[kernel] Change dill representation of doubles

Previously doubles was saved as strings in the string table,
with a DoubleLiteral holding a StringReference.

This can cause overhead in both computation time
(converting the double to and from string) as well as size
(e.g. a single usage of the previously unused double 1000000.42
would use (at least)
* 10 bytes for the characters
* 1 byte for the size
* 1 byte for the reference to the string

whereas saving it as a double would simply save the 8 bytes.

On the other hand the string table doesn't contain duplicates so
many usages of the same double will use more space.

The SDK dill file size decreases slightly (< 1 KB).

On a Dart file with 1M different doubles (0.42, 1.42, ..., 999999.42)
added to a list:

Before:

compile and write via fasta (non-strong-mode): 0:12.18
Reading (via dart, eager): 2500-2600 ms
Writing (to null sink) (after reading): 1600-1800 ms
Output dill file (via fasta compile): ~62 MB

After:

compile and write via fasta (non-strong-mode): 0:11.76
Reading (via dart, eager): 2050-2350 ms
Writing (to null sink) (after reading): 400-550 ms
Output dill file (via fasta compile): ~54 MB

Running the dill file is ~the same time, but "Maximum resident set size
(kbytes)" (from /usr/bin/time -v) decreases with ~4%.

On the other side, if it's 1M of the same doubles (0.42), while
compiling is ~the same speed, the output dill goes from 43MB to 50MB.
Surprisingly the "Maximum resident set size (kbytes)" still decreases
though (~3%).

Running flutter test in flutter/packages/flutter:

Before:
```
02:33 +2425 ~18: All tests passed!
02:28 +2425 ~18: All tests passed!
02:28 +2425 ~18: All tests passed!
```

After:
```
02:12 +2425 ~18: All tests passed!
02:11 +2425 ~18: All tests passed!
02:12 +2425 ~18: All tests passed!
```

So that's -12.0267% +/- 3.15253%

File size of a dill file of an arbitrary test using flutter is reduced by ~44 KB (~0.3%).

Change-Id: I64151376cde1dae6f0d02b3d96991bc432a994ae
Reviewed-on: https://dart-review.googlesource.com/41660
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>

pkg/kernel/binary.md

@@ -42,6 +42,8 @@ type UInt30 extends UInt {
 
 type UInt32 = big endian 32-bit unsigned integer
 
+type Double = Double-precision floating-point number.
+
 type List<T> {
   UInt length;
   T[length] items;
@@ -712,7 +714,7 @@ type BigIntLiteral extends Expression {
 
 type DoubleLiteral extends Expression {
   Byte tag = 40;
-  StringReference valueString;
+  Double value;
 }
 
 type TrueLiteral extends Expression {
@@ -875,7 +877,7 @@ type IntConstant extends Constant {
 
 type DoubleConstant extends Constant {
   Byte tag = 3;
-  StringReference value;
+  Double value;
 }
 
 type StringConstant extends Constant {

pkg/kernel/lib/binary/ast_from_binary.dart

@@ -97,6 +97,22 @@ class BinaryBuilder {
         readByte();
   }
 
+  final Float64List _doubleBuffer = new Float64List(1);
+  Uint8List _doubleBufferUint8;
+
+  double readDouble() {
+    _doubleBufferUint8 ??= _doubleBuffer.buffer.asUint8List();
+    _doubleBufferUint8[0] = readByte();
+    _doubleBufferUint8[1] = readByte();
+    _doubleBufferUint8[2] = readByte();
+    _doubleBufferUint8[3] = readByte();
+    _doubleBufferUint8[4] = readByte();
+    _doubleBufferUint8[5] = readByte();
+    _doubleBufferUint8[6] = readByte();
+    _doubleBufferUint8[7] = readByte();
+    return _doubleBuffer[0];
+  }
+
   List<int> readByteList() {
     List<int> bytes = new Uint8List(readUInt());
     bytes.setRange(0, bytes.length, _bytes, _byteOffset);
@@ -174,7 +190,7 @@ class BinaryBuilder {
       case ConstantTag.IntConstant:
         return new IntConstant((readExpression() as IntLiteral).value);
       case ConstantTag.DoubleConstant:
-        return new DoubleConstant(double.parse(readStringReference()));
+        return new DoubleConstant(readDouble());
       case ConstantTag.StringConstant:
         return new StringConstant(readStringReference());
       case ConstantTag.MapConstant:
@@ -1337,7 +1353,7 @@ class BinaryBuilder {
       case Tag.BigIntLiteral:
         return new IntLiteral(int.parse(readStringReference()));
       case Tag.DoubleLiteral:
-        return new DoubleLiteral(double.parse(readStringReference()));
+        return new DoubleLiteral(readDouble());
       case Tag.TrueLiteral:
         return new BoolLiteral(true);
       case Tag.FalseLiteral:

pkg/kernel/lib/binary/ast_to_binary.dart

@@ -147,7 +147,7 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
       writeInteger(constant.value);
     } else if (constant is DoubleConstant) {
       writeByte(ConstantTag.DoubleConstant);
-      writeStringReference('${constant.value}');
+      writeDouble(constant.value);
     } else if (constant is StringConstant) {
       writeByte(ConstantTag.StringConstant);
       writeStringReference(constant.value);
@@ -1240,13 +1240,12 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
 
   @override
   void visitDoubleLiteral(DoubleLiteral node) {
+    writeByte(Tag.DoubleLiteral);
     writeDouble(node.value);
   }
 
   writeDouble(double value) {
-    // TODO: Pick a better format for double literals.
-    writeByte(Tag.DoubleLiteral);
-    writeStringReference('$value');
+    _sink.addDouble(value);
   }
 
