[SimplifyCFG] cleanup code for converting switch to select (NFC)

[spatel]

Summary

This renames functions for more general usage (and current capitalization style) before a proposed logic change in D122485.

[bcl5980]

LGTM

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

static uintptr_t MapCaseToResult(ConstantInt *CaseVal,
   return 1;
 }
 
 // Helper function that initializes a map containing
 // results for the PHI node of the common destination block for a switch
 // instruction. Returns false if multiple PHI nodes have been found or if
 // there is not a common destination block for the switch.
 static bool
-InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
+initializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
                       SwitchCaseResultVectorTy &UniqueResults,
                       Constant *&DefaultResult, const DataLayout &DL,
                       const TargetTransformInfo &TTI,
                       uintptr_t MaxUniqueResults, uintptr_t MaxCasesPerResult) {
   for (auto &I : SI->cases()) {
     ConstantInt *CaseVal = I.getCaseValue();
 
     // Resulting value at phi nodes for this case value.

 // switch (a) {
 //   case 10:                %0 = icmp eq i32 %a, 10
 //     return 10;            %1 = select i1 %0, i32 10, i32 4
 //   case 20:        ---->   %2 = icmp eq i32 %a, 20
 //     return 2;             %3 = select i1 %2, i32 2, i32 %1
 //   default:
 //     return 4;
 // }
-static Value *ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy &ResultVector,
-                                   Constant *DefaultResult, Value *Condition,
-                                   IRBuilder<> &Builder) {
+// TODO: Handle switches with more than 2 cases that map to the same result.
+static Value *foldSwitchToSelect(const SwitchCaseResultVectorTy &ResultVector,
   // If we are selecting between only two cases transform into a simple
   // select or a two-way select if default is possible.
   if (ResultVector.size() == 2 && ResultVector[0].second.size() == 1 &&
       ResultVector[1].second.size() == 1) {
     ConstantInt *const FirstCase = ResultVector[0].second[0];
     ConstantInt *const SecondCase = ResultVector[1].second[0];
 
     bool DefaultCanTrigger = DefaultResult;
if (ResultVector.size() == 1 && ResultVector[0].second.size() == 2 &&
     return Builder.CreateSelect(Cmp, ResultVector[0].first, DefaultResult);
   }
 
   return nullptr;
 }
 
 // Helper function to cleanup a switch instruction that has been converted into
 // a select, fixing up PHI nodes and basic blocks.
-static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
-                                              Value *SelectValue,
-                                              IRBuilder<> &Builder,
-                                              DomTreeUpdater *DTU) {
+static void removeSwitchAfterSelectFold(SwitchInst *SI, PHINode *PHI,
   std::vector<DominatorTree::UpdateType> Updates;
 
   BasicBlock *SelectBB = SI->getParent();
   BasicBlock *DestBB = PHI->getParent();
 
   if (DTU && !is_contained(predecessors(DestBB), SelectBB))
     Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
   Builder.CreateBr(DestBB);
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
     if (DTU && RemovedSuccessors.insert(Succ).second)
       Updates.push_back({DominatorTree::Delete, SelectBB, Succ});
   }
   SI->eraseFromParent();
   if (DTU)
     DTU->applyUpdates(Updates);
 }
 
-/// If the switch is only used to initialize one or more
-/// phi nodes in a common successor block with only two different
-/// constant values, replace the switch with select.
-static bool switchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
+/// If a switch is only used to initialize one or more phi nodes in a common
+/// successor block with only two different constant values, try to replace the
+/// switch with a select. Returns true if the fold was made.
+static bool trySwitchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
                               DomTreeUpdater *DTU, const DataLayout &DL,
                               const TargetTransformInfo &TTI) {
   Value *const Cond = SI->getCondition();
   PHINode *PHI = nullptr;
   BasicBlock *CommonDest = nullptr;
   Constant *DefaultResult;
   SwitchCaseResultVectorTy UniqueResults;
   // Collect all the cases that will deliver the same value from the switch.
-  if (!InitializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
-                             DL, TTI, /*MaxUniqueResults*/2,
-                             /*MaxCasesPerResult*/2))
+  if (!initializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
+                             DL, TTI, /*MaxUniqueResults*/ 2,
+                             /*MaxCasesPerResult*/ 2))
     return false;
-  assert(PHI != nullptr && "PHI for value select not found");
 
+  assert(PHI != nullptr && "PHI for value select not found");
   Builder.SetInsertPoint(SI);
   Value *SelectValue =
-      ConvertTwoCaseSwitch(UniqueResults, DefaultResult, Cond, Builder);
-  if (SelectValue) {
-    RemoveSwitchAfterSelectConversion(SI, PHI, SelectValue, Builder, DTU);
-    return true;
-  }
-  // The switch couldn't be converted into a select.
+      foldSwitchToSelect(UniqueResults, DefaultResult, Cond, Builder);
+  if (!SelectValue)
     return false;
+
+  removeSwitchAfterSelectFold(SI, PHI, SelectValue, Builder, DTU);
+  return true;
 }
 
 namespace {
 
 /// This class represents a lookup table that can be used to replace a switch.
 class SwitchLookupTable {
 public:
   /// Create a lookup table to use as a switch replacement with the contents
bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   // impossible switch values, so disable it early in the pipeline.
   if (Options.ConvertSwitchRangeToICmp && TurnSwitchRangeIntoICmp(SI, Builder))
     return requestResimplify();
 
   // Remove unreachable cases.
   if (eliminateDeadSwitchCases(SI, DTU, Options.AC, DL))
     return requestResimplify();
 
-  if (switchToSelect(SI, Builder, DTU, DL, TTI))
+  if (trySwitchToSelect(SI, Builder, DTU, DL, TTI))
     return requestResimplify();
 
   if (Options.ForwardSwitchCondToPhi && ForwardSwitchConditionToPHI(SI))
     return requestResimplify();
 
   // The conversion from switch to lookup tables results in difficult-to-analyze
   // code and makes pruning branches much harder. This is a problem if the
   // switch expression itself can still be restricted as a result of inlining or