Introduce the type CounterPair for RegionCounterMap

`CounterPair` can hold `<uint32_t, uint32_t>` instead of current
`unsigned`, to hold also the counter number of SkipPath. For now, this
change provides the skeleton and only `CounterPair::first` is used.

Each counter number can have `None` to suppress emitting counter
increment. `second` is initialized as `None` by default, since most
`Stmt*` don't have a pair of counters.

This change also provides stubs for the verifyer. I'll provide the
impl of verifier for `+Asserts` later.

`markStmtAsUsed(bool, Stmt*)` may be used to inform that other side
counter may not emitted.

`markStmtMaybeUsed(S)` may be used for the `Stmt` and its inner will
be excluded for emission in the case of skipping by constant
folding. I put it into places where I found.

`verifyCounterMap()` will check the coverage map the counter map and
can be used to report inconsistency.

These verifier methods shall be eliminated in `-Asserts`.

https://discourse.llvm.org/t/rfc-integrating-singlebytecoverage-with-branch-coverage/82492

Author: chapuni

clang/lib/CodeGen/CGDecl.cpp

@@ -362,6 +362,8 @@ CodeGenFunction::AddInitializerToStaticVarDecl(const VarDecl &D,
     return GV;
   }
 
+  PGO.markStmtMaybeUsed(D.getInit()); // FIXME: Too lazy
+
 #ifndef NDEBUG
   CharUnits VarSize = CGM.getContext().getTypeSizeInChars(D.getType()) +
                       D.getFlexibleArrayInitChars(getContext());
@@ -1869,7 +1871,10 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
   // If we are at an unreachable point, we don't need to emit the initializer
   // unless it contains a label.
   if (!HaveInsertPoint()) {
-    if (!Init || !ContainsLabel(Init)) return;
+    if (!Init || !ContainsLabel(Init)) {
+      PGO.markStmtMaybeUsed(Init);
+      return;
+    }
     EnsureInsertPoint();
   }
 
@@ -1978,6 +1983,8 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
     return EmitExprAsInit(Init, &D, lv, capturedByInit);
   }
 
+  PGO.markStmtMaybeUsed(Init);
+
   if (!emission.IsConstantAggregate) {
     // For simple scalar/complex initialization, store the value directly.
     LValue lv = MakeAddrLValue(Loc, type);

clang/lib/CodeGen/CGExpr.cpp

@@ -5134,6 +5134,7 @@ std::optional<LValue> HandleConditionalOperatorLValueSimpleCase(
       // If the true case is live, we need to track its region.
       if (CondExprBool)
         CGF.incrementProfileCounter(E);
+      CGF.markStmtMaybeUsed(Dead);
       // If a throw expression we emit it and return an undefined lvalue
       // because it can't be used.
       if (auto *ThrowExpr = dyn_cast<CXXThrowExpr>(Live->IgnoreParens())) {

clang/lib/CodeGen/CGExprScalar.cpp

@@ -4982,8 +4982,10 @@ Value *ScalarExprEmitter::VisitBinLAnd(const BinaryOperator *E) {
     }
 
     // 0 && RHS: If it is safe, just elide the RHS, and return 0/false.
-    if (!CGF.ContainsLabel(E->getRHS()))
+    if (!CGF.ContainsLabel(E->getRHS())) {
+      CGF.markStmtMaybeUsed(E->getRHS());
       return llvm::Constant::getNullValue(ResTy);
+    }
   }
 
   // If the top of the logical operator nest, reset the MCDC temp to 0.
@@ -5122,8 +5124,10 @@ Value *ScalarExprEmitter::VisitBinLOr(const BinaryOperator *E) {
     }
 
     // 1 || RHS: If it is safe, just elide the RHS, and return 1/true.
-    if (!CGF.ContainsLabel(E->getRHS()))
+    if (!CGF.ContainsLabel(E->getRHS())) {
+      CGF.markStmtMaybeUsed(E->getRHS());
       return llvm::ConstantInt::get(ResTy, 1);
+    }
   }
 
   // If the top of the logical operator nest, reset the MCDC temp to 0.
@@ -5247,6 +5251,7 @@ VisitAbstractConditionalOperator(const AbstractConditionalOperator *E) {
         CGF.incrementProfileCounter(E);
       }
       Value *Result = Visit(live);
+      CGF.markStmtMaybeUsed(dead);
 
       // If the live part is a throw expression, it acts like it has a void
       // type, so evaluating it returns a null Value*.  However, a conditional

clang/lib/CodeGen/CGStmt.cpp

@@ -76,6 +76,7 @@ void CodeGenFunction::EmitStmt(const Stmt *S, ArrayRef<const Attr *> Attrs) {
       // Verify that any decl statements were handled as simple, they may be in
       // scope of subsequent reachable statements.
       assert(!isa<DeclStmt>(*S) && "Unexpected DeclStmt!");
+      PGO.markStmtMaybeUsed(S);
       return;
     }
 
