[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`

8 files changed, 61 insertions, 50 deletions

diff --git a/clang/lib/CodeGen/CodeGenPGO.cpp b/clang/lib/CodeGen/CodeGenPGO.cpp
index b5ce1aa..48c5e68 100644
--- a/clang/lib/CodeGen/CodeGenPGO.cpp
+++ b/clang/lib/CodeGen/CodeGenPGO.cpp
@@ -1031,7 +1031,7 @@ void CodeGenPGO::emitCounterRegionMapping(const Decl *D) {
 
   std::string CoverageMapping;
   llvm::raw_string_ostream OS(CoverageMapping);
-  RegionMCDCState->CondIDMap.clear();
+  RegionCondIDMap.reset(new llvm::DenseMap<const Stmt *, int16_t>);
   CoverageMappingGen MappingGen(
       *CGM.getCoverageMapping(), CGM.getContext().getSourceManager(),
       CGM.getLangOpts(), RegionCounterMap.get(), RegionMCDCState.get());
@@ -1195,8 +1195,8 @@ void CodeGenPGO::emitMCDCCondBitmapUpdate(CGBuilderTy &Builder, const Expr *S,
     return;
 
   // Extract the ID of the condition we are setting in the bitmap.
-  unsigned CondID = ExprMCDCConditionIDMapIterator->second;
-  assert(CondID > 0 && "Condition has no ID!");
+  auto CondID = ExprMCDCConditionIDMapIterator->second;
+  assert(CondID >= 0 && "Condition has no ID!");
 
   auto *I8PtrTy = llvm::PointerType::getUnqual(CGM.getLLVMContext());
 
@@ -1205,7 +1205,7 @@ void CodeGenPGO::emitMCDCCondBitmapUpdate(CGBuilderTy &Builder, const Expr *S,
   // the resulting value is used to update the boolean expression's bitmap.
   llvm::Value *Args[5] = {llvm::ConstantExpr::getBitCast(FuncNameVar, I8PtrTy),
                           Builder.getInt64(FunctionHash),
-                          Builder.getInt32(CondID - 1),
+                          Builder.getInt32(CondID),
                           MCDCCondBitmapAddr.getPointer(), Val};
   Builder.CreateCall(
       CGM.getIntrinsic(llvm::Intrinsic::instrprof_mcdc_condbitmap_update),
diff --git a/clang/lib/CodeGen/CodeGenPGO.h b/clang/lib/CodeGen/CodeGenPGO.h
index d3c2b27..369bf05 100644
--- a/clang/lib/CodeGen/CodeGenPGO.h
+++ b/clang/lib/CodeGen/CodeGenPGO.h
@@ -36,6 +36,8 @@ private:
   unsigned NumRegionCounters;
   uint64_t FunctionHash;
   std::unique_ptr<llvm::DenseMap<const Stmt *, unsigned>> RegionCounterMap;
+  std::unique_ptr<llvm::DenseMap<const Stmt *, unsigned>> RegionMCDCBitmapMap;
+  std::unique_ptr<llvm::DenseMap<const Stmt *, int16_t>> RegionCondIDMap;
   std::unique_ptr<llvm::DenseMap<const Stmt *, uint64_t>> StmtCountMap;
   std::unique_ptr<llvm::InstrProfRecord> ProfRecord;
   std::unique_ptr<MCDC::State> RegionMCDCState;
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)
diff --git a/llvm/include/llvm/ProfileData/Coverage/MCDCTypes.h b/llvm/include/llvm/ProfileData/Coverage/MCDCTypes.h
index 6127217..51f528b 100644
--- a/llvm/include/llvm/ProfileData/Coverage/MCDCTypes.h
+++ b/llvm/include/llvm/ProfileData/Coverage/MCDCTypes.h
@@ -19,7 +19,7 @@
 namespace llvm::coverage::mcdc {
 
 /// The ID for MCDCBranch.
-using ConditionID = unsigned int;
+using ConditionID = int16_t;
 using ConditionIDs = std::array<ConditionID, 2>;
 
 struct DecisionParameters {
@@ -27,7 +27,7 @@ struct DecisionParameters {
   unsigned BitmapIdx;
 
   /// Number of Conditions used for a Decision Region.
-  unsigned NumConditions;
+  uint16_t NumConditions;
 
   DecisionParameters() = delete;
   DecisionParameters(unsigned BitmapIdx, unsigned NumConditions)
diff --git a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
index 9adeceb..ddce758 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
@@ -272,17 +272,17 @@ private:
   // Walk the binary decision diagram and try assigning both false and true to
   // each node. When a terminal node (ID == 0) is reached, fill in the value in
   // the truth table.
-  void buildTestVector(MCDCRecord::TestVector &TV, unsigned ID,
+  void buildTestVector(MCDCRecord::TestVector &TV, mcdc::ConditionID ID,
                        unsigned Index) {
-    assert((Index & (1 << (ID - 1))) == 0);
+    assert((Index & (1 << ID)) == 0);
 
     for (auto MCDCCond : {MCDCRecord::MCDC_False, MCDCRecord::MCDC_True}) {
       static_assert(MCDCRecord::MCDC_False == 0);
       static_assert(MCDCRecord::MCDC_True == 1);
-      Index |= MCDCCond << (ID - 1);
-      TV[ID - 1] = MCDCCond;
+      Index |= MCDCCond << ID;
+      TV[ID] = MCDCCond;
       auto NextID = CondsMap[ID][MCDCCond];
-      if (NextID > 0) {
+      if (NextID >= 0) {
         buildTestVector(TV, NextID, Index);
         continue;
       }
@@ -299,17 +299,17 @@ private:
     }
 
