Implement a lot more functionality. Now loop invariant and linear

induction variables are correctly identified.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/InductionVars.cpp

@@ -3,26 +3,271 @@
 // This file implements induction variable cannonicalization of loops.
 //
 // Specifically, after this executes, the following is true:
-//   - There is a single induction variable for each loop (that used to contain
-//     at least one induction variable)
+//   - There is a single induction variable for each loop (at least loops that
+//     used to contain at least one induction variable)
 //   - This induction variable starts at 0 and steps by 1 per iteration
+//   - This induction variable is represented by the first PHI node in the
+//     Header block, allowing it to be found easily.
 //   - All other preexisting induction variables are adjusted to operate in
 //     terms of this primary induction variable
 //
+// This code assumes the following is true to perform its full job:
+//   - The CFG has been simplified to not have multiple entrances into an
+//     interval header.  Interval headers should only have two predecessors,
+//     one from inside of the loop and one from outside of the loop.
+//
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/Intervals.h"
 #include "llvm/Opt/AllOpts.h"
 #include "llvm/Assembly/Writer.h"
 #include "llvm/Tools/STLExtras.h"
+#include "llvm/iOther.h"
 
-static bool ProcessInterval(cfg::Interval *Int) {
-  if (!Int->isLoop()) return false;  // Not a loop?  Ignore it!
+// isLoopInvariant - Return true if the specified value/basic block source is 
+// an interval invariant computation.
+//
+static bool isLoopInvariant(cfg::Interval *Int, Value *V) {
+  assert(V->getValueType() == Value::ConstantVal ||
+	 V->getValueType() == Value::InstructionVal ||
+	 V->getValueType() == Value::MethodArgumentVal);
+
+  if (V->getValueType() != Value::InstructionVal)
+    return true;  // Constants and arguments are always loop invariant
+
+  BasicBlock *ValueBlock = ((Instruction*)V)->getParent();
+  assert(ValueBlock && "Instruction not embedded in basic block!");
+
+  // For now, only consider values from outside of the interval, regardless of
+  // whether the expression could be lifted out of the loop by some LICM.
+  //
+  // TODO: invoke LICM library if we find out it would be useful.
+  //
+  return !Int->contains(ValueBlock);
+}
+
+
+// isLinearInductionVariableH - Return isLIV if the expression V is a linear
+// expression defined in terms of loop invariant computations, and a single
+// instance of the PHI node PN.  Return isLIC if the expression V is a loop
+// invariant computation.  Return isNLIV if the expression is a negated linear
+// induction variable.  Return isOther if it is neither.
+//
+// Currently allowed operators are: ADD, SUB, NEG
+// TODO: This should allow casts!
+//
+enum LIVType { isLIV, isLIC, isNLIV, isOther };
+//
+// neg - Negate the sign of a LIV expression.
+inline LIVType neg(LIVType T) { 
+  assert(T == isLIV || T == isNLIV && "Negate Only works on LIV expressions");
+  return T == isLIV ? isNLIV : isLIV; 
+}
+//
+static LIVType isLinearInductionVariableH(cfg::Interval *Int, Value *V,
+					  PHINode *PN) {
+  if (V == PN) { return isLIV; }  // PHI node references are (0+PHI)
+  if (isLoopInvariant(Int, V)) return isLIC;
+
+  assert(V->getValueType() == Value::InstructionVal &&
+	 "loop noninvariant computations must be instructions!");
+
+  Instruction *I = (Instruction*)V;
+  switch (I->getInstType()) {       // Handle each instruction seperately
+  case Instruction::Neg: {
+    Value *SubV = ((UnaryOperator*)I)->getOperand(0);