@@ -845,6 +846,7 @@ void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
         RunCleanupsScope ExecutedScope(*this);
         EmitStmt(Executed);
       }
+      PGO.markStmtMaybeUsed(Skipped);
       return;
     }
   }
@@ -2170,6 +2172,7 @@ void CodeGenFunction::EmitSwitchStmt(const SwitchStmt &S) {
       for (unsigned i = 0, e = CaseStmts.size(); i != e; ++i)
         EmitStmt(CaseStmts[i]);
       incrementProfileCounter(&S);
+      PGO.markStmtMaybeUsed(S.getBody());
 
       // Now we want to restore the saved switch instance so that nested
       // switches continue to function properly

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -1606,6 +1606,8 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
   // Emit the standard function epilogue.
   FinishFunction(BodyRange.getEnd());
 
+  PGO.verifyCounterMap();
+
   // If we haven't marked the function nothrow through other means, do
   // a quick pass now to see if we can.
   if (!CurFn->doesNotThrow())
@@ -1728,6 +1730,7 @@ bool CodeGenFunction::ConstantFoldsToSimpleInteger(const Expr *Cond,
   if (!AllowLabels && CodeGenFunction::ContainsLabel(Cond))
     return false;  // Contains a label.
 
+  PGO.markStmtMaybeUsed(Cond);
   ResultInt = Int;
   return true;
 }

clang/lib/CodeGen/CodeGenFunction.h

@@ -1620,6 +1620,12 @@ class CodeGenFunction : public CodeGenTypeCache {
                                             uint64_t LoopCount) const;
 
 public:
+  std::pair<bool, bool> getIsCounterPair(const Stmt *S) const {
+    return PGO.getIsCounterPair(S);
+  }
+
+  void markStmtMaybeUsed(const Stmt *S) { PGO.markStmtMaybeUsed(S); }
+
   /// Increment the profiler's counter for the given statement by \p StepV.
   /// If \p StepV is null, the default increment is 1.
   void incrementProfileCounter(const Stmt *S, llvm::Value *StepV = nullptr) {

clang/lib/CodeGen/CodeGenModule.h

@@ -101,6 +101,25 @@ enum ForDefinition_t : bool {
   ForDefinition = true
 };
 
+class CounterPair : public std::pair<uint32_t, uint32_t> {
+private:
+  static constexpr uint32_t None = (1u << 31); /// None is set
+
+public:
+  static constexpr uint32_t Mask = None - 1;
+
+public:
+  CounterPair(unsigned Val = 0) {
+    assert(!(Val & ~Mask));
+    first = Val;
+    second = None;
+  }
+
+  std::pair<bool, bool> getIsCounterPair() const {
+    return {!(first & None), !(second & None)};
+  }
+};
+
 struct OrderGlobalInitsOrStermFinalizers {
   unsigned int priority;
   unsigned int lex_order;

clang/lib/CodeGen/CodeGenPGO.cpp

@@ -164,7 +164,7 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
   /// The function hash.
   PGOHash Hash;
   /// The map of statements to counters.
-  llvm::DenseMap<const Stmt *, unsigned> &CounterMap;
+  llvm::DenseMap<const Stmt *, CounterPair> &CounterMap;
   /// The state of MC/DC Coverage in this function.
   MCDC::State &MCDCState;
   /// Maximum number of supported MC/DC conditions in a boolean expression.
@@ -175,7 +175,7 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
   DiagnosticsEngine &Diag;
 
   MapRegionCounters(PGOHashVersion HashVersion, uint64_t ProfileVersion,
-                    llvm::DenseMap<const Stmt *, unsigned> &CounterMap,
+                    llvm::DenseMap<const Stmt *, CounterPair> &CounterMap,
                     MCDC::State &MCDCState, unsigned MCDCMaxCond,
                     DiagnosticsEngine &Diag)
       : NextCounter(0), Hash(HashVersion), CounterMap(CounterMap),
@@ -1084,7 +1084,7 @@ void CodeGenPGO::mapRegionCounters(const Decl *D) {
       (CGM.getCodeGenOpts().MCDCCoverage ? CGM.getCodeGenOpts().MCDCMaxConds
                                          : 0);
 
-  RegionCounterMap.reset(new llvm::DenseMap<const Stmt *, unsigned>);
+  RegionCounterMap.reset(new llvm::DenseMap<const Stmt *, CounterPair>);
   RegionMCDCState.reset(new MCDC::State);
   MapRegionCounters Walker(HashVersion, ProfileVersion, *RegionCounterMap,
                            *RegionMCDCState, MCDCMaxConditions, CGM.getDiags());
@@ -1186,12 +1186,18 @@ CodeGenPGO::applyFunctionAttributes(llvm::IndexedInstrProfReader *PGOReader,
   Fn->setEntryCount(FunctionCount);
 }
 
+std::pair<bool, bool> CodeGenPGO::getIsCounterPair(const Stmt *S) const {
+  if (!RegionCounterMap || RegionCounterMap->count(S) == 0)
+    return {false, false};
+  return (*RegionCounterMap)[S].getIsCounterPair();
+}
+
 void CodeGenPGO::emitCounterSetOrIncrement(CGBuilderTy &Builder, const Stmt *S,
                                            llvm::Value *StepV) {
   if (!RegionCounterMap || !Builder.GetInsertBlock())
     return;
 
-  unsigned Counter = (*RegionCounterMap)[S];
+  unsigned Counter = (*RegionCounterMap)[S].first;
 
   // Make sure that pointer to global is passed in with zero addrspace
   // This is relevant during GPU profiling

clang/lib/CodeGen/CodeGenPGO.h

@@ -35,7 +35,7 @@ class CodeGenPGO {
   std::array <unsigned, llvm::IPVK_Last + 1> NumValueSites;
   unsigned NumRegionCounters;
   uint64_t FunctionHash;
-  std::unique_ptr<llvm::DenseMap<const Stmt *, unsigned>> RegionCounterMap;
+  std::unique_ptr<llvm::DenseMap<const Stmt *, CounterPair>> RegionCounterMap;
   std::unique_ptr<llvm::DenseMap<const Stmt *, uint64_t>> StmtCountMap;
   std::unique_ptr<llvm::InstrProfRecord> ProfRecord;
   std::unique_ptr<MCDC::State> RegionMCDCState;
@@ -110,6 +110,7 @@ class CodeGenPGO {
   bool canEmitMCDCCoverage(const CGBuilderTy &Builder);
 
 public:
+  std::pair<bool, bool> getIsCounterPair(const Stmt *S) const;
   void emitCounterSetOrIncrement(CGBuilderTy &Builder, const Stmt *S,
                                  llvm::Value *StepV);
   void emitMCDCTestVectorBitmapUpdate(CGBuilderTy &Builder, const Expr *S,
@@ -122,6 +123,18 @@ class CodeGenPGO {
                                 Address MCDCCondBitmapAddr, llvm::Value *Val,
                                 CodeGenFunction &CGF);
 
+  void markStmtAsUsed(bool Skipped, const Stmt *S) {
+    // Do nothing.
+  }
+
+  void markStmtMaybeUsed(const Stmt *S) {
+    // Do nothing.
+  }
+
+  void verifyCounterMap() {
+    // Do nothing.
+  }
+
   /// Return the region count for the counter at the given index.
   uint64_t getRegionCount(const Stmt *S) {
     if (!RegionCounterMap)
@@ -130,7 +143,7 @@ class CodeGenPGO {
       return 0;
     // With profiles from a differing version of clang we can have mismatched
     // decl counts. Don't crash in such a case.
-    auto Index = (*RegionCounterMap)[S];
+    auto Index = (*RegionCounterMap)[S].first;
     if (Index >= RegionCounts.size())
       return 0;
     return RegionCounts[Index];

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -885,7 +885,7 @@ struct CounterCoverageMappingBuilder
     : public CoverageMappingBuilder,
       public ConstStmtVisitor<CounterCoverageMappingBuilder> {
   /// The map of statements to count values.
-  llvm::DenseMap<const Stmt *, unsigned> &CounterMap;
+  llvm::DenseMap<const Stmt *, CounterPair> &CounterMap;
 
   MCDC::State &MCDCState;
 
@@ -938,7 +938,7 @@ struct CounterCoverageMappingBuilder
   ///
   /// This should only be called on statements that have a dedicated counter.
   Counter getRegionCounter(const Stmt *S) {
-    return Counter::getCounter(CounterMap[S]);
+    return Counter::getCounter(CounterMap[S].first);
   }
 
   /// Push a region onto the stack.
@@ -1421,7 +1421,7 @@ struct CounterCoverageMappingBuilder
 
   CounterCoverageMappingBuilder(
       CoverageMappingModuleGen &CVM,
-      llvm::DenseMap<const Stmt *, unsigned> &CounterMap,
+      llvm::DenseMap<const Stmt *, CounterPair> &CounterMap,
       MCDC::State &MCDCState, SourceManager &SM, const LangOptions &LangOpts)
       : CoverageMappingBuilder(CVM, SM, LangOpts), CounterMap(CounterMap),
         MCDCState(MCDCState), MCDCBuilder(CVM.getCodeGenModule(), MCDCState) {}