Pretenure ASCII cons string in high promotion mode.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4048 matching lines @@
   // Set the map. The other fields are left uninitialized.
   LoadRoot(kScratchRegister, Heap::kConsStringMapRootIndex);
   movq(FieldOperand(result, HeapObject::kMapOffset), kScratchRegister);
 }
 
 
 void MacroAssembler::AllocateAsciiConsString(Register result,
                                              Register scratch1,
                                              Register scratch2,
                                              Label* gc_required) {
-  // Allocate heap number in new space.
-  Allocate(ConsString::kSize, result, scratch1, scratch2, gc_required,
-           TAG_OBJECT);
+  Label allocate_new_space, install_map;
+  AllocationFlags flags = TAG_OBJECT;
+
+  ExternalReference high_promotion_mode = ExternalReference::
+      new_space_high_promotion_mode_active_address(isolate());
+
+  Load(scratch1, high_promotion_mode);
+  testb(scratch1, Immediate(1));
+  j(zero, &allocate_new_space);
+  Allocate(ConsString::kSize,
+           result,
+           scratch1,
+           scratch2,
+           gc_required,
+           static_cast<AllocationFlags>(flags | PRETENURE_OLD_POINTER_SPACE));
+
+  jmp(&install_map);
+
+  bind(&allocate_new_space);
+  Allocate(ConsString::kSize,
+           result,
+           scratch1,
+           scratch2,
+           gc_required,
+           flags);
+
+  bind(&install_map);
 
   // Set the map. The other fields are left uninitialized.
   LoadRoot(kScratchRegister, Heap::kConsAsciiStringMapRootIndex);
   movq(FieldOperand(result, HeapObject::kMapOffset), kScratchRegister);
 }
 
 
 void MacroAssembler::AllocateTwoByteSlicedString(Register result,
                                           Register scratch1,
                                           Register scratch2,
@@ skipping 566 matching lines @@
   j(greater, &no_info_available);
   CompareRoot(MemOperand(scratch_reg, -AllocationSiteInfo::kSize),
               Heap::kAllocationSiteInfoMapRootIndex);
   bind(&no_info_available);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64