+    LIVType SubLIVType = isLinearInductionVariableH(Int, SubV, PN);
+    switch (SubLIVType) {
+    case isLIC:          // Loop invariant & other computations remain the same
+    case isOther: return SubLIVType;
+    case isLIV:          // Return the opposite signed LIV type
+    case isNLIV:  return neg(isLIV);
+    }
+  }
+  case Instruction::Add:
+  case Instruction::Sub: {
+    Value *SubV1 = ((BinaryOperator*)I)->getOperand(0);
+    Value *SubV2 = ((BinaryOperator*)I)->getOperand(1);
+    LIVType SubLIVType1 = isLinearInductionVariableH(Int, SubV1, PN);
+    if (SubLIVType1 == isOther) return isOther;  // Early bailout
+    LIVType SubLIVType2 = isLinearInductionVariableH(Int, SubV2, PN);
+
+    switch (SubLIVType2) {
+    case isOther: return isOther;      // Unknown subexpression type
+    case isLIC:   return SubLIVType1;  // Constant offset, return type #1
+    case isLIV:
+    case isNLIV:
+      // So now we know that we have a linear induction variable on the RHS of
+      // the ADD or SUB instruction.  SubLIVType1 cannot be isOther, so it is
+      // either a Loop Invariant computation, or a LIV type.
+      if (SubLIVType1 == isLIC) {
+	// Loop invariant computation, we know this is a LIV then.
+	return (I->getInstType() == Instruction::Add) ? 
+	               SubLIVType2 : neg(SubLIVType2);
+      }
+
+      // If the LHS is also a LIV Expression, we cannot add two LIVs together
+      if (I->getInstType() == Instruction::Add) return isOther;
+
+      // We can only subtract two LIVs if they are the same type, which yields
+      // a LIC, because the LIVs cancel each other out.
+      return (SubLIVType1 == SubLIVType2) ? isLIC : isOther;
+    }
+    // NOT REACHED
+  }
+
+  default:            // Any other instruction is not a LINEAR induction var
+    return isOther;
+  }
+}
+
+// isLinearInductionVariable - Return true if the specified expression is a
+// "linear induction variable", which is an expression involving a single 
+// instance of the PHI node and a loop invariant value that is added or
+// subtracted to the PHI node.  This is calculated by walking the SSA graph
+//
+static inline bool isLinearInductionVariable(cfg::Interval *Int, Value *V,
+					     PHINode *PN) {
+  return isLinearInductionVariableH(Int, V, PN) == isLIV;
+}
+
+
+// isSimpleInductionVar - Return true iff the cannonical induction variable PN
+// has an initializer of the constant value 0, and has a step size of constant 
+// 1.
+static inline bool isSimpleInductionVar(PHINode *PN) {
+  assert(PN->getNumIncomingValues() == 2 && "Must have cannonical PHI node!");
+  Value *Initializer = PN->getIncomingValue(0);
+  if (Initializer->getValueType() != Value::ConstantVal)
+    return false;
+
+  // How do I check for 0 for any integral value?  Use 
+  // ConstPoolVal::getNullConstant?
+
+  Value *StepExpr    = PN->getIncomingValue(1);
+  assert(StepExpr->getValueType() == Value::InstructionVal && "No ADD node?");
+  assert(((Instruction*)StepExpr)->getInstType() == Instruction::Add &&
+	 "No ADD node? Not a cannonical PHI!");
+  BinaryOperator *I = (BinaryOperator*)StepExpr;
+  assert(I->getOperand(0)->getValueType() == Value::InstructionVal && 
+      ((Instruction*)I->getOperand(0))->getInstType() == Instruction::PHINode &&
+	 "PHI node should be first operand of ADD instruction!");
+
+  // Get the right hand side of the ADD node.  See if it is a constant 1.
+  Value *StepSize = I->getOperand(1);
+  if (StepSize->getValueType() != Value::ConstantVal) return false;
+
+  // How do I check for 1 for any integral value?
 
-  cerr << "Found Looping Interval: " << Int; //->HeaderNode;
   return false;
 }
 
