[stubs] Port String.prototype.substr to TurboFan

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index e716dca73bfd562767759d07c71bfdb4812c5b91..3e1888be30c77bbe9abba11fb6b19caf02108e1c 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -873,6 +873,114 @@ BUILTIN(StringPrototypeNormalize) {
   return *string;
 }
 
+// ES6 section B.2.3.1 String.prototype.substr ( start, length )
+void Builtins::Generate_StringPrototypeSubstr(CodeStubAssembler* a) {
+  typedef CodeStubAssembler::Label Label;
+  typedef compiler::Node Node;
+  typedef CodeStubAssembler::Variable Variable;
+
+  Label out(a), handle_length(a);
+
+  Variable var_start(a, MachineRepresentation::kTagged);
+  Variable var_length(a, MachineRepresentation::kTagged);
+
+  Node* const receiver = a->Parameter(0);
+  Node* const start = a->Parameter(1);
+  Node* const length = a->Parameter(2);
+  Node* const context = a->Parameter(5);
+
+  // Check that {receiver} is coercible to Object and convert it to a String.
+  Node* const string =
+      a->ToThisString(context, receiver, "String.prototype.substr");
+
+  Node* const string_length = a->LoadStringLength(string);
+
+  // Conversions and bounds-checks for {start}.
+  {
+    Node* const start_int =
+        a->ToInteger(context, start, CodeStubAssembler::kTruncateMinusZero);
+
+    Label if_issmi(a), if_isheapnumber(a, Label::kDeferred);
+    a->Branch(a->WordIsSmi(start_int), &if_issmi, &if_isheapnumber);
+
+    a->Bind(&if_issmi);
+    {
+      Node* const zero = a->SmiConstant(Smi::FromInt(0));
+      Node* const length_plus_start = a->SmiAdd(string_length, start_int);
+      var_start.Bind(a->Select(a->SmiLessThan(start_int, zero),
+                               a->SmiMax(length_plus_start, zero), start_int));
+      a->Goto(&handle_length);
+    }
+
+    a->Bind(&if_isheapnumber);
+    {
+      // Set {start} to {string}.length; this ultimately returns "".
+      var_start.Bind(string_length);

[Igor Sheludko, 2016/09/29 10:59:44]

I think if the start index is < 0 then we should bind var_start to 0, like you
already do for the smi case.

[jgruber, 2016/09/29 12:13:43]

Done.

+      a->Goto(&handle_length);
+    }
+  }
+
+  // Conversions and bounds-checks for {length}.
+  a->Bind(&handle_length);
+  {
+    // Default to {string_length} if {length} is undefined.
+    {
+      Node* const length_int =
+          a->ToInteger(context, length, CodeStubAssembler::kTruncateMinusZero);
+      var_length.Bind(a->Select(a->WordEqual(length, a->UndefinedConstant()),

[Igor Sheludko, 2016/09/29 10:59:44]

Please check that we don't execute ToInteger(length) code in case of undefined
length in generated code.

[jgruber, 2016/09/29 12:13:43]

Done.

+                                string_length, length_int));
+    }
+
+    Label if_issmi(a), if_isheapnumber(a, Label::kDeferred);
+    a->Branch(a->WordIsSmi(var_length.value()), &if_issmi, &if_isheapnumber);
+
+    // Set {length} to min(max({length}, 0), {string_length} - {start}
+    a->Bind(&if_issmi);
+    {
+      Node* const zero = a->SmiConstant(Smi::FromInt(0));
+      Node* const positive_length = a->SmiMax(var_length.value(), zero);
+
+      Node* const minimal_length = a->SmiSub(string_length, var_start.value());
+      var_length.Bind(a->SmiMin(positive_length, minimal_length));
+
+      a->GotoUnless(a->SmiLessThanOrEqual(var_length.value(), zero), &out);
+      a->Return(a->EmptyStringConstant());
+    }
+
+    a->Bind(&if_isheapnumber);
+    {
+      // If {length} is a heap number, it is definitely out of bounds. There are
+      // two cases according to the spec: if it is negative, "" is returned; if
+      // it is positive, then length is set to {string_length} - {start}.
+
+      a->Assert(a->WordEqual(a->LoadMap(var_length.value()),
+                             a->HeapNumberMapConstant()));
+
+      Label if_isnegative(a), if_ispositive(a);
+      Node* const float_zero = a->Float64Constant(0.);
+      Node* const length_float = a->LoadHeapNumberValue(var_length.value());
+      a->Branch(a->Float64LessThan(length_float, float_zero), &if_isnegative,
+                &if_ispositive);
+
+      a->Bind(&if_isnegative);
+      a->Return(a->EmptyStringConstant());
+
+      a->Bind(&if_ispositive);
+      {
+        var_length.Bind(a->SmiSub(string_length, var_start.value()));
+        a->Goto(&out);
+      }
+    }
+  }
+
+  a->Bind(&out);
+  {
+    Node* const end = a->SmiAdd(var_start.value(), var_length.value());
+    Node* const result = a->SubString(context, string, var_start.value(), end);
+    a->Return(result);
+  }
+}
+
 namespace {
 
 compiler::Node* ToSmiBetweenZeroAnd(CodeStubAssembler* a,