[Coverage] Use gap regions to select better line exec counts

After clang started emitting deferred regions (r312818), llvm-cov has
had a hard time picking reasonable line execuction counts. There have
been one or two generic improvements in this area (e.g r310012), but
line counts can still report coverage for whitespace instead of code
(llvm.org/PR34612).

To fix the problem:

 * Introduce a new region kind so that frontends can explicitly label
   gap areas.

   This is done by changing the encoding of the columnEnd field of
   MappingRegion. This doesn't substantially increase binary size, and
   makes it easy to maintain backwards-compatibility.

 * Don't set the line count to a count from a gap area, unless the count
   comes from a wrapped segment.

 * Don't highlight gap areas as uncovered.

Fixes llvm.org/PR34612.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313597 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 7 deletions

diff --git a/tools/llvm-cov/SourceCoverageView.cpp b/tools/llvm-cov/SourceCoverageView.cpp
index 79c2c69571c..1965595cdb7 100644
--- a/tools/llvm-cov/SourceCoverageView.cpp
+++ b/tools/llvm-cov/SourceCoverageView.cpp
@@ -89,7 +89,7 @@ LineCoverageStats::LineCoverageStats(
   // Find the minimum number of regions which start in this line.
   unsigned MinRegionCount = 0;
   auto isStartOfRegion = [](const coverage::CoverageSegment *S) {
-    return S->HasCount && S->IsRegionEntry;
+    return !S->IsGapRegion && S->HasCount && S->IsRegionEntry;
   };
   for (unsigned I = 0; I < LineSegments.size() && MinRegionCount < 2; ++I)
     if (isStartOfRegion(LineSegments[I]))
@@ -112,16 +112,19 @@ LineCoverageStats::LineCoverageStats(
   // avoid erroneously using the wrapped count, and to avoid picking region
   // counts which come from deferred regions.
   if (LineSegments.size() > 1) {
-    for (unsigned I = 0; I < LineSegments.size() - 1; ++I)
-      ExecutionCount = std::max(ExecutionCount, LineSegments[I]->Count);
+    for (unsigned I = 0; I < LineSegments.size() - 1; ++I) {
+      if (!LineSegments[I]->IsGapRegion)
+        ExecutionCount = std::max(ExecutionCount, LineSegments[I]->Count);
+    }
     return;
   }
 
-  // Just pick the maximum count.
-  if (WrappedSegment && WrappedSegment->HasCount)
+  // If a non-gap region starts here, use its count. Otherwise use the wrapped
+  // count.
+  if (MinRegionCount == 1)
+    ExecutionCount = LineSegments[0]->Count;
+  else
     ExecutionCount = WrappedSegment->Count;
-  if (!LineSegments.empty())
-    ExecutionCount = std::max(ExecutionCount, LineSegments[0]->Count);
 }
 
 unsigned SourceCoverageView::getFirstUncoveredLineNo() {