Apply a short-vector optimization to EvalString.

This very often holds only a single RAW token, so we do not
need to allocate elements on an std::vector for it in the
common case.

For a no-op build of Chromium (Linux, Zen 2),
this reduces time spent from 5.48 to 5.14 seconds.

Note that this opens up for a potential optimization where
EvalString::Evaluate() could just return a StringPiece, without
making a std::string out of it (which requires allocation; this is
about 5% of remaining runtime). However, this would also require
that CanonicalizePath() somehow learned to work with StringPiece
(presumably allocating a new StringPiece if and only if changes
were needed).

src/eval_env.cc

@@ -99,6 +99,10 @@ string BindingEnv::LookupWithFallback(const string& var,
 }
 
 string EvalString::Evaluate(Env* env) const {
+  if (parsed_.empty()) {
+    return single_token_.AsString();
+  }
+
   string result;
   for (TokenList::const_iterator i = parsed_.begin(); i != parsed_.end(); ++i) {
     if (i->second == RAW)
@@ -112,34 +116,64 @@ string EvalString::Evaluate(Env* env) const {
 }
 
 void EvalString::AddText(StringPiece text) {
-  parsed_.push_back(std::make_pair(text, RAW));
+  if (parsed_.empty()) {
+    if (!single_token_.empty()) {
+      // Going from one to two tokens, so we can no longer apply
+      // our single_token_ optimization and need to push everything
+      // onto the vector.
+      parsed_.push_back(std::make_pair(single_token_, RAW));
+    } else {
+      // This is the first (nonempty) token, so we don't need to
+      // allocate anything on the vector (yet).
+      single_token_ = text;
+      return;
+    }
+  }
+  parsed_.push_back(make_pair(text, RAW));
 }
+
 void EvalString::AddSpecial(StringPiece text) {
+  if (parsed_.empty() && !single_token_.empty()) {
+    // Going from one to two tokens, so we can no longer apply
+    // our single_token_ optimization and need to push everything
+    // onto the vector.
+    parsed_.push_back(std::make_pair(single_token_, RAW));
+  }
   parsed_.push_back(std::make_pair(text, SPECIAL));
 }
 
 string EvalString::Serialize() const {
   string result;
-  for (const auto& pair : parsed_) {
+  if (parsed_.empty() && !single_token_.empty()) {
     result.append("[");
-    if (pair.second == SPECIAL)
-      result.append("$");
-    result.append(pair.first.begin(), pair.first.end());
+    result.append(single_token_.begin(), single_token_.end());
     result.append("]");
+  } else {
+    for (const auto& pair : parsed_) {
+      result.append("[");
+      if (pair.second == SPECIAL)
+        result.append("$");
+      result.append(pair.first.begin(), pair.first.end());
+      result.append("]");
+    }
   }
   return result;
 }
 
 string EvalString::Unparse() const {
   string result;
-  for (TokenList::const_iterator i = parsed_.begin();
-       i != parsed_.end(); ++i) {
-    bool special = (i->second == SPECIAL);
-    if (special)
-      result.append("${");
-    result.append(i->first.begin(), i->first.end());
-    if (special)
-      result.append("}");
+  if (parsed_.empty() && !single_token_.empty()) {
+    result.append(single_token_.begin(), single_token_.end());
+  } else {
+    for (TokenList::const_iterator i = parsed_.begin();
+         i != parsed_.end(); ++i) {
+      bool special = (i->second == SPECIAL);
+      if (special)
+        result.append("${");
+      result.append(i->first.begin(), i->first.end());
+      if (special)
+        result.append("}");
+    }
   }
   return result;
 }

src/eval_env.h

@@ -39,8 +39,8 @@ struct EvalString {
   /// @return The string with variables not expanded.
   std::string Unparse() const;
 
-  void Clear() { parsed_.clear(); }
-  bool empty() const { return parsed_.empty(); }
+  void Clear() { parsed_.clear(); single_token_ = StringPiece(); }
+  bool empty() const { return parsed_.empty() && single_token_.empty(); }
 
   void AddText(StringPiece text);
   void AddSpecial(StringPiece text);
@@ -53,6 +53,12 @@ struct EvalString {
   enum TokenType { RAW, SPECIAL };
   typedef std::vector<std::pair<StringPiece, TokenType> > TokenList;
   TokenList parsed_;
+
+  // If we hold only a single RAW token, then we keep it here instead of
+  // pushing it on TokenList. This saves a bunch of allocations for
+  // what is a common case. If parsed_ is nonempty, then this value
+  // must be ignored.
+  StringPiece single_token_;
 };
 
 /// An invocable build command and associated metadata (description, etc.).

src/string_piece.h

@@ -63,6 +63,10 @@ struct StringPiece {
     return len_;
   }
 
+  size_t empty() const {
+    return len_ == 0;
+  }
+
   const char* str_;
   size_t len_;
 };