Index: llvm/tools/dsymutil/DwarfLinker.cpp
===================================================================
--- llvm/tools/dsymutil/DwarfLinker.cpp
+++ llvm/tools/dsymutil/DwarfLinker.cpp
@@ -100,6 +100,31 @@
 
 namespace {
 
+/// Small helper that resolves and caches file paths.
+class CachedPathResolver {
+public:
+  /// Resolve a path and cache its result.
+  std::string resolve(std::string Path) {
+    StringRef FileName = sys::path::filename(Path);
+    SmallString<256> ParentPath = sys::path::parent_path(Path);
+
+    // If the ParentPath has not yet been resolved, resolve and cache it for
+    // future look-ups.
+    if(!ResolvedPaths.count(ParentPath)) {
+      SmallString<256> RealPath;
+      sys::fs::real_path(ParentPath, RealPath);
+      ResolvedPaths.insert({ParentPath, RealPath});
+    }
+
+    // Join the file name again with the resolved path.
+    SmallString<256> ResolvedPath = ResolvedPaths[ParentPath];
+    sys::path::append(ResolvedPath, FileName);
+    return StringRef(ResolvedPath);
+  }
+private:
+  StringMap<SmallString<256>> ResolvedPaths;
+};
+
 /// Retrieve the section named \a SecName in \a Obj.
 ///
 /// To accommodate for platform discrepancies, the name passed should be
@@ -234,6 +259,8 @@
   DeclContext Root;
   DeclContext::Map Contexts;
 
+  /// Cache resolved paths from the line table.
+  CachedPathResolver PathResolver;
 public:
   /// Get the child of \a Context described by \a DIE in \a Unit. The
   /// required strings will be interned in \a StringPool.
@@ -428,21 +455,6 @@
   const std::vector<AccelInfo> &getNamespaces() const { return Namespaces; }
   const std::vector<AccelInfo> &getObjC() const { return ObjC; }
 
-  /// Get the full path for file \a FileNum in the line table
-  StringRef getResolvedPath(unsigned FileNum) {
-    if (FileNum >= ResolvedPaths.size())
-      return StringRef();
-    return ResolvedPaths[FileNum];
-  }
-
-  /// Set the fully resolved path for the line-table's file \a FileNum
-  /// to \a Path.
-  void setResolvedPath(unsigned FileNum, StringRef Path) {
-    if (ResolvedPaths.size() <= FileNum)
-      ResolvedPaths.resize(FileNum + 1);
-    ResolvedPaths[FileNum] = Path;
-  }
-
 private:
   DWARFUnit &OrigUnit;
   unsigned ID;
@@ -493,12 +505,6 @@
   std::vector<AccelInfo> ObjC;
   /// @}
 
-  /// Cached resolved paths from the line table.
-  /// Note, the StringRefs here point in to the intern (uniquing) string pool.
-  /// This means that a StringRef returned here doesn't need to then be uniqued
-  /// for the purposes of getting a unique address for each string.
-  std::vector<StringRef> ResolvedPaths;
-
   /// Is this unit subject to the ODR rule?
   bool HasODR;
 
@@ -1380,7 +1386,6 @@
 
   void reportWarning(const Twine &Warning,
                      const DWARFDie *DIE = nullptr) const;
-
 private:
   /// Called at the start of a debug object link.
   void startDebugObject(DWARFContext &, DebugMapObject &);
@@ -1763,6 +1768,7 @@
 
   bool ModuleCacheHintDisplayed = false;
   bool ArchiveHintDisplayed = false;
+
 };
 
 } // end anonymous namespace
@@ -1933,32 +1939,21 @@
           if (!Name && Tag == dwarf::DW_TAG_namespace)
             FileNum = 1;
 
-          // FIXME: Passing U.getOrigUnit().getCompilationDir()
-          // instead of "" would allow more uniquing, but for now, do
-          // it this way to match dsymutil-classic.
           if (LT->hasFileAtIndex(FileNum)) {
             Line = dwarf::toUnsigned(DIE.find(dwarf::DW_AT_decl_line), 0);
-            // Cache the resolved paths, because calling realpath is expansive.
-            StringRef ResolvedPath = U.getResolvedPath(FileNum);
-            if (!ResolvedPath.empty()) {
-              FileRef = ResolvedPath;
-            } else {
-              std::string File;
-              bool gotFileName =
-                LT->getFileNameByIndex(FileNum, "",
-                        DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath,
-                        File);
-              (void)gotFileName;
-              assert(gotFileName && "Must get file name from line table");
-#ifdef HAVE_REALPATH
-              char RealPath[PATH_MAX + 1];
-              RealPath[PATH_MAX] = 0;
-              if (::realpath(File.c_str(), RealPath))
-                File = RealPath;
-#endif
-              FileRef = StringPool.internString(File);
-              U.setResolvedPath(FileNum, FileRef);
-            }
+
+            // FIXME: Passing U.getOrigUnit().getCompilationDir()
+            // instead of "" would allow more uniquing, but for now, do
+            // it this way to match dsymutil-classic.
+            std::string File;
+            bool FoundFile =
+              LT->getFileNameByIndex(FileNum, "",
+                      DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath,
+                      File);
+            (void)FoundFile;
+            assert(FoundFile && "Must get file name from line table");
+
+            FileRef = StringPool.internString(PathResolver.resolve(File));
           }
         }
       }