Add initial hydrogenized NewStringAddStub.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 307595256c626a056f70b09f4face412ec312507..e96a992e01f9b3bcd319993b403ac00287f26970 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1454,6 +1454,346 @@ HValue* HGraphBuilder::BuildNumberToString(HValue* object,
 }
 
 
+void HGraphBuilder::BuildCopySeqStringChars(HValue* src,
+                                            HValue* src_offset,
+                                            String::Encoding src_encoding,
+                                            HValue* dst,
+                                            HValue* dst_offset,
+                                            String::Encoding dst_encoding,
+                                            HValue* length) {
+  ASSERT(dst_encoding != String::ONE_BYTE_ENCODING ||
+         src_encoding == String::ONE_BYTE_ENCODING);
+  LoopBuilder loop(this, context(), LoopBuilder::kPostIncrement);
+  HValue* index = loop.BeginBody(graph()->GetConstant0(), length, Token::LT);
+  {
+    HValue* src_index = Add<HAdd>(src_offset, index);
+    HValue* value = Add<HSeqStringGetChar>(src_encoding, src, src_index);
+    HValue* dst_index = Add<HAdd>(dst_offset, index);
+    Add<HSeqStringSetChar>(dst_encoding, dst, dst_index, value);
+  }
+  loop.EndBody();
+}
+
+
+HValue* HGraphBuilder::BuildUncheckedStringAdd(HValue* left,
+                                               HValue* right,
+                                               PretenureFlag pretenure_flag) {
+  // Determine the string lengths.
+  HValue* left_length = Add<HLoadNamedField>(
+      left, HObjectAccess::ForStringLength());
+  HValue* right_length = Add<HLoadNamedField>(
+      right, HObjectAccess::ForStringLength());
+
+  // Check if we concatenated the strings here, or if we have to resort to the
+  // runtime function.
+  HIfContinuation handled(graph()->CreateBasicBlock(),
+                          graph()->CreateBasicBlock());
+
+  // Check if both parameters do not exceed half the max string length, because
+  // exceptionally long strings should be handled in the runtime. This may be
+  // too pessimistic, but it's the only portable way to express this in Hydrogen
+  // right now.

[mvstanton, 2013/11/08 10:17:50]

I appreciate the detailed comments on this long and tricky method! 
Instead of saying "this may be too pessimistic," could you say what you would
prefer to do, and then lament that it's non-portable right now.

[Benedikt Meurer, 2013/11/08 13:21:08]

Done.

+  HConstant* max_length = Add<HConstant>(
+      static_cast<int32_t>(String::kMaxLength / 2));
+  IfBuilder if_nooverflow(this);
+  if_nooverflow.If<HCompareNumericAndBranch>(
+      left_length, max_length, Token::LTE);
+  if_nooverflow.AndIf<HCompareNumericAndBranch>(
+      right_length, max_length, Token::LTE);
+  if_nooverflow.Then();
+  {
+    // Determine the string instance types.
+    HLoadNamedField* left_instance_type = Add<HLoadNamedField>(
+        Add<HLoadNamedField>(left, HObjectAccess::ForMap()),
+        HObjectAccess::ForMapInstanceType());
+    HLoadNamedField* right_instance_type = Add<HLoadNamedField>(
+        Add<HLoadNamedField>(right, HObjectAccess::ForMap()),
+        HObjectAccess::ForMapInstanceType());
+
+    // Compute the length of the resulting string.
+    HValue* length = Add<HAdd>(left_length, right_length);
+
+    // Check if we should create a cons string.
+    IfBuilder if_createcons(this);
+    if_createcons.If<HCompareNumericAndBranch>(
+        length,
+        Add<HConstant>(static_cast<int32_t>(ConsString::kMinLength)),
+        Token::GTE);
+    if_createcons.Then();
+    {
+      // Allocate the cons string object. HAllocate does not care whether we
+      // pass CONS_STRING_TYPE or CONS_ASCII_STRING_TYPE here, so we just use
+      // CONS_STRING_TYPE here. Below we decide whether the cons string is
+      // one-byte or two-byte and set the appropriate map.
+      HAllocate* string = Add<HAllocate>(Add<HConstant>(ConsString::kSize),
+                                         HType::String(), pretenure_flag,
+                                         CONS_STRING_TYPE);
+
+      // We create a one-byte cons string if
+      // 1. both strings are one-byte, or
+      // 2. at least one of the strings is two-byte, but happens to contain only
+      //    one-byte characters.
+      // To do this, we check
+      // 1. if both strings are one-byte, or
+      // 2. if the one-byte data hint is set in both strings, or
+      // 3. if one of the strings has the one-byte data hint set and the other
+      //    string is one-byte.
+      IfBuilder if_onebyte(this);
+      STATIC_ASSERT(kTwoByteStringTag == 0);
+      if_onebyte.If<HCompareNumericAndBranch>(
+          Add<HBitwise>(
+              Token::BIT_AND,
+              Add<HBitwise>(
+                  Token::BIT_AND, left_instance_type, right_instance_type),
+              Add<HConstant>(static_cast<int32_t>(kStringEncodingMask))),
+          graph()->GetConstant0(),
+          Token::NE);
+      if_onebyte.Or();
+      STATIC_ASSERT(kOneByteDataHintMask != 0);
+      if_onebyte.If<HCompareNumericAndBranch>(
+          Add<HBitwise>(
+              Token::BIT_AND,
+              Add<HBitwise>(
+                  Token::BIT_AND, left_instance_type, right_instance_type),
+              Add<HConstant>(static_cast<int32_t>(kOneByteDataHintMask))),

