Add a new Sparse Conditional Constant Propogation pass

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 502 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/SCCP.cpp b/lib/Transforms/Scalar/SCCP.cpp
new file mode 100644
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+++ b/lib/Transforms/Scalar/SCCP.cpp
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+//===- SCCP.cpp - Sparse Conditional Constant Propogation -----------------===//
+//
+// This file implements sparse conditional constant propogation and merging:
+//
+// Specifically, this:
+//   * Assumes values are constant unless proven otherwise
+//   * Assumes BasicBlocks are dead unless proven otherwise
+//   * Proves values to be constant, and replaces them with constants
+//   . Proves conditional branches constant, and unconditionalizes them
+//   * Folds multiple identical constants in the constant pool together
+//
+// Notice that:
+//   * This pass has a habit of making definitions be dead.  It is a good idea
+//     to to run a DCE pass sometime after running this pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Opt/AllOpts.h"
+#include "llvm/Method.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/ConstPoolVals.h"
+#include "llvm/ConstantPool.h"
+#include "llvm/Opt/ConstantHandling.h"
+#include "llvm/InstrTypes.h"
+#include "llvm/iOther.h"
+#include "llvm/iTerminators.h"
+//#include "llvm/Assembly/Writer.h"
+#include <algorithm>
+#include <map>
+#include <set>
+
+
+// InstVal class - This class represents the different lattice values that an 
+// instruction may occupy.  It is a simple class with value semantics.  The
+// potential constant value that is pointed to is owned by the constant pool
+// for the method being optimized.
+//
+class InstVal {
+  enum { 
+    Undefined,           // This instruction has no known value
+    Constant,            // This instruction has a constant value
+    // Range,            // This instruction is known to fall within a range
+    Overdefined          // This instruction has an unknown value
+  } LatticeValue;    // The current lattice position
+  ConstPoolVal *ConstantVal;     // If Constant value, the current value
+public:
+  inline InstVal() : LatticeValue(Undefined), ConstantVal(0) {}
+
+  // markOverdefined - Return true if this is a new status to be in...
+  inline bool markOverdefined() {
+    if (LatticeValue != Overdefined) {
+      LatticeValue = Overdefined;
+      return true;
+    }
+    return false;
+  }
+
+  // markConstant - Return true if this is a new status for us...
+  inline bool markConstant(ConstPoolVal *V) {
+    if (LatticeValue != Constant) {
+      LatticeValue = Constant;
+      ConstantVal = V;
+      return true;
+    } else {
+      assert(ConstantVal->equals(V) && "Marking constant with different value");
+    }
+    return false;
+  }
+
+  inline bool isUndefined()   const { return LatticeValue == Undefined; }
+  inline bool isConstant()    const { return LatticeValue == Constant; }
+  inline bool isOverdefined() const { return LatticeValue == Overdefined; }
+
+  inline ConstPoolVal *getConstant() const { return ConstantVal; }
+};
+
+
+
+//===----------------------------------------------------------------------===//
+// SCCP Class
+//
+// This class does all of the work of Sparse Conditional Constant Propogation.
+// It's public interface consists of a constructor and a doSCCP() method.
+//
+class SCCP {
+  Method *M;                            // The method that we are working on...
+
+  set<BasicBlock*>       BBExecutable;  // The basic blocks that are executable
+  map<Value*, InstVal>   ValueState;    // The state each value is in...
+
+  vector<Instruction*>   InstWorkList;  // The instruction work list
+  vector<BasicBlock*>    BBWorkList;    // The BasicBlock work list
+
+  //===--------------------------------------------------------------------===//
+  // The public interface for this class
+  //
+public:
+
+  // SCCP Ctor - Save the method to operate on...
+  inline SCCP(Method *m) : M(m) {}
+
+  // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and 
+  // return true if the method was modified.
+  bool doSCCP();
+
+  //===--------------------------------------------------------------------===//
+  // The implementation of this class
+  //
+private:
+
+  // markValueOverdefined - Make a value be marked as "constant".  If the value
+  // is not already a constant, add it to the instruction work list so that 
+  // the users of the instruction are updated later.
+  //
+  inline bool markConstant(Instruction *I, ConstPoolVal *V) {
+    //cerr << "markConstant: " << V << " = " << I;
+    if (ValueState[I].markConstant(V)) {
+      InstWorkList.push_back(I);
+      return true;
+    }
+    return false;
+  }
+
+  // markValueOverdefined - Make a value be marked as "overdefined". If the
+  // value is not already overdefined, add it to the instruction work list so
+  // that the users of the instruction are updated later.
+  //
+  inline bool markOverdefined(Value *V) {
+    if (ValueState[V].markOverdefined()) {
+      if (Instruction *I = V->castInstruction()) {
+	//cerr << "markOverdefined: " << V;
+	InstWorkList.push_back(I);  // Only instructions go on the work list
+      }
+      return true;
+    }
+    return false;
+  }
+
+  // getValueState - Return the InstVal object that corresponds to the value.
+  // This function is neccesary because not all values should start out in the
+  // underdefined state... MethodArgument's should be overdefined, and constants
+  // should be marked as constants.  If a value is not known to be an
+  // Instruction object, then use this accessor to get its value from the map.
+  //
+  inline InstVal &getValueState(Value *V) {
+    map<Value*, InstVal>::iterator I = ValueState.find(V);
+    if (I != ValueState.end()) return I->second;  // Common case, in the map
+      
+    if (ConstPoolVal *CPV = V->castConstant()) {  // Constants are constant
+      ValueState[CPV].markConstant(CPV);
+    } else if (V->isMethodArgument()) {           // MethodArgs are overdefined
+      ValueState[V].markOverdefined();
+    } 
+    // All others are underdefined by default...
+    return ValueState[V];
+  }
+
+  // markExecutable - Mark a basic block as executable, adding it to the BB 
+  // work list if it is not already executable...
+  // 
+  void markExecutable(BasicBlock *BB) {
+    if (BBExecutable.count(BB)) return;
+    //cerr << "Marking BB Executable: " << BB;
+    BBExecutable.insert(BB);   // Basic block is executable!
+    BBWorkList.push_back(BB);  // Add the block to the work list!
+  }
+
+  void OperandChangedState(User *U);
+  void UpdateInstruction(Instruction *I);
+};
+
+
+
+//===----------------------------------------------------------------------===//
+// SCCP Class Implementation
+
+
+// doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and 
+// return true if the method was modified.
+//
+bool SCCP::doSCCP() {
+  // Mark the first block of the method as being executable...
+  markExecutable(M->front());
+
+  // Process the work lists until their are empty!
+  while (!BBWorkList.empty() || !InstWorkList.empty()) {
+    // Process the instruction work list...
+    while (!InstWorkList.empty()) {
+      Instruction *I = InstWorkList.back();
+      InstWorkList.pop_back();
+
+      //cerr << "\nPopped off I-WL: " << I;
+
+      
+      // "I" got into the work list because it either made the transition from
+      // bottom to constant, or to Overdefined.
+      //
+      // Update all of the users of this instruction's value...
+      //
+      for_each(I->use_begin(), I->use_end(),
+	       bind_obj(this, &SCCP::OperandChangedState));
+    }
+
+    // Process the basic block work list...
+    while (!BBWorkList.empty()) {
+      BasicBlock *BB = BBWorkList.back();
+      BBWorkList.pop_back();
+
+      //cerr << "\nPopped off BBWL: " << BB;
+
+      // If this block only has a single successor, mark it as executable as
+      // well... if not, terminate the do loop.
+      //
+      if (BB->getTerminator()->getNumSuccessors() == 1)
+	markExecutable(BB->getTerminator()->getSuccessor(0));
+
+      // Loop over all of the instructions and notify them that they are newly
+      // executable...
+      for_each(BB->begin(), BB->end(),
+	       bind_obj(this, &SCCP::UpdateInstruction));
+    }
+  }
+
+#if 0
+  for (Method::iterator BBI = M->begin(), BBEnd = M->end(); BBI != BBEnd; ++BBI)
+    if (!BBExecutable.count(*BBI))
+      cerr << "BasicBlock Dead:" << *BBI;
+#endif
+
+
+  // Iterate over all of the instructions in a method, replacing them with
+  // constants if we have found them to be of constant values.
+  //
+  bool MadeChanges = false;
+  for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
+    Instruction *Inst = *II;
+    InstVal &IV = ValueState[Inst];
+    if (IV.isConstant()) {
+      ConstPoolVal *Const = IV.getConstant();
+      // cerr << "Constant: " << Inst << "  is: " << Const;
+      
+      // Replaces all of the uses of a variable with uses of the constant.
+      Inst->replaceAllUsesWith(Const);
+
+      // Remove the operator from the list of definitions...
+      Inst->getParent()->getInstList().remove(II.getInstructionIterator());
+      
+      // The new constant inherits the old name of the operator...
+      if (Inst->hasName() && !Const->hasName())
+	Const->setName(Inst->getName());
+  
+      // Delete the operator now...
+      delete Inst;
+
+      // Incrementing the iterator in an unchecked manner could mess up the
+      // internals of 'II'.  To make sure everything is happy, tell it we might
+      // have broken it.
