[MC/DC] Refactor: Let MCDCConditionID int16_t with zero-origin (#81257)

Also, Let `NumConditions` `uint16_t`.

It is smarter to handle the ID as signed.
Narrowing to `int16_t` will reduce costs of handling byvalue. (See also
#81221 and #81227)

External behavior doesn't change. They below handle values as internal
values plus 1.
* `-dump-coverage-mapping`
* `CoverageMappingReader.cpp`
* `CoverageMappingWriter.cpp`

1 files changed, 15 insertions, 13 deletions

diff --git a/clang/lib/CodeGen/CoverageMappingGen.cpp b/clang/lib/CodeGen/CoverageMappingGen.cpp
index 93c3c31..fdf821a 100644
--- a/clang/lib/CodeGen/CoverageMappingGen.cpp
+++ b/clang/lib/CodeGen/CoverageMappingGen.cpp
@@ -686,11 +686,12 @@ private:
   llvm::SmallVector<mcdc::ConditionIDs> DecisionStack;
   MCDC::State &MCDCState;
   llvm::DenseMap<const Stmt *, mcdc::ConditionID> &CondIDs;
-  mcdc::ConditionID NextID = 1;
+  mcdc::ConditionID NextID = 0;
   bool NotMapped = false;
 
-  /// Represent a sentinel value of [0,0] for the bottom of DecisionStack.
-  static constexpr mcdc::ConditionIDs DecisionStackSentinel{0, 0};
+  /// Represent a sentinel value as a pair of final decisions for the bottom
+  // of DecisionStack.
+  static constexpr mcdc::ConditionIDs DecisionStackSentinel{-1, -1};
 
   /// Is this a logical-AND operation?
   bool isLAnd(const BinaryOperator *E) const {
@@ -705,12 +706,12 @@ public:
   /// Return whether the build of the control flow map is at the top-level
   /// (root) of a logical operator nest in a boolean expression prior to the
   /// assignment of condition IDs.
-  bool isIdle() const { return (NextID == 1 && !NotMapped); }
+  bool isIdle() const { return (NextID == 0 && !NotMapped); }
 
   /// Return whether any IDs have been assigned in the build of the control
   /// flow map, indicating that the map is being generated for this boolean
   /// expression.
-  bool isBuilding() const { return (NextID > 1); }
+  bool isBuilding() const { return (NextID > 0); }
 
   /// Set the given condition's ID.
   void setCondID(const Expr *Cond, mcdc::ConditionID ID) {
@@ -721,7 +722,7 @@ public:
   mcdc::ConditionID getCondID(const Expr *Cond) const {
     auto I = CondIDs.find(CodeGenFunction::stripCond(Cond));
     if (I == CondIDs.end())
-      return 0;
+      return -1;
     else
       return I->second;
   }
@@ -789,15 +790,15 @@ public:
     // Reset state if not doing mapping.
     if (NotMapped) {
       NotMapped = false;
-      assert(NextID == 1);
+      assert(NextID == 0);
       return 0;
     }
 
     // Set number of conditions and reset.
-    unsigned TotalConds = NextID - 1;
+    unsigned TotalConds = NextID;
 
     // Reset ID back to beginning.
-    NextID = 1;
+    NextID = 0;
 
     return TotalConds;
   }
@@ -889,7 +890,7 @@ struct CounterCoverageMappingBuilder
     return RegionStack.size() - 1;
   }
 
-  size_t pushRegion(unsigned BitmapIdx, unsigned Conditions,
+  size_t pushRegion(unsigned BitmapIdx, uint16_t Conditions,
                     std::optional<SourceLocation> StartLoc = std::nullopt,
                     std::optional<SourceLocation> EndLoc = std::nullopt) {
 
@@ -1038,7 +1039,7 @@ struct CounterCoverageMappingBuilder
     if (CodeGenFunction::isInstrumentedCondition(C)) {
       mcdc::Parameters BranchParams;
       mcdc::ConditionID ID = MCDCBuilder.getCondID(C);
-      if (ID > 0)
+      if (ID >= 0)
         BranchParams = mcdc::BranchParameters{ID, Conds};
 
       // If a condition can fold to true or false, the corresponding branch
@@ -2125,8 +2126,9 @@ static void dump(llvm::raw_ostream &OS, StringRef FunctionName,
 
     if (const auto *BranchParams =
             std::get_if<mcdc::BranchParameters>(&R.MCDCParams)) {
-      OS << " [" << BranchParams->ID << "," << BranchParams->Conds[true];
-      OS << "," << BranchParams->Conds[false] << "] ";
+      OS << " [" << BranchParams->ID + 1 << ","
+         << BranchParams->Conds[true] + 1;
+      OS << "," << BranchParams->Conds[false] + 1 << "] ";
     }
 
     if (R.Kind == CounterMappingRegion::ExpansionRegion)