[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`

3 files changed, 33 insertions, 26 deletions

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: