[Constants] extend getBinOpIdentity(); NFC

The enhanced version will be used in D48893 and related patches
and an almost identical (fadd is different) version is proposed 
in D28907, so adding this as a preliminary step.

llvm-svn: 336444

include/llvm/IR/Constants.h

@@ -1018,10 +1018,15 @@ public:
     return getLShr(C1, C2, true);
   }
 
-  /// 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
-  /// returns null if the operator doesn't have an identity.
-  static Constant *getBinOpIdentity(unsigned Opcode, Type *Ty);
+  /// Return the identity constant for a binary opcode.
+  /// The identity constant C is defined as X op C = X and C op X = X for every
+  /// X when the binary operation is commutative. If the binop is not
+  /// commutative, callers can acquire the operand 1 identity constant by
+  /// setting AllowRHSConstant to true. For example, any shift has a zero
+  /// identity constant for operand 1: X shift 0 = X.
+  /// Return nullptr if the operator does not have an identity constant.
+  static Constant *getBinOpIdentity(unsigned Opcode, Type *Ty,
+                                    bool AllowRHSConstant = false);
 
   /// 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

lib/IR/Constants.cpp

@@ -2271,32 +2271,49 @@ Constant *ConstantExpr::getAShr(Constant *C1, Constant *C2, bool isExact) {
              isExact ? PossiblyExactOperator::IsExact : 0);
 }
 
-// FIXME: Add a parameter to specify the operand number for non-commutative ops.
-// For example, the operand 1 identity constant for any shift is the null value
-// because shift-by-0 always returns operand 0.
-Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty) {
-  switch (Opcode) {
-  default:
-    // Doesn't have an identity.
+Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty,
+                                         bool AllowRHSConstant) {
+  assert(Instruction::isBinaryOp(Opcode) && "Only binops allowed");
+
+  // Commutative opcodes: it does not matter if AllowRHSConstant is set.
+  if (Instruction::isCommutative(Opcode)) {
+    switch (Opcode) {
+      case Instruction::Add: // X + 0 = X
+      case Instruction::Or:  // X | 0 = X
+      case Instruction::Xor: // X ^ 0 = X
+        return Constant::getNullValue(Ty);
+      case Instruction::Mul: // X * 1 = X
+        return ConstantInt::get(Ty, 1);
+      case Instruction::And: // X & -1 = X
+        return Constant::getAllOnesValue(Ty);
+      case Instruction::FAdd: // X + -0.0 = X
+        // TODO: If the fadd has 'nsz', should we return +0.0?
+        return ConstantFP::getNegativeZero(Ty);
+      case Instruction::FMul: // X * 1.0 = X
+        return ConstantFP::get(Ty, 1.0);
+      default:
+        llvm_unreachable("Every commutative binop has an identity constant");
+    }
+  }
+
+  // Non-commutative opcodes: AllowRHSConstant must be set.
+  if (!AllowRHSConstant)
     return nullptr;
 
-  case Instruction::Add:
-  case Instruction::Or:
-  case Instruction::Xor:
-    return Constant::getNullValue(Ty);
-
-  case Instruction::Mul:
-    return ConstantInt::get(Ty, 1);
-
-  case Instruction::And:
-    return Constant::getAllOnesValue(Ty);
-
-  // TODO: If the fadd has 'nsz', should we return +0.0?
-  case Instruction::FAdd:
-    return ConstantFP::getNegativeZero(Ty);
-
-  case Instruction::FMul:
-    return ConstantFP::get(Ty, 1.0);
+  switch (Opcode) {
+    case Instruction::Sub:  // X - 0 = X
+    case Instruction::Shl:  // X << 0 = X
+    case Instruction::LShr: // X >>u 0 = X
+    case Instruction::AShr: // X >> 0 = X
+    case Instruction::FSub: // X - 0.0 = X
+      return Constant::getNullValue(Ty);
+    case Instruction::SDiv: // X / 1 = X
+    case Instruction::UDiv: // X /u 1 = X
+      return ConstantInt::get(Ty, 1);
+    case Instruction::FDiv: // X / 1.0 = X
+      return ConstantFP::get(Ty, 1.0);
+    default:
+      return nullptr;
   }
 }