     // Reset back to DontCare.
-    TV[ID - 1] = MCDCRecord::MCDC_DontCare;
+    TV[ID] = MCDCRecord::MCDC_DontCare;
   }
 
   /// Walk the bits in the bitmap.  A bit set to '1' indicates that the test
   /// vector at the corresponding index was executed during a test run.
   void findExecutedTestVectors() {
     // Walk the binary decision diagram to enumerate all possible test vectors.
-    // We start at the root node (ID == 1) with all values being DontCare.
+    // We start at the root node (ID == 0) with all values being DontCare.
     // `Index` encodes the bitmask of true values and is initially 0.
     MCDCRecord::TestVector TV(NumConditions, MCDCRecord::MCDC_DontCare);
-    buildTestVector(TV, 1, 0);
+    buildTestVector(TV, 0, 0);
   }
 
   // Find an independence pair for each condition:
@@ -371,7 +371,7 @@ public:
     for (const auto *B : Branches) {
       const auto &BranchParams = B->getBranchParams();
       CondsMap[BranchParams.ID] = BranchParams.Conds;
-      PosToID[I] = BranchParams.ID - 1;
+      PosToID[I] = BranchParams.ID;
       CondLoc[I] = B->startLoc();
       Folded[I++] = (B->Count.isZero() && B->FalseCount.isZero());
     }
@@ -566,10 +566,10 @@ private:
       assert(Branch.Kind == CounterMappingRegion::MCDCBranchRegion);
 
       auto ConditionID = Branch.getBranchParams().ID;
-      assert(ConditionID > 0 && "ConditionID should begin with 1");
+      assert(ConditionID >= 0 && "ConditionID should be positive");
 
       if (ConditionIDs.contains(ConditionID) ||
-          ConditionID > DecisionParams.NumConditions)
+          ConditionID >= DecisionParams.NumConditions)
         return NotProcessed;
 
       if (!this->dominates(Branch))
@@ -577,9 +577,9 @@ private:
 
       assert(MCDCBranches.size() < DecisionParams.NumConditions);
 
-      // Put `ID=1` in front of `MCDCBranches` for convenience
+      // Put `ID=0` in front of `MCDCBranches` for convenience
       // even if `MCDCBranches` is not topological.
-      if (ConditionID == 1)
+      if (ConditionID == 0)
         MCDCBranches.insert(MCDCBranches.begin(), &Branch);
       else
         MCDCBranches.push_back(&Branch);
diff --git a/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp b/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
index de7be52..d328460 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
@@ -244,7 +244,9 @@ Error RawCoverageMappingReader::readMappingRegionsSubArray(
   unsigned LineStart = 0;
   for (size_t I = 0; I < NumRegions; ++I) {
     Counter C, C2;
-    uint64_t BIDX, NC, ID, TID, FID;
+    uint64_t BIDX, NC;
+    // They are stored as internal values plus 1 (min is -1)
+    uint64_t ID1, TID1, FID1;
     mcdc::Parameters Params;
     CounterMappingRegion::RegionKind Kind = CounterMappingRegion::CodeRegion;
 
