[X86] Estimate the codesize cost of switch

Author: XChy

llvm/lib/Target/X86/X86TargetTransformInfo.cpp

@@ -6155,6 +6155,23 @@ X86TTIImpl::getIntImmCostIntrin(Intrinsic::ID IID, unsigned Idx,
 InstructionCost X86TTIImpl::getCFInstrCost(unsigned Opcode,
                                            TTI::TargetCostKind CostKind,
                                            const Instruction *I) const {
+  if (Opcode == Instruction::Switch && CostKind == TTI::TCK_CodeSize) {
+    unsigned JumpTableSize, NumSuccs = I->getNumSuccessors();
+    getEstimatedNumberOfCaseClusters(*cast<SwitchInst>(I), JumpTableSize,
+                                     nullptr, nullptr);
+    // A trivial unconditional branch.
+    if (NumSuccs == 1)
+      return TTI::TCC_Basic;
+
+    // Assume that lowering the switch block is implemented by binary search if
+    // no jump table is generated.
+    if (JumpTableSize == 0)
+      return llvm::Log2_32_Ceil(NumSuccs) * 2 * TTI::TCC_Basic;
+
+    // Indirect branch + default compare + default jump
+    return 3 * TTI::TCC_Basic;
+  }
+
   if (CostKind != TTI::TCK_RecipThroughput)
     return Opcode == Instruction::PHI ? TTI::TCC_Free : TTI::TCC_Basic;
   // Branches are assumed to be predicted.