+// ProcessInterval - This function is invoked once for each interval in the 
+// IntervalPartition of the program.  It looks for auxilliary induction
+// variables in loops.  If it finds one, it:
+// * Cannonicalizes the induction variable.  This consists of:
+//   A. Making the first element of the PHI node be the loop invariant 
+//      computation, and the second element be the linear induction portion.
+//   B. Changing the first element of the linear induction portion of the PHI 
+//      node to be of the form ADD(PHI, <loop invariant expr>).
+// * Add the induction variable PHI to a list of induction variables found.
+//
+// After this, a list of cannonical induction variables is known.  This list
+// is searched to see if there is an induction variable that counts from 
+// constant 0 with a step size of constant 1.  If there is not one, one is
+// injected into the loop.  Thus a "simple" induction variable is always known
+//
+// One a simple induction variable is known, all other induction variables are
+// modified to refer to the "simple" induction variable.
+//
+static bool ProcessInterval(cfg::Interval *Int) {
+  if (!Int->isLoop()) return false;  // Not a loop?  Ignore it!
+
+  vector<PHINode *> InductionVars;
+
+  BasicBlock *Header = Int->getHeaderNode();
+  // Loop over all of the PHI nodes in the interval header...
+  for (BasicBlock::InstListType::iterator I = Header->getInstList().begin(),
+	 E = Header->getInstList().end(); 
+       I != E && (*I)->getInstType() == Instruction::PHINode; ++I) {
+
+    PHINode *PN = (PHINode*)*I;
+    if (PN->getNumIncomingValues() != 2) { // These should be eliminated by now.
+      cerr << "Found interval header with more than 2 predecessors! Ignoring\n";
+      return false;    // Todo, make an assertion.
+    }
+
+    // For this to be an induction variable, one of the arguments must be a
+    // loop invariant expression, and the other must be an expression involving
+    // the PHI node, along with possible additions and subtractions of loop
+    // invariant values.
+    //
+    BasicBlock *BB1 = PN->getIncomingBlock(0);
+    Value      *V1  = PN->getIncomingValue(0);
+    BasicBlock *BB2 = PN->getIncomingBlock(1);
+    Value      *V2  = PN->getIncomingValue(1);
+
+    // Figure out which computation is loop invariant...
+    if (!isLoopInvariant(Int, V1)) {
+      // V1 is *not* loop invariant.  Check to see if V2 is:
+      if (isLoopInvariant(Int, V2)) {
+	// They *are* loop invariant.  Exchange BB1/BB2 and V1/V2 so that
+	// V1 is always the loop invariant computation.
+	swap(V1, V2); swap(BB1, BB2);
+      } else {
+	// Neither value is loop invariant.  Must not be an induction variable.
+	// This case can happen if there is an unreachable loop in the CFG that
+	// has two tail loops in it that was not split by the cleanup phase
+	// before.
+	continue;
+      }      
+    }
+
+    // At this point, we know that BB1/V1 are loop invariant.  We don't know
+    // anything about BB2/V2.  Check now to see if V2 is a linear induction
+    // variable.
+    //
+    cerr << "Found loop invariant computation: " << V1;
+    
+    if (!isLinearInductionVariable(Int, V2, PN))
+      continue;         // No, it is not a linear ind var, ignore the PHI node.
+    cerr << "Found linear induction variable: " << V2;
+
+    // TODO: Cannonicalize V2
+
+    // Add this PHI node to the list of induction variables found...
+    InductionVars.push_back(PN);    
+  }
+
+  // No induction variables found?
+  if (InductionVars.empty()) return false;
+
+  cerr << "Found Interval Header with indvars: \n" << Header;
+
+  // Search to see if there is already a "simple" induction variable.
+  vector<PHINode*>::iterator It = 
+    find_if(InductionVars.begin(), InductionVars.end(), isSimpleInductionVar);
+  
+  // A simple induction variable was not found, inject one now...
+  if (It == InductionVars.end()) {
+    cerr << "WARNING, Induction variable injection not implemented yet!\n";
+    // TODO: Inject induction variable
+    It = InductionVars.end();  --It; // Point it at the new indvar
+  }
+
+  // Now we know that there is a simple induction variable *It.  Simplify all
+  // of the other induction variables to use this induction variable as their
+  // counter, and destroy the PHI nodes that correspond to the old indvars.
+  //
+  // TODO
+
+  return false;  // TODO: true;
+}
+
+
+// ProcessIntervalPartition - This function loops over the interval partition
+// processing each interval with ProcessInterval
+//
 static bool ProcessIntervalPartition(cfg::IntervalPartition &IP) {
   // This currently just prints out information about the interval structure
   // of the method...
@@ -39,7 +284,10 @@ static bool ProcessIntervalPartition(cfg::IntervalPartition &IP) {
 		      ptr_fun(ProcessInterval));
 }
 
-// DoInductionVariableCannonicalize - Simplify induction variables in loops
+
+// DoInductionVariableCannonicalize - Simplify induction variables in loops.
+// This function loops over an interval partition of a program, reducing it
+// until the graph is gone.
 //
 bool DoInductionVariableCannonicalize(Method *M) {
   cfg::IntervalPartition *IP = new cfg::IntervalPartition(M);