Implement select.ll:test12*

This transforms code like this:

   %C = or %A, %B
   %D = select %cond, %C, %A
into:
   %C = select %cond, %B, 0
   %D = or %A, %C

Since B is often a constant, the select can often be eliminated.  In any case,
this reduces the usage count of A, allowing subsequent optimizations to happen.

This xform applies when the operator is any of:
  add, sub, mul, or, xor, and, shl, shr

llvm-svn: 12800

Author: lattner

llvm/lib/Transforms/Scalar/InstructionCombining.cpp

@@ -2091,6 +2091,54 @@ Instruction *InstCombiner::visitCastInst(CastInst &CI) {
   return 0;
 }
 
+/// GetSelectFoldableOperands - We want to turn code that looks like this:
+///   %C = or %A, %B
+///   %D = select %cond, %C, %A
+/// into:
+///   %C = select %cond, %B, 0
+///   %D = or %A, %C
+///
+/// Assuming that the specified instruction is an operand to the select, return
+/// a bitmask indicating which operands of this instruction are foldable if they
+/// equal the other incoming value of the select.
+///
+static unsigned GetSelectFoldableOperands(Instruction *I) {
+  switch (I->getOpcode()) {
+  case Instruction::Add:
+  case Instruction::Mul:
+  case Instruction::And:
+  case Instruction::Or:
+  case Instruction::Xor:
+    return 3;              // Can fold through either operand.
+  case Instruction::Sub:   // Can only fold on the amount subtracted.
+  case Instruction::Shl:   // Can only fold on the shift amount.
+  case Instruction::Shr:
+    return 1;           
+  default:
+    return 0;              // Cannot fold
+  }
+}
+
+/// GetSelectFoldableConstant - For the same transformation as the previous
+/// function, return the identity constant that goes into the select.
+static Constant *GetSelectFoldableConstant(Instruction *I) {
+  switch (I->getOpcode()) {
+  default: assert(0 && "This cannot happen!"); abort();
+  case Instruction::Add:
+  case Instruction::Sub:
+  case Instruction::Or:
+  case Instruction::Xor:
+    return Constant::getNullValue(I->getType());
+  case Instruction::Shl:
+  case Instruction::Shr:
+    return Constant::getNullValue(Type::UByteTy);
+  case Instruction::And:
+    return ConstantInt::getAllOnesValue(I->getType());
+  case Instruction::Mul:
+    return ConstantInt::get(I->getType(), 1);
+  }
+}
+
 Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
   Value *CondVal = SI.getCondition();
   Value *TrueVal = SI.getTrueValue();
@@ -2150,6 +2198,66 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
       }
     }
 
+  // See if we can fold the select into one of our operands.
+  if (SI.getType()->isInteger()) {
+    // See the comment above GetSelectFoldableOperands for a description of the
+    // transformation we are doing here.
+    if (Instruction *TVI = dyn_cast<Instruction>(TrueVal))
+      if (TVI->hasOneUse() && TVI->getNumOperands() == 2 &&
+          !isa<Constant>(FalseVal))
+        if (unsigned SFO = GetSelectFoldableOperands(TVI)) {
+          unsigned OpToFold = 0;
+          if ((SFO & 1) && FalseVal == TVI->getOperand(0)) {
+            OpToFold = 1;
+          } else  if ((SFO & 2) && FalseVal == TVI->getOperand(1)) {
+            OpToFold = 2;
+          }
+
+          if (OpToFold) {
+            Constant *C = GetSelectFoldableConstant(TVI);
+            std::string Name = TVI->getName(); TVI->setName("");
+            Instruction *NewSel =
+              new SelectInst(SI.getCondition(), TVI->getOperand(2-OpToFold), C,
+                             Name);
+            InsertNewInstBefore(NewSel, SI);
+            if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TVI))
+              return BinaryOperator::create(BO->getOpcode(), FalseVal, NewSel);
+            else if (ShiftInst *SI = dyn_cast<ShiftInst>(TVI))
+              return new ShiftInst(SI->getOpcode(), FalseVal, NewSel);
+            else {
+              assert(0 && "Unknown instruction!!");
+            }
+          }
+        }
+    
+    if (Instruction *FVI = dyn_cast<Instruction>(FalseVal))
+      if (FVI->hasOneUse() && FVI->getNumOperands() == 2 &&
+          !isa<Constant>(TrueVal))
+        if (unsigned SFO = GetSelectFoldableOperands(FVI)) {
+          unsigned OpToFold = 0;
+          if ((SFO & 1) && TrueVal == FVI->getOperand(0)) {
+            OpToFold = 1;
+          } else  if ((SFO & 2) && TrueVal == FVI->getOperand(1)) {
+            OpToFold = 2;
+          }
+
+          if (OpToFold) {
+            Constant *C = GetSelectFoldableConstant(FVI);
+            std::string Name = FVI->getName(); FVI->setName("");
+            Instruction *NewSel =
+              new SelectInst(SI.getCondition(), C, FVI->getOperand(2-OpToFold),
+                             Name);
+            InsertNewInstBefore(NewSel, SI);
+            if (BinaryOperator *BO = dyn_cast<BinaryOperator>(FVI))
+              return BinaryOperator::create(BO->getOpcode(), TrueVal, NewSel);
+            else if (ShiftInst *SI = dyn_cast<ShiftInst>(FVI))
+              return new ShiftInst(SI->getOpcode(), TrueVal, NewSel);
+            else {
+              assert(0 && "Unknown instruction!!");
+            }
+          }
+        }
+  }
   return 0;
 }