[C++] Add a fast path for interning lhs of assignments

5 files changed, 26 insertions, 1 deletions

diff --git a/eval.cc b/eval.cc
index 3dfc0e6..be4bb34 100644
--- a/eval.cc
+++ b/eval.cc
@@ -96,7 +96,7 @@ Var* Evaluator::EvalRHS(Symbol lhs, Value* rhs_v, StringPiece orig_rhs,
 void Evaluator::EvalAssign(const AssignStmt* stmt) {
   loc_ = stmt->loc();
   last_rule_ = NULL;
-  Symbol lhs = Intern(stmt->lhs->Eval(this));
+  Symbol lhs = stmt->GetLhsSymbol(this);
   if (lhs.empty())
     Error("*** empty variable name.");
   Var* rhs = EvalRHS(lhs, stmt->rhs, stmt->orig_rhs, stmt->op,
diff --git a/expr.cc b/expr.cc
index 3da740e..a629352 100644
--- a/expr.cc
+++ b/expr.cc
@@ -63,6 +63,7 @@ class Literal : public Value {
   }
 
   virtual bool IsLiteral() const override { return true; }
+  virtual StringPiece GetLiteralValueUnsafe() const { return s_; }
 
   virtual string DebugString_() const override {
     return s_.as_string();
diff --git a/expr.h b/expr.h
index e479ed8..fdd6d36 100644
--- a/expr.h
+++ b/expr.h
@@ -42,6 +42,8 @@ class Value : public Evaluable {
   virtual Value* Compact() { return this; }
 
   virtual bool IsLiteral() const { return false; }
+  // Only safe after IsLiteral() returns true.
+  virtual StringPiece GetLiteralValueUnsafe() const { return ""; }
 
   string DebugString() const;
 
diff --git a/stmt.cc b/stmt.cc
index 1abab32..155e0cd 100644
--- a/stmt.cc
+++ b/stmt.cc
@@ -55,6 +55,19 @@ string AssignStmt::DebugString() const {
                       opstr, dirstr, LOCF(loc()));
 }
 
+Symbol AssignStmt::GetLhsSymbol(Evaluator* ev) const {
+  if (!lhs->IsLiteral()) {
+    string buf;
+    lhs->Eval(ev, &buf);
+    return Intern(buf);
+  }
+
+  if (!lhs_sym_cache_.IsValid()) {
+    lhs_sym_cache_ = Intern(lhs->GetLiteralValueUnsafe());
+  }
+  return lhs_sym_cache_;
+}
+
 string CommandStmt::DebugString() const {
   return StringPrintf("CommandStmt(%s, loc=%s:%d)",
                       expr->DebugString().c_str(), LOCF(loc()));
diff --git a/stmt.h b/stmt.h
index 4d4c5eb..3b6feeb 100644
--- a/stmt.h
+++ b/stmt.h
@@ -20,6 +20,7 @@
 
 #include "loc.h"
 #include "string_piece.h"
+#include "symtab.h"
 
 using namespace std;
 
@@ -85,11 +86,19 @@ struct AssignStmt : public Stmt {
   AssignOp op;
   AssignDirective directive;
 
+  AssignStmt()
+      : lhs_sym_cache_(Symbol::IsUninitialized{}) {
+  }
   virtual ~AssignStmt();
 
   virtual void Eval(Evaluator* ev) const;
 
   virtual string DebugString() const;
+
+  Symbol GetLhsSymbol(Evaluator* ev) const;
+
+ private:
+  mutable Symbol lhs_sym_cache_;
 };
 
 struct CommandStmt : public Stmt {