[mvstanton, 2013/11/08 10:17:50]

These hurt my eyes. :)
How about a helper function that consolidates this kind of typical bitwise
operation for this method. The load of Add, <, >, static_cast, obscures the
overall operation.

[Benedikt Meurer, 2013/11/08 13:21:08]

I'm not sure that's going to make it better. Tried that already, but it's even
less obvious what's happening then, IMHO.

+          graph()->GetConstant0(),
+          Token::NE);
+      if_onebyte.Or();
+      STATIC_ASSERT(kOneByteStringTag != 0 &&
+                    kOneByteDataHintTag != 0 &&
+                    kOneByteDataHintTag != kOneByteStringTag);
+      if_onebyte.If<HCompareNumericAndBranch>(
+          Add<HBitwise>(
+              Token::BIT_AND,
+              Add<HBitwise>(
+                  Token::BIT_XOR, left_instance_type, right_instance_type),
+              Add<HConstant>(static_cast<int32_t>(
+                      kOneByteStringTag | kOneByteDataHintTag))),
+          Add<HConstant>(static_cast<int32_t>(
+                  kOneByteStringTag | kOneByteDataHintTag)),
+          Token::EQ);
+      if_onebyte.Then();
+      {
+        // We can safely skip the write barrier for storing the map here.
+        HStoreNamedField* store_map = AddStoreMapConstant(
+            string, isolate()->factory()->cons_ascii_string_map());
+        store_map->SkipWriteBarrier();
+      }
+      if_onebyte.Else();
+      {
+        // We can safely skip the write barrier for storing the map here.
+        HStoreNamedField* store_map = AddStoreMapConstant(
+            string, isolate()->factory()->cons_string_map());
+        store_map->SkipWriteBarrier();
+      }
+      if_onebyte.End();
+
+      // Initialize the cons string fields.
+      Add<HStoreNamedField>(string, HObjectAccess::ForStringHashField(),
+                            Add<HConstant>(String::kEmptyHashField));
+      Add<HStoreNamedField>(string, HObjectAccess::ForStringLength(), length);
+      Add<HStoreNamedField>(string, HObjectAccess::ForConsStringFirst(), left);
+      Add<HStoreNamedField>(string, HObjectAccess::ForConsStringSecond(),
+                            right);
+
+      // Cons string is result.
+      Push(string);
+    }
+    if_createcons.Else();
+    {
+      // Check if both strings have the same encoding and both are
+      // sequential.
+      IfBuilder if_sameencodingandsequential(this);
+      if_sameencodingandsequential.If<HCompareNumericAndBranch>(
+          Add<HBitwise>(
+              Token::BIT_AND,
+              Add<HBitwise>(
+                  Token::BIT_XOR, left_instance_type, right_instance_type),
+              Add<HConstant>(static_cast<int32_t>(kStringEncodingMask))),

[mvstanton, 2013/11/08 10:17:50]

More instances of your typical pattern here, OP1(OP2(hvalue1, hvalue2), mask).

