[Turbofan] Enable fast-path for String.p.codePointAt

Bug: v8:7270
Change-Id: Ia7fcd230adbf0f81a99a7f34b554c00c07649bfa
Reviewed-on: https://chromium-review.googlesource.com/888742Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51043}

src/compiler/effect-control-linearizer.cc

@@ -858,7 +858,7 @@ bool EffectControlLinearizer::TryWireInStateEffect(Node* node,
       result = LowerStringCodePointAt(node, UnicodeEncodingOf(node->op()));
       break;
     case IrOpcode::kSeqStringCodePointAt:
-      result = LowerSeqStringCharCodeAt(node);
+      result = LowerSeqStringCodePointAt(node, UnicodeEncodingOf(node->op()));
       break;
     case IrOpcode::kStringToLowerCaseIntl:
       result = LowerStringToLowerCaseIntl(node);
@@ -2908,17 +2908,18 @@ Node* EffectControlLinearizer::LowerSeqStringCodePointAt(
       __ Word32Equal(__ Word32And(first_char_code, __ Int32Constant(0xFC00)),
                      __ Int32Constant(0xD800));
   // Return first character code.
-  __ GotoIf(first_out, &return_result, first_char_code);
+  __ GotoIfNot(first_out, &return_result, first_char_code);
   // Check if position + 1 is still in range.
-  Node* length = __ LoadField(AccessBuilder::ForStringLength(), receiver);
+  Node* length = ChangeSmiToInt32(
+      __ LoadField(AccessBuilder::ForStringLength(), receiver));
   Node* next_position = __ Int32Add(position, __ Int32Constant(1));
   Node* next_position_in_range = __ Int32LessThan(next_position, length);
-  __ GotoIf(next_position_in_range, &return_result, first_char_code);
+  __ GotoIfNot(next_position_in_range, &return_result, first_char_code);
 
   // Load second character code.
   Node* second_char_code =
       LoadFromSeqString(receiver, next_position, is_one_byte);
-  // Check if first character code is outside of interval [0xD800, 0xDBFF].
+  // Check if second character code is outside of interval [0xDC00, 0xDFFF].
   Node* second_out =
       __ Word32Equal(__ Word32And(second_char_code, __ Int32Constant(0xFC00)),
                      __ Int32Constant(0xDC00));

src/compiler/simplified-lowering.cc

@@ -2358,9 +2358,19 @@ class RepresentationSelector {
         return;
       }
       case IrOpcode::kStringCodePointAt: {
-        // TODO(turbofan): Allow builtins to return untagged values.
-        VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
-                   MachineRepresentation::kTaggedSigned);
+        Type* string_type = TypeOf(node->InputAt(0));
+        if (string_type->Is(Type::SeqString())) {
+          VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
+                     MachineRepresentation::kWord32);
+          if (lower()) {
+            UnicodeEncoding encoding = UnicodeEncodingOf(node->op());
+            NodeProperties::ChangeOp(
+                node, simplified()->SeqStringCodePointAt(encoding));
+          }
+        } else {
+          VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
+                     MachineRepresentation::kTaggedSigned);
+        }
         return;
       }
       case IrOpcode::kStringFromCharCode: {