Allocate per-edge BindingEnvs lazily.

In chrome, only 2000 of 22000 build edges have bindings.  A BindingEnv is
64 bytes, so allocating these only when needed saves a bit over 1 MB of
memory.  Since env chains are shorter for lookups, builds also become a
tiny bit faster.

1 files changed, 5 insertions, 4 deletions

diff --git a/src/manifest_parser.cc b/src/manifest_parser.cc
index 20be7f3..6fa4f7c 100644
--- a/src/manifest_parser.cc
+++ b/src/manifest_parser.cc
@@ -296,16 +296,17 @@ bool ManifestParser::ParseEdge(string* err) {
   if (!ExpectToken(Lexer::NEWLINE, err))
     return false;
 
-  // XXX scoped_ptr to handle error case.
-  BindingEnv* env = new BindingEnv(env_);
-
-  while (lexer_.PeekToken(Lexer::INDENT)) {
+  // Bindings on edges are rare, so allocate per-edge envs only when needed.
+  bool hasIdent = lexer_.PeekToken(Lexer::INDENT);
+  BindingEnv* env = hasIdent ? new BindingEnv(env_) : env_;
+  while (hasIdent) {
     string key;
     EvalString val;
     if (!ParseLet(&key, &val, err))
       return false;
 
     env->AddBinding(key, val.Evaluate(env_));
+    hasIdent = lexer_.PeekToken(Lexer::INDENT);
   }
 
   Edge* edge = state_->AddEdge(rule);