diff --git a/clang-tools-extra/clangd/ConfigProvider.cpp b/clang-tools-extra/clangd/ConfigProvider.cpp
--- a/clang-tools-extra/clangd/ConfigProvider.cpp
+++ b/clang-tools-extra/clangd/ConfigProvider.cpp
@@ -10,8 +10,10 @@
 #include "Config.h"
 #include "ConfigFragment.h"
 #include "support/FileCache.h"
+#include "support/Path.h"
 #include "support/ThreadsafeFS.h"
 #include "support/Trace.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
@@ -96,16 +98,10 @@
         return {};
 
       // Compute absolute paths to all ancestors (substrings of P.Path).
-      llvm::StringRef Parent = path::parent_path(P.Path);
       llvm::SmallVector<llvm::StringRef, 8> Ancestors;
-      for (auto I = path::begin(Parent, path::Style::posix),
-                E = path::end(Parent);
-           I != E; ++I) {
-        // Avoid weird non-substring cases like phantom "." components.
-        // In practice, Component is a substring for all "normal" ancestors.
-        if (I->end() < Parent.begin() || I->end() > Parent.end())
-          continue;
-        Ancestors.emplace_back(Parent.begin(), I->end() - Parent.begin());
+      for (auto Ancestor = absoluteParent(P.Path); !Ancestor.empty();
+           Ancestor = absoluteParent(Ancestor)) {
+        Ancestors.emplace_back(Ancestor);
       }
       // Ensure corresponding cache entries exist in the map.
       llvm::SmallVector<FileConfigCache *, 8> Caches;
@@ -127,7 +123,7 @@
       // Finally query each individual file.
       // This will take a (per-file) lock for each file that actually exists.
       std::vector<CompiledFragment> Result;
-      for (FileConfigCache *Cache : Caches)
+      for (FileConfigCache *Cache : llvm::reverse(Caches))
         Cache->get(FS, DC, P.FreshTime, Result);
       return Result;
     };
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -43,21 +43,6 @@
 namespace clangd {
 namespace {
 
-// Variant of parent_path that operates only on absolute paths.
-PathRef absoluteParent(PathRef Path) {
-  assert(llvm::sys::path::is_absolute(Path));
-#if defined(_WIN32)
-  // llvm::sys says "C:\" is absolute, and its parent is "C:" which is relative.
-  // This unhelpful behavior seems to have been inherited from boost.
-  if (llvm::sys::path::relative_path(Path).empty()) {
-    return PathRef();
-  }
-#endif
-  PathRef Result = llvm::sys::path::parent_path(Path);
-  assert(Result.empty() || llvm::sys::path::is_absolute(Result));
-  return Result;
-}
-
 // Runs the given action on all parent directories of filename, starting from
 // deepest directory and going up to root. Stops whenever action succeeds.
 void actOnAllParentDirectories(PathRef FileName,
diff --git a/clang-tools-extra/clangd/TidyProvider.cpp b/clang-tools-extra/clangd/TidyProvider.cpp
--- a/clang-tools-extra/clangd/TidyProvider.cpp
+++ b/clang-tools-extra/clangd/TidyProvider.cpp
@@ -11,6 +11,7 @@
 #include "Config.h"
 #include "support/FileCache.h"
 #include "support/Logger.h"
+#include "support/Path.h"
 #include "support/ThreadsafeFS.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
@@ -102,23 +103,12 @@
 
     // Compute absolute paths to all ancestors (substrings of P.Path).
     // Ensure cache entries for each ancestor exist in the map.
-    llvm::StringRef Parent = path::parent_path(AbsPath);
     llvm::SmallVector<DotClangTidyCache *> Caches;
     {
       std::lock_guard<std::mutex> Lock(Mu);
-      for (auto I = path::rbegin(Parent), E = path::rend(Parent); I != E; ++I) {
-        assert(I->end() >= Parent.begin() && I->end() <= Parent.end() &&
-               "Canonical path components should be substrings");
-        llvm::StringRef Ancestor(Parent.begin(), I->end() - Parent.begin());
-#ifdef _WIN32
-        // C:\ is an ancestor, but skip its (relative!) parent C:.
-        if (Ancestor.size() == 2 && Ancestor.back() == ':')
-          continue;
-#endif
-        assert(path::is_absolute(Ancestor));
-
+      for (auto Ancestor = absoluteParent(AbsPath); !Ancestor.empty();
+           Ancestor = absoluteParent(Ancestor)) {
         auto It = Cache.find(Ancestor);
-
         // Assemble the actual config file path only if needed.
         if (It == Cache.end()) {
           llvm::SmallString<256> ConfigPath = Ancestor;
diff --git a/clang-tools-extra/clangd/support/Path.h b/clang-tools-extra/clangd/support/Path.h
--- a/clang-tools-extra/clangd/support/Path.h
+++ b/clang-tools-extra/clangd/support/Path.h
@@ -28,6 +28,9 @@
 std::string maybeCaseFoldPath(PathRef Path);
 bool pathEqual(PathRef, PathRef);
 
+/// Variant of parent_path that operates only on absolute paths.
+/// Unlike parent_path doesn't consider C: a parent of C:\.
+PathRef absoluteParent(PathRef Path);
 } // namespace clangd
 } // namespace clang
 
diff --git a/clang-tools-extra/clangd/support/Path.cpp b/clang-tools-extra/clangd/support/Path.cpp
--- a/clang-tools-extra/clangd/support/Path.cpp
+++ b/clang-tools-extra/clangd/support/Path.cpp
@@ -7,9 +7,25 @@
 //===----------------------------------------------------------------------===//
 
 #include "support/Path.h"
+#include "llvm/Support/Path.h"
+
 namespace clang {
 namespace clangd {
 
+PathRef absoluteParent(PathRef Path) {
+  assert(llvm::sys::path::is_absolute(Path));
+#if defined(_WIN32)
+  // llvm::sys says "C:\" is absolute, and its parent is "C:" which is relative.
+  // This unhelpful behavior seems to have been inherited from boost.
+  if (llvm::sys::path::relative_path(Path).empty()) {
+    return PathRef();
+  }
+#endif
+  PathRef Result = llvm::sys::path::parent_path(Path);
+  assert(Result.empty() || llvm::sys::path::is_absolute(Result));
+  return Result;
+}
+
 std::string maybeCaseFoldPath(PathRef Path) {
 #if defined(_WIN32) || defined(__APPLE__)
   return Path.lower();