[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.

llvm-svn: 313597

1 files changed, 14 insertions, 2 deletions

diff --git a/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp b/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
index 9bf70c2e3aa..467a36ca748 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp
@@ -216,6 +216,13 @@ Error RawCoverageMappingReader::readMappingRegionsSubArray(
     if (auto Err = readIntMax(ColumnEnd, std::numeric_limits<unsigned>::max()))
       return Err;
     LineStart += LineStartDelta;
+
+    // If the high bit of ColumnEnd is set, this is a gap region.
+    if (ColumnEnd & (1U << 31)) {
+      Kind = CounterMappingRegion::GapRegion;
+      ColumnEnd &= ~(1U << 31);
+    }
+
     // Adjust the column locations for the empty regions that are supposed to
     // cover whole lines. Those regions should be encoded with the
     // column range (1 -> std::numeric_limits<unsigned>::max()), but because
@@ -534,11 +541,16 @@ Expected<std::unique_ptr<CovMapFuncRecordReader>> CovMapFuncRecordReader::get(
     return llvm::make_unique<VersionedCovMapFuncRecordReader<
         CovMapVersion::Version1, IntPtrT, Endian>>(P, R, F);
   case CovMapVersion::Version2:
+  case CovMapVersion::Version3:
     // Decompress the name data.
     if (Error E = P.create(P.getNameData()))
       return std::move(E);
-    return llvm::make_unique<VersionedCovMapFuncRecordReader<
-        CovMapVersion::Version2, IntPtrT, Endian>>(P, R, F);
+    if (Version == CovMapVersion::Version2)
+      return llvm::make_unique<VersionedCovMapFuncRecordReader<
+          CovMapVersion::Version2, IntPtrT, Endian>>(P, R, F);
+    else
+      return llvm::make_unique<VersionedCovMapFuncRecordReader<
+          CovMapVersion::Version3, IntPtrT, Endian>>(P, R, F);
   }
   llvm_unreachable("Unsupported version");
 }