[Coverage] Resurrect Branch:FalseCnt in SwitchStmt that was pruned in #112694 (#120418)

I missed that FalseCnt for each Case was used to calculate percentage in
the SwitchStmt. At the moment I resurrect them.

In `!HasDefaultCase`, the pair of Counters shall be `[CaseCountSum,
FalseCnt]`. (Reversal of before #112694)
I think it can be considered as the False count on SwitchStmt.

FalseCnt shall be folded (same as current impl) in the coming
SingleByteCoverage changes, since percentage would not make sense.

1 files changed, 15 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CoverageMappingGen.cpp b/clang/lib/CodeGen/CoverageMappingGen.cpp
index 7248abe..cda218e 100644
--- a/clang/lib/CodeGen/CoverageMappingGen.cpp
+++ b/clang/lib/CodeGen/CoverageMappingGen.cpp
@@ -1146,12 +1146,15 @@ struct CounterCoverageMappingBuilder
 
   /// Create a Branch Region around a SwitchCase for code coverage
   /// and add it to the function's SourceRegions.
-  void createSwitchCaseRegion(const SwitchCase *SC, Counter TrueCnt) {
+  /// Returns Counter that corresponds to SC.
+  Counter createSwitchCaseRegion(const SwitchCase *SC, Counter ParentCount) {
     // Push region onto RegionStack but immediately pop it (which adds it to
     // the function's SourceRegions) because it doesn't apply to any other
     // source other than the SwitchCase.
+    Counter TrueCnt = getRegionCounter(SC);
     popRegions(pushRegion(TrueCnt, getStart(SC), SC->getColonLoc(),
-                          Counter::getZero()));
+                          subtractCounters(ParentCount, TrueCnt)));
+    return TrueCnt;
   }
 
   /// Check whether a region with bounds \c StartLoc and \c EndLoc
@@ -1863,16 +1866,22 @@ struct CounterCoverageMappingBuilder
     const SwitchCase *Case = S->getSwitchCaseList();
     for (; Case; Case = Case->getNextSwitchCase()) {
       HasDefaultCase = HasDefaultCase || isa<DefaultStmt>(Case);
-      auto CaseCount = getRegionCounter(Case);
+      auto CaseCount = createSwitchCaseRegion(Case, ParentCount);
       CaseCountSum = addCounters(CaseCountSum, CaseCount, /*Simplify=*/false);
-      createSwitchCaseRegion(Case, CaseCount);
     }
     // If no explicit default case exists, create a branch region to represent
     // the hidden branch, which will be added later by the CodeGen. This region
     // will be associated with the switch statement's condition.
     if (!HasDefaultCase) {
-      Counter DefaultCount = subtractCounters(ParentCount, CaseCountSum);
-      createBranchRegion(S->getCond(), Counter::getZero(), DefaultCount);
+      // Simplify is skipped while building the counters above: it can get
+      // really slow on top of switches with thousands of cases. Instead,
+      // trigger simplification by adding zero to the last counter.
+      CaseCountSum =
+          addCounters(CaseCountSum, Counter::getZero(), /*Simplify=*/true);
+
+      // This is considered as the False count on SwitchStmt.
+      Counter SwitchFalse = subtractCounters(ParentCount, CaseCountSum);
+      createBranchRegion(S->getCond(), CaseCountSum, SwitchFalse);
     }
   }