Pretenure ASCII cons string in high promotion mode.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index d9db3ad346db03739e0e5faa66c65678c906fa4f..691894c0e2cb4eb92fdad51bf9cbbed5c0c29fc1 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -4066,9 +4066,33 @@ 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);