+          graph()->GetConstant0(), Token::EQ);
+      if_sameencodingandsequential.And();
+      if_sameencodingandsequential.If<HCompareNumericAndBranch>(
+          Add<HBitwise>(
+              Token::BIT_AND,
+              Add<HBitwise>(
+                  Token::BIT_OR, left_instance_type, right_instance_type),
+              Add<HConstant>(static_cast<int32_t>(kStringRepresentationMask))),
+          Add<HConstant>(static_cast<int32_t>(kSeqStringTag)), Token::EQ);
+      if_sameencodingandsequential.Then();
+      {
+        // Check if the result is a one-byte string.
+        IfBuilder if_onebyte(this);
+        if_onebyte.If<HCompareNumericAndBranch>(
+            Add<HBitwise>(
+                Token::BIT_AND,
+                Add<HBitwise>(
+                    Token::BIT_OR, left_instance_type, right_instance_type),
+                Add<HConstant>(static_cast<int32_t>(kStringEncodingMask))),
+            Add<HConstant>(static_cast<int32_t>(kTwoByteStringTag)), Token::NE);
+        if_onebyte.Then();
+        {
+          // Calculate the number of bytes needed for the characters in the
+          // string while observing object alignment.
+          STATIC_ASSERT(kOneByteSize == 1);
+          STATIC_ASSERT((SeqOneByteString::kHeaderSize &
+                        kObjectAlignmentMask) == 0);
+          HValue* size = Add<HAdd>(
+              length, Add<HConstant>(static_cast<int32_t>(
+                      SeqOneByteString::kHeaderSize + kObjectAlignmentMask)));
+          size->ClearFlag(HValue::kCanOverflow);
+          size = Add<HBitwise>(
+              Token::BIT_AND, size, Add<HConstant>(static_cast<int32_t>(
+                      ~kObjectAlignmentMask)));
+

[mvstanton, 2013/11/08 10:17:50]

The size operand calculation above is very tricky and very similar to the block
below (under if_onebyte.Else()). Can it be moved into a function?

[Benedikt Meurer, 2013/11/08 13:21:08]

Done.

+          // Allocate the ASCII string object.
+          Handle<Map> map = isolate()->factory()->ascii_string_map();
+          HAllocate* string = Add<HAllocate>(size, HType::String(),
+                                             pretenure_flag, ASCII_STRING_TYPE);
+          string->set_known_initial_map(map);
+
+          // We can safely skip the write barrier for storing map here.
+          HStoreNamedField* store_map = AddStoreMapConstant(string, map);

[mvstanton, 2013/11/08 10:17:50]

AddStoreMapConstantNoWriteBarrier() again, or as a mode of
AddStoreMapConstant().

[Benedikt Meurer, 2013/11/08 13:21:08]

Done.

+          store_map->SkipWriteBarrier();
+
+          // Copy bytes from the left string.
+          BuildCopySeqStringChars(
+              left, graph()->GetConstant0(), String::ONE_BYTE_ENCODING,
+              string, graph()->GetConstant0(), String::ONE_BYTE_ENCODING,
+              left_length);
+
+          // Copy bytes from the right string.
+          BuildCopySeqStringChars(
+              right, graph()->GetConstant0(), String::ONE_BYTE_ENCODING,
+              string, left_length, String::ONE_BYTE_ENCODING,
+              right_length);
+
+          // Return the string.
+          Push(string);
+        }
+        if_onebyte.Else();
+        {
+          // Calculate the number of bytes needed for the characters in the
+          // string while observing object alignment.
+          STATIC_ASSERT(kUC16Size == 2);
+          STATIC_ASSERT((SeqTwoByteString::kHeaderSize &
+                         kObjectAlignmentMask) == 0);
+          HValue* size = Add<HShl>(length, graph()->GetConstant1());
+          size->ClearFlag(HValue::kCanOverflow);
+          size->SetFlag(HValue::kUint32);
+          size = Add<HAdd>(
+              size, Add<HConstant>(static_cast<int32_t>(
+                      SeqTwoByteString::kHeaderSize + kObjectAlignmentMask)));
+          size->ClearFlag(HValue::kCanOverflow);
+          size = Add<HBitwise>(
+              Token::BIT_AND, size, Add<HConstant>(static_cast<int32_t>(
+                      ~kObjectAlignmentMask)));
+
+          // Allocate the two-byte string object.
+          Handle<Map> map = isolate()->factory()->string_map();
+          HAllocate* string = Add<HAllocate>(size, HType::String(),
+                                             pretenure_flag, STRING_TYPE);
+          string->set_known_initial_map(map);
+
+          // We can safely skip the write barrier for storing map here.
+          HStoreNamedField* store_map = AddStoreMapConstant(string, map);

