Division by a constant

Author: AZero13

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -454,16 +454,21 @@ InstructionCost AArch64TTIImpl::getIntImmCostInst(unsigned Opcode, unsigned Idx,
   case Instruction::Add:
   case Instruction::Sub:
   case Instruction::Mul:
-  case Instruction::UDiv:
-  case Instruction::SDiv:
-  case Instruction::URem:
-  case Instruction::SRem:
   case Instruction::And:
   case Instruction::Or:
   case Instruction::Xor:
   case Instruction::ICmp:
     ImmIdx = 1;
     break;
+
+  // Division by a constant can be turned into multiplication, but only if we
+  // know it's constant. So it's not so much that the immediate is cheap (it's
+  // not), but that the alternative is worse.
+  case Instruction::UDiv:
+  case Instruction::SDiv:
+  case Instruction::URem:
+  case Instruction::SRem:
+
   // Always return TCC_Free for the shift value of a shift instruction.
   case Instruction::Shl:
   case Instruction::LShr: