Reintroduce compile-time --string-slices flag.

This partially reverts commit aaebbbaa59,
which removed the --string-slices flag. We reintroduce the flag as a
build time flag for an experiment to gather information of how much
SliceStrings help with throughput and effective memory use.

Bug: v8:7025
Change-Id: I529da91bb7501fe93d83891abf560710f3ecb9d0
Reviewed-on: https://chromium-review.googlesource.com/750681
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49068}

src/code-stub-assembler.cc

@@ -4625,37 +4625,39 @@ Node* CodeStubAssembler::SubString(Node* context, Node* string, Node* from,
   // encoding at this point.
   Label external_string(this);
   {
-    Label next(this);
+    if (FLAG_string_slices) {
+      Label next(this);
 
-    // Short slice.  Copy instead of slicing.
-    GotoIf(SmiLessThan(substr_length, SmiConstant(SlicedString::kMinLength)),
-           &next);
+      // Short slice.  Copy instead of slicing.
+      GotoIf(SmiLessThan(substr_length, SmiConstant(SlicedString::kMinLength)),
+             &next);
 
-    // Allocate new sliced string.
+      // Allocate new sliced string.
 
-    Counters* counters = isolate()->counters();
-    IncrementCounter(counters->sub_string_native(), 1);
+      Counters* counters = isolate()->counters();
+      IncrementCounter(counters->sub_string_native(), 1);
 
-    Label one_byte_slice(this), two_byte_slice(this);
-    Branch(IsOneByteStringInstanceType(to_direct.instance_type()),
-           &one_byte_slice, &two_byte_slice);
+      Label one_byte_slice(this), two_byte_slice(this);
+      Branch(IsOneByteStringInstanceType(to_direct.instance_type()),
+             &one_byte_slice, &two_byte_slice);
 
-    BIND(&one_byte_slice);
-    {
-      var_result.Bind(
-          AllocateSlicedOneByteString(substr_length, direct_string, offset));
-      Goto(&end);
+      BIND(&one_byte_slice);
+      {
+        var_result.Bind(
+            AllocateSlicedOneByteString(substr_length, direct_string, offset));
+        Goto(&end);
+      }
+
+      BIND(&two_byte_slice);
+      {
+        var_result.Bind(
+            AllocateSlicedTwoByteString(substr_length, direct_string, offset));
+        Goto(&end);
+      }
+
+      BIND(&next);
     }
 
-    BIND(&two_byte_slice);
-    {
-      var_result.Bind(
-          AllocateSlicedTwoByteString(substr_length, direct_string, offset));
-      Goto(&end);
-    }
-
-    BIND(&next);
-
     // The subject string can only be external or sequential string of either
     // encoding at this point.
     GotoIf(to_direct.is_external(), &external_string);
@@ -4663,6 +4665,7 @@ Node* CodeStubAssembler::SubString(Node* context, Node* string, Node* from,
     var_result.Bind(AllocAndCopyStringCharacters(
         context, direct_string, instance_type, offset, substr_length));
 
+    Counters* counters = isolate()->counters();
     IncrementCounter(counters->sub_string_native(), 1);
 
     Goto(&end);
@@ -4790,7 +4793,7 @@ Node* ToDirectStringAssembler::TryToDirect(Label* if_bailout) {
   // Sliced string. Fetch parent and correct start index by offset.
   BIND(&if_issliced);
   {
-    if (flags_ & kDontUnpackSlicedStrings) {
+    if (!FLAG_string_slices || (flags_ & kDontUnpackSlicedStrings)) {
       Goto(if_bailout);
     } else {
       Node* const string = var_string_.value();

src/factory.cc

@@ -835,7 +835,7 @@ Handle<String> Factory::NewProperSubString(Handle<String> str,
     return MakeOrFindTwoCharacterString(isolate(), c1, c2);
   }
 
-  if (length < SlicedString::kMinLength) {
+  if (!FLAG_string_slices || length < SlicedString::kMinLength) {
     if (str->IsOneByteRepresentation()) {
       Handle<SeqOneByteString> result =
           NewRawOneByteString(length).ToHandleChecked();

src/flag-definitions.h

@@ -316,6 +316,7 @@ DEFINE_VALUE_IMPLICATION(optimize_for_size, max_semi_space_size, 1)
 
 // Flags for data representation optimizations
 DEFINE_BOOL(unbox_double_arrays, true, "automatically unbox arrays of doubles")
+DEFINE_BOOL_READONLY(string_slices, true, "use string slices")
 
 // Flags for Ignition.
 DEFINE_BOOL(ignition_elide_noneffectful_bytecodes, true,

test/cctest/test-heap-profiler.cc

@@ -402,6 +402,7 @@ TEST(HeapSnapshotHeapNumbers) {
 
 
 TEST(HeapSnapshotSlicedString) {
+  if (!i::FLAG_string_slices) return;
   LocalContext env;
   v8::HandleScope scope(env->GetIsolate());
   v8::HeapProfiler* heap_profiler = env->GetIsolate()->GetHeapProfiler();

test/cctest/test-strings.cc

@@ -1070,6 +1070,7 @@ TEST(ExternalShortStringAdd) {
 
 
 TEST(JSONStringifySliceMadeExternal) {
+  if (!FLAG_string_slices) return;
   CcTest::InitializeVM();
   // Create a sliced string from a one-byte string.  The latter is turned
   // into a two-byte external string.  Check that JSON.stringify works.
@@ -1155,6 +1156,7 @@ TEST(CachedHashOverflow) {
 
 
 TEST(SliceFromCons) {
+  if (!FLAG_string_slices) return;
   CcTest::InitializeVM();
   Factory* factory = CcTest::i_isolate()->factory();
   v8::HandleScope scope(CcTest::isolate());
@@ -1221,6 +1223,7 @@ TEST(InternalizeExternal) {
 }
 
 TEST(SliceFromExternal) {
+  if (!FLAG_string_slices) return;
   CcTest::InitializeVM();
   Factory* factory = CcTest::i_isolate()->factory();
   v8::HandleScope scope(CcTest::isolate());
@@ -1241,6 +1244,7 @@ TEST(SliceFromExternal) {
 TEST(TrivialSlice) {
   // This tests whether a slice that contains the entire parent string
   // actually creates a new string (it should not).
+  if (!FLAG_string_slices) return;
   CcTest::InitializeVM();
   Factory* factory = CcTest::i_isolate()->factory();
   v8::HandleScope scope(CcTest::isolate());
@@ -1270,6 +1274,7 @@ TEST(TrivialSlice) {
 TEST(SliceFromSlice) {
   // This tests whether a slice that contains the entire parent string
   // actually creates a new string (it should not).
+  if (!FLAG_string_slices) return;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   v8::Local<v8::Value> result;