[mvstanton, 2013/11/08 10:17:50]

Could you add a variant to AddStoreMapConstant either with a parameter or a new
name (like AddStoreMapConstantNoWriteBarrier) that encapsulates these? As
discussed earlier, this "special case" of skipping write barriers can be
dangerous to program, and it would be nice to consolidate and better name those
instances when we do this.

[Benedikt Meurer, 2013/11/08 13:21:08]

Done.

+          store_map->SkipWriteBarrier();
+
+          // Copy bytes from the left string.
+          BuildCopySeqStringChars(
+              left, graph()->GetConstant0(), String::TWO_BYTE_ENCODING,
+              string, graph()->GetConstant0(), String::TWO_BYTE_ENCODING,
+              left_length);
+
+          // Copy bytes from the right string.
+          BuildCopySeqStringChars(
+              right, graph()->GetConstant0(), String::TWO_BYTE_ENCODING,
+              string, left_length, String::TWO_BYTE_ENCODING,
+              right_length);
+
+          // Return the string.
+          Push(string);
+        }
+        if_onebyte.End();
+
+        // Initialize the (common) string fields.
+        HValue* string = Pop();
+        Add<HStoreNamedField>(string, HObjectAccess::ForStringHashField(),
+                              Add<HConstant>(String::kEmptyHashField));
+        Add<HStoreNamedField>(string, HObjectAccess::ForStringLength(),
+                              length);
+        Push(string);
+      }
+      if_sameencodingandsequential.JoinContinuation(&handled);
+    }
+    if_createcons.JoinContinuation(&handled);
+  }
+  if_nooverflow.JoinContinuation(&handled);
+
+  // Check if the strings were concatenated successfully, otherwise fallback to
+  // add the strings in the runtime.
+  IfBuilder if_handled(this, &handled);
+  if_handled.Then();
+  {
+    // Count the native string addition.
+    AddIncrementCounter(isolate()->counters()->string_add_native());
+  }
+  if_handled.Else();
+  {
+    // Fallback to the runtime to add the two strings.
+    Add<HPushArgument>(left);
+    Add<HPushArgument>(right);
+    Push(Add<HCallRuntime>(isolate()->factory()->empty_string(),
+                           Runtime::FunctionForId(Runtime::kStringAdd),
+                           2));
+  }
+  if_handled.End();
+
+  return Pop();
+}
+
+
+HValue* HGraphBuilder::BuildStringAdd(HValue* left,
+                                      HValue* right,
+                                      PretenureFlag pretenure_flag) {
+  // Determine the string lengths.
+  HValue* left_length = Add<HLoadNamedField>(
+      left, HObjectAccess::ForStringLength());
+  HValue* right_length = Add<HLoadNamedField>(
+      right, HObjectAccess::ForStringLength());
+
+  // Check if left string is empty.
+  IfBuilder if_leftisempty(this);
+  if_leftisempty.If<HCompareNumericAndBranch>(
+      left_length, graph()->GetConstant0(), Token::EQ);
+  if_leftisempty.Then();
+  {
+    // Just return the right string.
+    Push(right);
+  }
+  if_leftisempty.Else();
+  {
+    // Check if right string is empty.
+    IfBuilder if_rightisempty(this);
+    if_rightisempty.If<HCompareNumericAndBranch>(
+        right_length, graph()->GetConstant0(), Token::EQ);
+    if_rightisempty.Then();
+    {
+      // Just return the left string.
+      Push(left);
+    }
+    if_rightisempty.Else();
+    {
+      // Concatenate the two non-empty strings.
+      Push(BuildUncheckedStringAdd(left, right, pretenure_flag));
+    }
+    if_rightisempty.End();
+  }
+  if_leftisempty.End();
+
+  return Pop();
+}
+
+
 HInstruction* HGraphBuilder::BuildUncheckedMonomorphicElementAccess(
     HValue* checked_object,
     HValue* key,