When linearizing a multiplication, return at once if we see a factor of zero,

since then the entire expression must equal zero (similarly for other operations
with an absorbing element).  With this in place a bunch of reassociate code for
handling constants is dead since it is all taken care of when linearizing.  No
intended functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158398 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 37 insertions, 42 deletions

diff --git a/lib/Transforms/Scalar/Reassociate.cpp b/lib/Transforms/Scalar/Reassociate.cpp
index 8cace5eebd..ed7340f7e6 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/lib/Transforms/Scalar/Reassociate.cpp
@@ -455,6 +455,10 @@ static bool LinearizeExprTree(BinaryOperator *I,
   assert(Instruction::isAssociative(Opcode) &&
          Instruction::isCommutative(Opcode) &&
          "Expected an associative and commutative operation!");
+  // If we see an absorbing element then the entire expression must be equal to
+  // it.  For example, if this is a multiplication expression and zero occurs as
+  // an operand somewhere in it then the result of the expression must be zero.
+  Constant *Absorber = ConstantExpr::getBinOpAbsorber(Opcode, I->getType());
 
   // Visit all operands of the expression, keeping track of their weight (the
   // number of paths from the expression root to the operand, or if you like
@@ -502,6 +506,13 @@ static bool LinearizeExprTree(BinaryOperator *I,
       DEBUG(dbgs() << "OPERAND: " << *Op << " (" << Weight << ")\n");
       assert(!Op->use_empty() && "No uses, so how did we get to it?!");
 
+      // If the expression contains an absorbing element then there is no need
+      // to analyze it further: it must evaluate to the absorbing element.
+      if (Op == Absorber && !Weight.isMinValue()) {
+        Ops.push_back(std::make_pair(Absorber, APInt(Bitwidth, 1)));
+        return MadeChange;
+      }
+
       // If this is a binary operation of the right kind with only one use then
       // add its operands to the expression.
       if (BinaryOperator *BO = isReassociableOp(Op, Opcode)) {
@@ -617,14 +628,15 @@ static bool LinearizeExprTree(BinaryOperator *I,
 
   // Add any constants back into Ops, all globbed together and reduced to having
   // weight 1 for the convenience of users.
-  if (Cst && Cst != ConstantExpr::getBinOpIdentity(Opcode, I->getType()))
+  Constant *Identity = ConstantExpr::getBinOpIdentity(Opcode, I->getType());
+  if (Cst && Cst != Identity)
     Ops.push_back(std::make_pair(Cst, APInt(Bitwidth, 1)));
 
   // For nilpotent operations or addition there may be no operands, for example
   // because the expression was "X xor X" or consisted of 2^Bitwidth additions:
   // in both cases the weight reduces to 0 causing the value to be skipped.
   if (Ops.empty()) {
-    Constant *Identity = ConstantExpr::getBinOpIdentity(Opcode, I->getType());
+    assert(Identity && "Associative operation without identity!");
     Ops.push_back(std::make_pair(Identity, APInt(Bitwidth, 1)));
   }
 
@@ -1426,44 +1438,6 @@ Value *Reassociate::OptimizeExpression(BinaryOperator *I,
 
   unsigned Opcode = I->getOpcode();
 
-  if (Constant *V1 = dyn_cast<Constant>(Ops[Ops.size()-2].Op))
-    if (Constant *V2 = dyn_cast<Constant>(Ops.back().Op)) {
-      Ops.pop_back();
-      Ops.back().Op = ConstantExpr::get(Opcode, V1, V2);
-      return OptimizeExpression(I, Ops);
-    }
-
-  // Check for destructive annihilation due to a constant being used.
-  if (ConstantInt *CstVal = dyn_cast<ConstantInt>(Ops.back().Op))
-    switch (Opcode) {
-    default: break;
-    case Instruction::And:
-      if (CstVal->isZero())                  // X & 0 -> 0
-        return CstVal;
-      if (CstVal->isAllOnesValue())          // X & -1 -> X
-        Ops.pop_back();
-      break;
-    case Instruction::Mul:
-      if (CstVal->isZero()) {                // X * 0 -> 0
-        ++NumAnnihil;
-        return CstVal;
-      }
-
-      if (cast<ConstantInt>(CstVal)->isOne())
-        Ops.pop_back();                      // X * 1 -> X
-      break;
-    case Instruction::Or:
-      if (CstVal->isAllOnesValue())          // X | -1 -> -1
-        return CstVal;
-      // FALLTHROUGH!
-    case Instruction::Add:
-    case Instruction::Xor:
-      if (CstVal->isZero())                  // X [|^+] 0 -> X
-        Ops.pop_back();
-      break;
-    }
-  if (Ops.size() == 1) return Ops[0].Op;
-
   // Handle destructive annihilation due to identities between elements in the
   // argument list here.
   unsigned NumOps = Ops.size();
diff --git a/lib/VMCore/Constants.cpp b/lib/VMCore/Constants.cpp
index 17bad97e6f..400b2741cd 100644
--- a/lib/VMCore/Constants.cpp
+++ b/lib/VMCore/Constants.cpp
@@ -2009,11 +2009,13 @@ Constant *ConstantExpr::getAShr(Constant *C1, Constant *C2, bool isExact) {
 
 /// getBinOpIdentity - Return the identity for the given binary operation,
 /// i.e. a constant C such that X op C = X and C op X = X for every X.  It
-/// is an error to call this for an operation that doesn't have an identity.
+/// returns null if the operator doesn't have an identity.
 Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty) {
   switch (Opcode) {
   default:
-    llvm_unreachable("Not a binary operation with identity");
+    // Doesn't have an identity.
+    return 0;
+
   case Instruction::Add:
   case Instruction::Or:
   case Instruction::Xor:
@@ -2027,6 +2029,25 @@ Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty) {
   }
 }
 
+/// getBinOpAbsorber - Return the absorbing element for the given binary
+/// operation, i.e. a constant C such that X op C = C and C op X = C for
+/// every X.  For example, this returns zero for integer multiplication.
+/// It returns null if the operator doesn't have an absorbing element.
+Constant *ConstantExpr::getBinOpAbsorber(unsigned Opcode, Type *Ty) {
+  switch (Opcode) {
+  default:
+    // Doesn't have an absorber.
+    return 0;
+
+  case Instruction::Or:
+    return Constant::getAllOnesValue(Ty);
+
+  case Instruction::And:
+  case Instruction::Mul:
+    return Constant::getNullValue(Ty);
+  }
+}
+
 // destroyConstant - Remove the constant from the constant table...
 //
 void ConstantExpr::destroyConstant() {