Initial checkin of expression reassociation pass

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2559 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/Reassociate.cpp

@@ -0,0 +1,198 @@
+//===- 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"
+
+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;
+    //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);
+
+        //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()+".add");
+      // Everyone now refers to the add instruction...
+      Inst->replaceAllUsesWith(New);
+      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;
+}