[clang][Tooling] Try to avoid file system access if there is no recor…

…d for the file in compile_commads.json

Summary:
If there is no record in compile_commands.json, we try to find suitable record with `MatchTrie.findEquivalent()` call.
This is very expensive operation with a lot of `llvm::sys::fs::equivalent()` calls in some cases.

This patch disables file symlinks for performance reasons.

Example scenario without this patch:
- compile_commands.json generated at clangd build (contains ~3000 files).
- it tooks more than 1 second to get compile command for newly created file in the root folder of LLVM project.
- we wait for 1 second every time when clangd requests compile command for this file (at file change).

Reviewers: sammccall, kadircet, hokein

Reviewed By: sammccall

Subscribers: chandlerc, djasper, klimek, ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83621

clang/lib/Tooling/FileMatchTrie.cpp

@@ -105,8 +105,13 @@ class FileMatchTrieNode {
                            StringRef FileName,
                            bool &IsAmbiguous,
                            unsigned ConsumedLength = 0) const {
+    // Note: we support only directory symlinks for performance reasons.
     if (Children.empty()) {
-      if (Comparator.equivalent(StringRef(Path), FileName))
+      // As far as we do not support file symlinks, compare
+      // basenames here to avoid request to file system.
+      if (llvm::sys::path::filename(Path) ==
+              llvm::sys::path::filename(FileName) &&
+          Comparator.equivalent(StringRef(Path), FileName))
         return StringRef(Path);
       return {};
     }
@@ -121,6 +126,13 @@ class FileMatchTrieNode {
       if (!Result.empty() || IsAmbiguous)
         return Result;
     }
+
+    // If `ConsumedLength` is zero, this is the root and we have no filename
+    // match. Give up in this case, we don't try to find symlinks with
+    // different names.
+    if (ConsumedLength == 0)
+      return {};
+
     std::vector<StringRef> AllChildren;
     getAll(AllChildren, MatchingChild);
     StringRef Result;

clang/unittests/Tooling/CompilationDatabaseTest.cpp

@@ -281,6 +281,15 @@ TEST_F(FileMatchTrieTest, CannotResolveRelativePath) {
   EXPECT_EQ("Cannot resolve relative paths", Error);
 }
 
+TEST_F(FileMatchTrieTest, SingleFile) {
+  Trie.insert("/root/RootFile.cc");
+  EXPECT_EQ("", find("/root/rootfile.cc"));
+  // Add subpath to avoid `if (Children.empty())` special case
+  // which we hit at previous `find()`.
+  Trie.insert("/root/otherpath/OtherFile.cc");
+  EXPECT_EQ("", find("/root/rootfile.cc"));
+}
+
 TEST(findCompileArgsInJsonDatabase, FindsNothingIfEmpty) {
   std::string ErrorMessage;
   CompileCommand NotFound = findCompileArgsInJsonDatabase(