+      II.resyncInstructionIterator();
+
+      // Hey, we just changed something!
+      MadeChanges = true;
+    } else {
+      ++II;
+    }
+  }
+
+  // Merge identical constants last: this is important because we may have just
+  // introduced constants that already exist, and we don't want to pollute later
+  // stages with extraneous constants.
+  //
+  return MadeChanges | DoConstantPoolMerging(M->getConstantPool());
+}
+
+
+// UpdateInstruction - Something changed in this instruction... Either an 
+// operand made a transition, or the instruction is newly executable.  Change
+// the value type of I to reflect these changes if appropriate.  This method
+// makes sure to do the following actions:
+//
+// 1. If a phi node merges two constants in, and has conflicting value coming
+//    from different branches, or if the PHI node merges in an overdefined
+//    value, then the PHI node becomes overdefined.
+// 2. If a phi node merges only constants in, and they all agree on value, the
+//    PHI node becomes a constant value equal to that.
+// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
+// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
+// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
+// 6. If a conditional branch has a value that is constant, make the selected
+//    destination executable
+// 7. If a conditional branch has a value that is overdefined, make all
+//    successors executable.
+//
+void SCCP::UpdateInstruction(Instruction *I) {
+  InstVal &IValue = ValueState[I];
+  if (IValue.isOverdefined())
+    return; // If already overdefined, we aren't going to effect anything
+
+  switch (I->getInstType()) {
+    //===-----------------------------------------------------------------===//
+    // Handle PHI nodes...
+    //
+  case Instruction::PHINode: {
+    PHINode *PN = (PHINode*)I;
+    unsigned NumValues = PN->getNumIncomingValues(), i;
+    InstVal *OperandIV = 0;
+
+    // Look at all of the executable operands of the PHI node.  If any of them
+    // are overdefined, the PHI becomes overdefined as well.  If they are all
+    // constant, and they agree with each other, the PHI becomes the identical
+    // constant.  If they are constant and don't agree, the PHI is overdefined.
+    // If there are no executable operands, the PHI remains undefined.
+    //
+    for (i = 0; i < NumValues; ++i) {
+      if (BBExecutable.count(PN->getIncomingBlock(i))) {
+	InstVal &IV = getValueState(PN->getIncomingValue(i));
+	if (IV.isUndefined()) continue;  // Doesn't influence PHI node.
+	if (IV.isOverdefined()) {   // PHI node becomes overdefined!
+	  markOverdefined(PN);
+	  return;
+	}
+
+	if (OperandIV == 0) {   // Grab the first value...
+	  OperandIV = &IV;
+	} else {                // Another value is being merged in!
+	  // There is already a reachable operand.  If we conflict with it,
+	  // then the PHI node becomes overdefined.  If we agree with it, we
+	  // can continue on.
+	  
+	  // Check to see if there are two different constants merging...
+	  if (!IV.getConstant()->equals(OperandIV->getConstant())) {
+	    // Yes there is.  This means the PHI node is not constant.
+	    // You must be overdefined poor PHI.
+	    //
+	    markOverdefined(I);         // The PHI node now becomes overdefined
+	    return;    // I'm done analyzing you
+	  }
+	}
+      }
+    }
+
+    // If we exited the loop, this means that the PHI node only has constant
+    // arguments that agree with each other(and OperandIV is a pointer to one
+    // of their InstVal's) or OperandIV is null because there are no defined
+    // incoming arguments.  If this is the case, the PHI remains undefined.
+    //
+    if (OperandIV) {
+      assert(OperandIV->isConstant() && "Should only be here for constants!");
+      markConstant(I, OperandIV->getConstant());  // Aquire operand value
+    }
+    return;
+  }
+
+    //===-----------------------------------------------------------------===//
+    // Handle instructions that unconditionally provide overdefined values...
+    //
+  case Instruction::Malloc:
+  case Instruction::Free:
+  case Instruction::Alloca:
+  case Instruction::Load:
+  case Instruction::Store:
+    // TODO: getfield/putfield?
+  case Instruction::Call:
+    markOverdefined(I);          // Memory and call's are all overdefined
+    return;
+
+    //===-----------------------------------------------------------------===//
+    // Handle Terminator instructions...
+    //
+  case Instruction::Ret: return;  // Method return doesn't affect anything
+  case Instruction::Br: {        // Handle conditional branches...
+    BranchInst *BI = (BranchInst*)I;
+    if (BI->isUnconditional()) 
+      return; // Unconditional branches are already handled!
+
+    InstVal &BCValue = getValueState(BI->getCondition());
+    if (BCValue.isOverdefined()) {
+      // Overdefined condition variables mean the branch could go either way.