   @override
@@ -2193,8 +2192,20 @@ class BufferedSink {
   int length = 0;
   int flushedLength = 0;
 
+  Float64List _doubleBuffer = new Float64List(1);
+  Uint8List _doubleBufferUint8;
+
   BufferedSink(this._sink);
 
+  void addDouble(double d) {
+    _doubleBufferUint8 ??= _doubleBuffer.buffer.asUint8List();
+    _doubleBuffer[0] = d;
+    addByte4(_doubleBufferUint8[0], _doubleBufferUint8[1],
+        _doubleBufferUint8[2], _doubleBufferUint8[3]);
+    addByte4(_doubleBufferUint8[4], _doubleBufferUint8[5],
+        _doubleBufferUint8[6], _doubleBufferUint8[7]);
+  }
+
   void addByte(int byte) {
     _buffer[length++] = byte;
     if (length == SIZE) {

pkg/kernel/lib/binary/tag.dart

@@ -135,7 +135,7 @@ class Tag {
   /// Internal version of kernel binary format.
   /// Bump it when making incompatible changes in kernel binaries.
   /// Keep in sync with runtime/vm/kernel_binary.h.
-  static const int BinaryFormatVersion = 4;
+  static const int BinaryFormatVersion = 5;
 }
 
 abstract class ConstantTag {

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -1552,7 +1552,7 @@ void StreamingScopeBuilder::VisitExpression() {
       builder_->ReadUInt();  // read value.
       return;
     case kDoubleLiteral:
-      builder_->SkipStringReference();  // read index into string table.
+      builder_->ReadDouble();  // read value.
       return;
     case kTrueLiteral:
       return;
@@ -3546,8 +3546,7 @@ void StreamingConstantEvaluator::EvaluateIntLiteral(bool is_negative) {
 }
 
 void StreamingConstantEvaluator::EvaluateDoubleLiteral() {
-  result_ = Double::New(H.DartString(builder_->ReadStringReference()),
-                        Heap::kOld);  // read string reference.
+  result_ = Double::New(builder_->ReadDouble(), Heap::kOld);  // read value.
   result_ = H.Canonicalize(result_);
 }
 
@@ -6027,6 +6026,10 @@ uint32_t KernelReaderHelper::PeekUInt() {
   return reader_.ReadUInt();
 }
 
+double KernelReaderHelper::ReadDouble() {
+  return reader_.ReadDouble();
+}
+
 uint32_t KernelReaderHelper::PeekListLength() {
   AlternativeReadingScope alt(&reader_);
   return reader_.ReadListLength();
@@ -6494,7 +6497,7 @@ void KernelReaderHelper::SkipExpression() {
       ReadUInt();  // read value.
       return;
     case kDoubleLiteral:
-      SkipStringReference();  // read index into string table.
+      ReadDouble();  // read value.
       return;
     case kTrueLiteral:
       return;
@@ -8987,8 +8990,7 @@ Fragment StreamingFlowGraphBuilder::BuildDoubleLiteral(
   if (position != NULL) *position = TokenPosition::kNoSource;
 
   Double& constant = Double::ZoneHandle(
-      Z, Double::NewCanonical(
-             H.DartString(ReadStringReference())));  // read string reference.
+      Z, Double::NewCanonical(ReadDouble()));  // read double.
   return Constant(constant);
 }
 
@@ -10822,8 +10824,7 @@ const Array& ConstantHelper::ReadConstantTable() {
         break;
       }
       case kDoubleConstant: {
-        temp_instance_ = Double::New(
-            H.DartString(builder_.ReadStringReference()), Heap::kOld);
+        temp_instance_ = Double::New(builder_.ReadDouble(), Heap::kOld);
         temp_instance_ = H.Canonicalize(temp_instance_);
         break;
       }

runtime/vm/compiler/frontend/kernel_binary_flowgraph.h

@@ -1049,6 +1049,7 @@ class KernelReaderHelper {
   uint32_t ReadUInt();
   uint32_t ReadUInt32();
   uint32_t PeekUInt();
+  double ReadDouble();
   uint32_t PeekListLength();
   StringIndex ReadStringReference();
   NameIndex ReadCanonicalNameReference();

runtime/vm/kernel_binary.h

@@ -19,7 +19,7 @@ namespace kernel {
 // Keep in sync with package:kernel/lib/binary/tag.dart.
 
 static const uint32_t kMagicProgramFile = 0x90ABCDEFu;
-static const uint32_t kBinaryFormatVersion = 4;
+static const uint32_t kBinaryFormatVersion = 5;
 
 // Keep in sync with package:kernel/lib/binary/tag.dart
 #define KERNEL_TAG_LIST(V)                                                     \
@@ -213,6 +213,14 @@ class Reader : public ValueObject {
     return value;
   }
 
+  double ReadDouble() {
+    ASSERT((size_ >= 8) && (offset_ >= 0) && (offset_ <= size_ - 8));
+    double value = ReadUnaligned(
+        reinterpret_cast<const double*>(&this->buffer()[offset_]));
+    offset_ += 8;
+    return value;
+  }
+
   uint32_t ReadUInt() {
     ASSERT((size_ >= 1) && (offset_ >= 0) && (offset_ <= size_ - 1));
 

runtime/vm/kernel_loader.cc

@@ -82,8 +82,7 @@ class SimpleExpressionConverter {
         return true;
       case kDoubleLiteral:
         simple_value_ = &Double::ZoneHandle(
-            Z, Double::New(H.DartString(builder_->ReadStringReference()),
-                           Heap::kOld));  // read string reference.
+            Z, Double::New(builder_->ReadDouble(), Heap::kOld));  // read value.
         *simple_value_ = H.Canonicalize(*simple_value_);
         return true;
       case kTrueLiteral: