[Coverage] getMaxBitmapSize: Scan `max(BitmapIdx)` instead of the last `Decision` (#78963)

In `CoverageMapping.cpp:getMaxBitmapSize()`, 
this assumed that the last `Decision` has the maxmum `BitmapIdx`.

Let it scan `max(BitmapIdx)`.

Note that `<=` is used insted of `<`, because `BitmapIdx == 0` is valid
and `MaxBitmapID` is `unsigned`. `BitmapIdx` is unique in the record.

Fixes #78922

1 files changed, 6 insertions, 5 deletions

diff --git a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
index 9c95cd5..da8e1d8 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
@@ -229,6 +229,7 @@ Expected<BitVector> CounterMappingContext::evaluateBitmap(
   unsigned SizeInBits = llvm::alignTo(uint64_t(1) << NC, CHAR_BIT);
   unsigned SizeInBytes = SizeInBits / CHAR_BIT;
 
+  assert(ID + SizeInBytes <= BitmapBytes.size() && "BitmapBytes overrun");
   ArrayRef<uint8_t> Bytes(&BitmapBytes[ID], SizeInBytes);
 
   // Mask each bitmap byte into the BitVector. Go in reverse so that the
@@ -568,14 +569,14 @@ static unsigned getMaxBitmapSize(const CounterMappingContext &Ctx,
                                  const CoverageMappingRecord &Record) {
   unsigned MaxBitmapID = 0;
   unsigned NumConditions = 0;
-  // The last DecisionRegion has the highest bitmap byte index used in the
-  // function, which when combined with its number of conditions, yields the
-  // full bitmap size.
+  // Scan max(BitmapIdx).
+  // Note that `<=` is used insted of `<`, because `BitmapIdx == 0` is valid
+  // and `MaxBitmapID is `unsigned`. `BitmapIdx` is unique in the record.
   for (const auto &Region : reverse(Record.MappingRegions)) {
-    if (Region.Kind == CounterMappingRegion::MCDCDecisionRegion) {
+    if (Region.Kind == CounterMappingRegion::MCDCDecisionRegion &&
+        MaxBitmapID <= Region.MCDCParams.BitmapIdx) {
       MaxBitmapID = Region.MCDCParams.BitmapIdx;
       NumConditions = Region.MCDCParams.NumConditions;
-      break;
     }
   }
   unsigned SizeInBits = llvm::alignTo(uint64_t(1) << NumConditions, CHAR_BIT);