+      markExecutable(BI->getSuccessor(0));
+      markExecutable(BI->getSuccessor(1));
+    } else if (BCValue.isConstant()) {
+      // Constant condition variables mean the branch can only go a single way.
+      ConstPoolBool *CPB = (ConstPoolBool*)BCValue.getConstant();
+      if (CPB->getValue())       // If the branch condition is TRUE...
+	markExecutable(BI->getSuccessor(0));
+      else                       // Else if the br cond is FALSE...
+	markExecutable(BI->getSuccessor(1));
+    }
+    return;
+  }
+
+  case Instruction::Switch: {
+    SwitchInst *SI = (SwitchInst*)I;
+    InstVal &SCValue = getValueState(SI->getCondition());
+    if (SCValue.isOverdefined()) {  // Overdefined condition?  All dests are exe
+      for(unsigned i = 0; BasicBlock *Succ = SI->getSuccessor(i); ++i)
+	markExecutable(Succ);
+    } else if (SCValue.isConstant()) {
+      ConstPoolVal *CPV = SCValue.getConstant();
+      for (SwitchInst::dest_iterator I = SI->dest_begin(), E = SI->dest_end();
+	   I != E; ++I) {
+	if (I->first->equals(CPV)) {   // Found the right branch...
+	  markExecutable(I->second);
+	  return;
+	}
+      }
+      
+      // Constant value not equal to any of the branches... must execute 
+      // default branch then...
+      markExecutable(SI->getDefaultDest());
+    }
+    return;
+  }
+
+  default: break;  // Handle math operators as groups.
+  } // end switch(I->getInstType())
+
+  
+  //===-------------------------------------------------------------------===//
+  // Handle Unary instructions...
+  //
+  if (I->isUnaryOp()) {
+    Value *V = I->getOperand(0);
+    InstVal &VState = getValueState(V);
+    if (VState.isOverdefined()) {        // Inherit overdefinedness of operand
+      markOverdefined(I);
+    } else if (VState.isConstant()) {    // Propogate constant value
+      ConstPoolVal *Result = 
+	ConstantFoldUnaryInstruction(I->getInstType(), VState.getConstant());
+
+      if (Result) {
+	// This instruction constant folds!  The only problem is that the value
+	// returned is newly allocated.  Make sure to stick it into the methods
+	// constant pool...
+	M->getConstantPool().insert(Result);
+	markConstant(I, Result);
+      } else {
+	markOverdefined(I);   // Don't know how to fold this instruction.  :(
+      }
+    }
+    return;
+  }
+
+  //===-----------------------------------------------------------------===//
+  // Handle Binary instructions...
+  //
+  if (I->isBinaryOp()) {
+    Value *V1 = I->getOperand(0);
+    Value *V2 = I->getOperand(1);
+
+    InstVal &V1State = getValueState(V1);
+    InstVal &V2State = getValueState(V2);
+    if (V1State.isOverdefined() || V2State.isOverdefined()) {
+      markOverdefined(I);
+    } else if (V1State.isConstant() && V2State.isConstant()) {
+      ConstPoolVal *Result = 
+	ConstantFoldBinaryInstruction(I->getInstType(), V1State.getConstant(),
+				      V2State.getConstant());
+
+      if (Result) {
+	// This instruction constant folds!  The only problem is that the value
+	// returned is newly allocated.  Make sure to stick it into the methods
+	// constant pool...
+	M->getConstantPool().insert(Result);
+	markConstant(I, Result);
+      } else {
+	markOverdefined(I);   // Don't know how to fold this instruction.  :(
+      }
+    }
+    return;
+  }
+  
+  // Shouldn't get here... either the switch statement or one of the group
+  // handlers should have kicked in...
+  //
+  cerr << "SCCP: Don't know how to handle: " << I;
+  markOverdefined(I);   // Just in case
+}
+
+
+
+// OperandChangedState - This method is invoked on all of the users of an
+// instruction that was just changed state somehow....  Based on this
+// information, we need to update the specified user of this instruction.
+//
+void SCCP::OperandChangedState(User *U) {
+  // Only instructions use other variable values!
+  Instruction *I = U->castInstructionAsserting();
+  if (!BBExecutable.count(I->getParent())) return;  // Inst not executable yet!
+
+  UpdateInstruction(I);
+}
+
+
+
+// DoSparseConditionalConstantProp - Use Sparse Conditional Constant Propogation
+// to prove whether a value is constant and whether blocks are used.
+//
+bool DoSparseConditionalConstantProp(Method *M) {
+  SCCP S(M);
+  return S.doSCCP();
+}
+