Speed up conversion of untagged int32 result if we know it must be a smi.

src/ast.cc

Index: src/ast.cc
===================================================================
--- src/ast.cc	(revision 4271)
+++ src/ast.cc	(working copy)
@@ -172,6 +172,72 @@
 }
 
 
+bool Expression::GuaranteedSmiResult() {
+  BinaryOperation* node = AsBinaryOperation();
+  if (node == NULL) return false;
+  Token::Value op = node->op();
+  switch (op) {
+    case Token::COMMA:
+    case Token::OR:
+    case Token::AND:
+    case Token::ADD:
+    case Token::SUB:
+    case Token::MUL:
+    case Token::DIV:
+    case Token::MOD:
+    case Token::BIT_XOR:
+    case Token::SHL:
+      return false;
+      break;
+    case Token::BIT_OR:
+    case Token::BIT_AND: {
+      Literal* left = node->left()->AsLiteral();
+      Literal* right = node->right()->AsLiteral();
+      if (left != NULL && left->handle()->IsSmi()) {
+        int value = Smi::cast(*left->handle())->value();
+        if (op == Token::BIT_OR && ((value & 0xc0000000) == 0xc0000000)) {
+          // Result of bitwise or is always a negative Smi.
+          return true;
+        }
+        if (op == Token::BIT_AND && ((value & 0xc0000000) == 0)) {
+          // Result of bitwise and is always a positive Smi.
+          return true;
+        }
+      }
+      if (right != NULL && right->handle()->IsSmi()) {
+        int value = Smi::cast(*right->handle())->value();
+        if (op == Token::BIT_OR && ((value & 0xc0000000) == 0xc0000000)) {
+          // Result of bitwise or is always a negative Smi.
+          return true;
+        }
+        if (op == Token::BIT_AND && ((value & 0xc0000000) == 0)) {
+          // Result of bitwise and is always a positive Smi.
+          return true;
+        }
+      }
+      return false;
+      break;
+    }
+    case Token::SAR:
+    case Token::SHR: {
+      Literal* right = node->right()->AsLiteral();
+       if (right != NULL && right->handle()->IsSmi()) {
+        int value = Smi::cast(*right->handle())->value();
+        if ((value & 0x1F) > 1 ||
+            (op == Token::SAR && (value & 0x1F) == 1)) {
+          return true;
+        }
+       }
+       return false;
+       break;
+    }
+    default:
+      UNREACHABLE();
+      break;
+  }
+  return false;
+}
+
 // ----------------------------------------------------------------------------
 // Implementation of AstVisitor