[ConstantFold][RFC] Add AllowLHSConstant parameter in getBinOpAbsorber (llvm#109736)

Add a AllowLHSConstant parameter in getBinOpAbsorber function for
supporting more binary operators.

Author: eric-xtang1008

llvm/include/llvm/IR/Constants.h

@@ -1210,8 +1210,11 @@ class ConstantExpr : public Constant {
   /// 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.
-  static Constant *getBinOpAbsorber(unsigned Opcode, Type *Ty);
+  /// If AllowLHSConstant is true, the LHS operand is a constant C that must be
+  /// defined as C op X = C. It returns null if the operator doesn't have
+  /// an absorbing element.
+  static Constant *getBinOpAbsorber(unsigned Opcode, Type *Ty,
+                                    bool AllowLHSConstant = false);
 
   /// Transparently provide more efficient getOperand methods.
   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Constant);

llvm/lib/IR/ConstantFold.cpp

@@ -731,11 +731,11 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
 
   // Handle simplifications when the RHS is a constant int.
   if (ConstantInt *CI2 = dyn_cast<ConstantInt>(C2)) {
+    if (C2 == ConstantExpr::getBinOpAbsorber(Opcode, C2->getType(),
+                                             /*AllowLHSConstant*/ false))
+      return C2;
+
     switch (Opcode) {
-    case Instruction::Mul:
-      if (CI2->isZero())
-        return C2; // X * 0 == 0
-      break;
     case Instruction::UDiv:
     case Instruction::SDiv:
       if (CI2->isZero())
@@ -749,9 +749,7 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
         return PoisonValue::get(CI2->getType());              // X % 0 == poison
       break;
     case Instruction::And:
-      if (CI2->isZero())
-        return C2; // X & 0 == 0
-
+      assert(!CI2->isZero() && "And zero handled above");
       if (ConstantExpr *CE1 = dyn_cast<ConstantExpr>(C1)) {
         // If and'ing the address of a global with a constant, fold it.
         if (CE1->getOpcode() == Instruction::PtrToInt &&
@@ -791,10 +789,6 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
         }
       }
       break;
-    case Instruction::Or:
-      if (CI2->isMinusOne())
-        return C2; // X | -1 == -1
-      break;
     }
   } else if (isa<ConstantInt>(C1)) {
     // If C1 is a ConstantInt and C2 is not, swap the operands.
@@ -854,19 +848,9 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
       }
     }
 
-    switch (Opcode) {
-    case Instruction::SDiv:
-    case Instruction::UDiv:
-    case Instruction::URem:
-    case Instruction::SRem:
-    case Instruction::LShr:
-    case Instruction::AShr:
-    case Instruction::Shl:
-      if (CI1->isZero()) return C1;
-      break;
-    default:
-      break;
-    }
+    if (C1 == ConstantExpr::getBinOpAbsorber(Opcode, C1->getType(),
+                                             /*AllowLHSConstant*/ true))
+      return C1;
   } else if (ConstantFP *CFP1 = dyn_cast<ConstantFP>(C1)) {
     if (ConstantFP *CFP2 = dyn_cast<ConstantFP>(C2)) {
       const APFloat &C1V = CFP1->getValueAPF();

llvm/lib/IR/Constants.cpp

@@ -2735,17 +2735,34 @@ Constant *ConstantExpr::getIdentity(Instruction *I, Type *Ty,
   return nullptr;
 }
 
-Constant *ConstantExpr::getBinOpAbsorber(unsigned Opcode, Type *Ty) {
+Constant *ConstantExpr::getBinOpAbsorber(unsigned Opcode, Type *Ty,
+                                         bool AllowLHSConstant) {
   switch (Opcode) {
   default:
-    // Doesn't have an absorber.
-    return nullptr;
+    break;
 
-  case Instruction::Or:
+  case Instruction::Or: // -1 | X = -1
     return Constant::getAllOnesValue(Ty);
 
-  case Instruction::And:
-  case Instruction::Mul:
+  case Instruction::And: // 0 & X = 0
+  case Instruction::Mul: // 0 * X = 0
+    return Constant::getNullValue(Ty);
+  }
+
+  // AllowLHSConstant must be set.
+  if (!AllowLHSConstant)
+    return nullptr;
+
+  switch (Opcode) {
+  default:
+    return nullptr;
+  case Instruction::Shl:  // 0 << X = 0
+  case Instruction::LShr: // 0 >>l X = 0
+  case Instruction::AShr: // 0 >>a X = 0
+  case Instruction::SDiv: // 0 /s X = 0
+  case Instruction::UDiv: // 0 /u X = 0
+  case Instruction::URem: // 0 %u X = 0
+  case Instruction::SRem: // 0 %s X = 0
     return Constant::getNullValue(Ty);
   }
 }