1 files changed, 44 insertions, 14 deletions

diff --git a/llvm/tools/llvm-cov/CoverageReport.cpp b/llvm/tools/llvm-cov/CoverageReport.cpp
index 78333d09ad8..6ae64ba8a3a 100644
--- a/llvm/tools/llvm-cov/CoverageReport.cpp
+++ b/llvm/tools/llvm-cov/CoverageReport.cpp
@@ -118,19 +118,51 @@ raw_ostream::Colors determineCoveragePercentageColor(const T &Info) {
                                           : raw_ostream::RED;
 }
 
-/// \brief Determine the length of the longest common prefix of the strings in
-/// \p Strings.
-unsigned getLongestCommonPrefixLen(ArrayRef<std::string> Strings) {
-  unsigned LCP = Strings[0].size();
-  for (unsigned I = 1, E = Strings.size(); LCP > 0 && I < E; ++I) {
-    unsigned Cursor;
-    StringRef S = Strings[I];
-    for (Cursor = 0; Cursor < LCP && Cursor < S.size(); ++Cursor)
-      if (Strings[0][Cursor] != S[Cursor])
+/// \brief Get the number of redundant path components in each path in \p Paths.
+unsigned getNumRedundantPathComponents(ArrayRef<std::string> Paths) {
+  // To start, set the number of redundant path components to the maximum
+  // possible value.
+  SmallVector<StringRef, 8> FirstPathComponents{sys::path::begin(Paths[0]),
+                                                sys::path::end(Paths[0])};
+  unsigned NumRedundant = FirstPathComponents.size();
+
+  for (unsigned I = 1, E = Paths.size(); NumRedundant > 0 && I < E; ++I) {
+    StringRef Path = Paths[I];
+    for (const auto &Component :
+         enumerate(make_range(sys::path::begin(Path), sys::path::end(Path)))) {
+      // Do not increase the number of redundant components: that would remove
+      // useful parts of already-visited paths.
+      if (Component.Index >= NumRedundant)
         break;
-    LCP = std::min(LCP, Cursor);
+
+      // Lower the number of redundant components when there's a mismatch
+      // between the first path, and the path under consideration.
+      if (FirstPathComponents[Component.Index] != Component.Value) {
+        NumRedundant = Component.Index;
+        break;
+      }
+    }
+  }
+
+  return NumRedundant;
+}
+
+/// \brief Determine the length of the longest redundant prefix of the paths in
+/// \p Paths.
+unsigned getRedundantPrefixLen(ArrayRef<std::string> Paths) {
+  // If there's at most one path, no path components are redundant.
+  if (Paths.size() <= 1)
+    return 0;
+
+  unsigned PrefixLen = 0;
+  unsigned NumRedundant = getNumRedundantPathComponents(Paths);
+  auto Component = sys::path::begin(Paths[0]);
+  for (unsigned I = 0; I < NumRedundant; ++I) {
+    auto LastComponent = Component;
+    ++Component;
+    PrefixLen += Component - LastComponent;
   }
-  return LCP;
+  return PrefixLen;
 }
 
 } // end anonymous namespace
@@ -280,9 +312,7 @@ CoverageReport::prepareFileReports(const coverage::CoverageMapping &Coverage,
                                    FileCoverageSummary &Totals,
                                    ArrayRef<std::string> Files) {
   std::vector<FileCoverageSummary> FileReports;
-  unsigned LCP = 0;
-  if (Files.size() > 1)
-    LCP = getLongestCommonPrefixLen(Files);
+  unsigned LCP = getRedundantPrefixLen(Files);
 
   for (StringRef Filename : Files) {
     FileCoverageSummary Summary(Filename.drop_front(LCP));