1 files changed, 39 insertions, 0 deletions
diff --git a/deps/v8/src/debug/debug-coverage.cc b/deps/v8/src/debug/debug-coverage.cc
index a71c9e572b..f8b716f7c9 100644
--- a/deps/v8/src/debug/debug-coverage.cc
+++ b/deps/v8/src/debug/debug-coverage.cc
@@ -171,6 +171,12 @@ class CoverageBlockIterator final {
     return function_->blocks[read_index_ + 1];
   }
 
+  CoverageBlock& GetPreviousBlock() {
+    DCHECK(IsActive());
+    DCHECK_GT(read_index_, 0);
+    return function_->blocks[read_index_ - 1];
+  }
+
   CoverageBlock& GetParent() {
     DCHECK(IsActive());
     return nesting_stack_.back();
@@ -325,6 +331,30 @@ void MergeNestedRanges(CoverageFunction* function) {
   }
 }
 
+void FilterAliasedSingletons(CoverageFunction* function) {
+  CoverageBlockIterator iter(function);
+
+  iter.Next();  // Advance once since we reference the previous block later.
+
+  while (iter.Next()) {
+    CoverageBlock& previous_block = iter.GetPreviousBlock();
+    CoverageBlock& block = iter.GetBlock();
+
+    bool is_singleton = block.end == kNoSourcePosition;
+    bool aliases_start = block.start == previous_block.start;
+
+    if (is_singleton && aliases_start) {
+      // The previous block must have a full range since duplicate singletons
+      // have already been merged.
+      DCHECK_NE(previous_block.end, kNoSourcePosition);
+      // Likewise, the next block must have another start position since
+      // singletons are sorted to the end.
+      DCHECK_IMPLIES(iter.HasNext(), iter.GetNextBlock().start != block.start);
+      iter.DeleteBlock();
+    }
+  }
+}
+
 void FilterUncoveredRanges(CoverageFunction* function) {
   CoverageBlockIterator iter(function);
 
@@ -397,6 +427,15 @@ void CollectBlockCoverage(CoverageFunction* function, SharedFunctionInfo* info,
   // Remove duplicate singleton ranges, keeping the max count.
   MergeDuplicateSingletons(function);
 
+  // Remove singleton ranges with the same start position as a full range and
+  // throw away their counts.
+  // Singleton ranges are only intended to split existing full ranges and should
+  // never expand into a full range. Consider 'if (cond) { ... } else { ... }'
+  // as a problematic example; if the then-block produces a continuation
+  // singleton, it would incorrectly expand into the else range.
+  // For more context, see https://crbug.com/v8/8237.
+  FilterAliasedSingletons(function);
+
   // Rewrite all singletons (created e.g. by continuations and unconditional
   // control flow) to ranges.
   RewritePositionSingletonsToRanges(function);