@@ -303,28 +305,29 @@ Error RawCoverageMappingReader::readMappingRegionsSubArray(
             return Err;
           if (auto Err = readCounter(C2))
             return Err;
-          if (auto Err = readIntMax(ID, std::numeric_limits<unsigned>::max()))
+          if (auto Err = readIntMax(ID1, std::numeric_limits<int16_t>::max()))
             return Err;
-          if (auto Err = readIntMax(TID, std::numeric_limits<unsigned>::max()))
+          if (auto Err = readIntMax(TID1, std::numeric_limits<int16_t>::max()))
             return Err;
-          if (auto Err = readIntMax(FID, std::numeric_limits<unsigned>::max()))
+          if (auto Err = readIntMax(FID1, std::numeric_limits<int16_t>::max()))
             return Err;
-          if (ID == 0)
+          if (ID1 == 0)
             return make_error<CoverageMapError>(
                 coveragemap_error::malformed,
                 "MCDCConditionID shouldn't be zero");
           Params = mcdc::BranchParameters{
-              static_cast<unsigned>(ID),
-              {static_cast<unsigned>(FID), static_cast<unsigned>(TID)}};
+              static_cast<int16_t>(static_cast<int16_t>(ID1) - 1),
+              {static_cast<int16_t>(static_cast<int16_t>(FID1) - 1),
+               static_cast<int16_t>(static_cast<int16_t>(TID1) - 1)}};
           break;
         case CounterMappingRegion::MCDCDecisionRegion:
           Kind = CounterMappingRegion::MCDCDecisionRegion;
           if (auto Err = readIntMax(BIDX, std::numeric_limits<unsigned>::max()))
             return Err;
-          if (auto Err = readIntMax(NC, std::numeric_limits<unsigned>::max()))
+          if (auto Err = readIntMax(NC, std::numeric_limits<int16_t>::max()))
             return Err;
           Params = mcdc::DecisionParameters{static_cast<unsigned>(BIDX),
-                                            static_cast<unsigned>(NC)};
+                                            static_cast<uint16_t>(NC)};
           break;
         default:
           return make_error<CoverageMapError>(coveragemap_error::malformed,
diff --git a/llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp b/llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp
index 6125cce..5036bde 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp
@@ -253,12 +253,16 @@ void CoverageMappingWriter::write(raw_ostream &OS) {
       writeCounter(MinExpressions, Count, OS);
       writeCounter(MinExpressions, FalseCount, OS);
       {
+        // They are written as internal values plus 1.
         const auto &BranchParams = I->getBranchParams();
         ParamsShouldBeNull = false;
-        assert(BranchParams.ID > 0);
-        encodeULEB128(static_cast<unsigned>(BranchParams.ID), OS);
-        encodeULEB128(static_cast<unsigned>(BranchParams.Conds[true]), OS);
-        encodeULEB128(static_cast<unsigned>(BranchParams.Conds[false]), OS);
+        assert(BranchParams.ID >= 0);
+        unsigned ID1 = BranchParams.ID + 1;
+        unsigned TID1 = BranchParams.Conds[true] + 1;
+        unsigned FID1 = BranchParams.Conds[false] + 1;
+        encodeULEB128(ID1, OS);
+        encodeULEB128(TID1, OS);
+        encodeULEB128(FID1, OS);
       }
       break;
     case CounterMappingRegion::MCDCDecisionRegion:
diff --git a/llvm/unittests/ProfileData/CoverageMappingTest.cpp b/llvm/unittests/ProfileData/CoverageMappingTest.cpp
index db6689b..425b3d1 100644
--- a/llvm/unittests/ProfileData/CoverageMappingTest.cpp
+++ b/llvm/unittests/ProfileData/CoverageMappingTest.cpp
@@ -192,7 +192,7 @@ struct CoverageMappingTest : ::testing::TestWithParam<std::tuple<bool, bool>> {
     addCMR(Counter::getZero(), File, LS, CS, LE, CE, true);
   }
 
-  void addMCDCDecisionCMR(unsigned Mask, unsigned NC, StringRef File,
+  void addMCDCDecisionCMR(unsigned Mask, uint16_t NC, StringRef File,
                           unsigned LS, unsigned CS, unsigned LE, unsigned CE) {
     auto &Regions = InputFunctions.back().Regions;
     unsigned FileID = getFileIndexForFunction(File);
@@ -872,9 +872,9 @@ TEST_P(CoverageMappingTest, non_code_region_bitmask) {
   addCMR(Counter::getCounter(3), "file", 1, 1, 5, 5);
 
   addMCDCDecisionCMR(0, 2, "file", 7, 1, 7, 6);
-  addMCDCBranchCMR(Counter::getCounter(0), Counter::getCounter(1), 1, {0, 2},
+  addMCDCBranchCMR(Counter::getCounter(0), Counter::getCounter(1), 0, {-1, 1},
                    "file", 7, 2, 7, 3);
-  addMCDCBranchCMR(Counter::getCounter(2), Counter::getCounter(3), 2, {0, 0},
+  addMCDCBranchCMR(Counter::getCounter(2), Counter::getCounter(3), 1, {-1, -1},
                    "file", 7, 4, 7, 5);
 
   EXPECT_THAT_ERROR(loadCoverageMapping(), Succeeded());
@@ -900,10 +900,10 @@ TEST_P(CoverageMappingTest, decision_before_expansion) {
   addExpansionCMR("foo", "B", 4, 19, 4, 20);
   addCMR(Counter::getCounter(0), "A", 1, 14, 1, 17);
   addCMR(Counter::getCounter(0), "A", 1, 14, 1, 17);
-  addMCDCBranchCMR(Counter::getCounter(0), Counter::getCounter(1), 1, {0, 2},
+  addMCDCBranchCMR(Counter::getCounter(0), Counter::getCounter(1), 0, {-1, 1},
                    "A", 1, 14, 1, 17);
   addCMR(Counter::getCounter(1), "B", 1, 14, 1, 17);
-  addMCDCBranchCMR(Counter::getCounter(1), Counter::getCounter(2), 2, {0, 0},
+  addMCDCBranchCMR(Counter::getCounter(1), Counter::getCounter(2), 1, {-1, -1},
                    "B", 1, 14, 1, 17);
 
   // InputFunctionCoverageData::Regions is rewritten after the write.