[builtins] Create StringToNumber helper.

src/code-stub-assembler.cc

Index: src/code-stub-assembler.cc
diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index a9bb408048773244644c417279111be93ff571b6..debbc30e70f474ec3e383b1573bc4ecda9fa3387 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -2216,6 +2216,35 @@ Node* CodeStubAssembler::StringFromCharCode(Node* code) {
   return var_result.value();
 }
 
+Node* CodeStubAssembler::StringToNumber(Node* context, Node* input) {
+  Label runtime(this, Label::kDeferred);
+  Label end(this);
+
+  Variable var_result(this, MachineRepresentation::kTagged);
+
+  // Check if string has a cached array index.
+  Node* hash = LoadNameHashField(input);
+  Node* bit =
+      Word32And(hash, Int32Constant(String::kContainsCachedArrayIndexMask));
+  GotoIf(Word32NotEqual(bit, Int32Constant(0)), &runtime);
+
+  var_result.Bind(SmiTag(BitFieldDecode<String::ArrayIndexValueBits>(hash)));
+  Goto(&end);
+
+  Bind(&runtime);
+  {
+    // Note: We cannot tail call to the runtime here, as js-to-wasm

[Benedikt Meurer, 2016/09/02 04:23:48]

Nit: remove this note as now with the helper we cannot tail call anyways.

+    // trampolines also use this code currently, and they declare all
+    // outgoing parameters as untagged, while we would push a tagged
+    // object here.
+    var_result.Bind(CallRuntime(Runtime::kStringToNumber, context, input));
+    Goto(&end);
+  }
+
+  Bind(&end);
+  return var_result.value();
+}
+
 Node* CodeStubAssembler::BitFieldDecode(Node* word32, uint32_t shift,
                                         uint32_t mask) {
   return Word32Shr(Word32And(word32, Int32Constant(mask)),