReassociableOp(Sub->getOperand(1), Instruction::Add) &&
+ !(Sub->hasOneUse() &&isReassociableOp(Sub->use_back(), Instruction::Add)))
+ return 0;
+
+ // 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 = Sub->getName();
+ Sub->setName("");
+ Value *NegVal = NegateValue(Sub->getOperand(1), Sub);
+ Instruction *New =
+ BinaryOperator::createAdd(Sub->getOperand(0), NegVal, Name, Sub);
+
+ // Everyone now refers to the add instruction.
+ Sub->replaceAllUsesWith(New);
+ Sub->eraseFromParent();
+
+ DEBUG(std::cerr << "Negated: " << *New);
+ return New;
+}
+
+
+/// ReassociateBB - Inspect all of the instructions in this basic block,
+/// reassociating them as we go.
bool Reassociate::ReassociateBB(BasicBlock *BB) {
bool Changed = false;
for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) {
-
- DEBUG(std::cerr << "Reassociating: " << *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 is a subtract instruction which is not already in negate form,
+ // see if we can convert it to X+-Y.
+ if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI))
+ if (Instruction *NI = BreakUpSubtract(BI)) {
+ Changed = true;
+ BI = NI;
+ }
// If this instruction is a commutative binary operator, and the ranks of
// the two operands are sorted incorrectly, fix it now.
//
if (BI->isAssociative()) {
+ DEBUG(std::cerr << "Reassociating: " << *BI);
BinaryOperator *I = cast<BinaryOperator>(BI);
if (!I->use_empty()) {
// Make sure that we don't have a tree-shaped computation. If we do,
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