[base::FilePath] Optimize FilePath::ReferencesParent

base/files/file_path.cc

Index: base/files/file_path.cc
diff --git a/base/files/file_path.cc b/base/files/file_path.cc
index 0b06493c8831f485a06e169167dfb802fc188dd5..cff862ae19da14bd59cffe64724027a3f2c16ed6 100644
--- a/base/files/file_path.cc
+++ b/base/files/file_path.cc
@@ -560,6 +560,12 @@ FilePath FilePath::StripTrailingSeparators() const {
 }
 
 bool FilePath::ReferencesParent() const {
+  if (path_.find(kParentDirectory) == StringType::npos) {
+    // GetComponents is quite expensive, so avoid calling it in the majority
+    // of cases where there isn't a kParentDirectory anywhere in the path.
+    return false;
+  }
+
   std::vector<StringType> components;

[danakj, 2016/11/11 19:17:40]

This is great!

Question tho: We should probably use StringPiece instead StringType here too, so
we don't have to do all this mallocing and copying to do GetComponents. Would
that give the same wins in your case but not need an extra walk over the string?

[jkarlin, 2016/11/11 19:38:24]

Are you suggesting that GetComponents should take a vector<StringPieceType> and
fill it accordingly? That would be better, but it's non-trivial as GetComponents
depends on BaseName() and DirName() which both return FilePath types.

We could also rewrite this function to just walk down the path scanning for
'..'s and ignoring the whitespace on either side. That would result in a fairly
fast function, but I bet you that average case this function is still faster. I
don't think we're going to see '..' in paths very often.

[danakj, 2016/11/11 19:57:27]

Ah ok, it's not trivial at all np. This LGTM!
For most users I think you're right. Maybe someone will have a .. in their
XDG_CONFIG_PATH or something and then they'll hit it all the time. But it seems
corner-casey.

   GetComponents(&components);