Runtime that makes Dart tick
by Slava Egorov
I am
a SWE
on the Dart team
I am
a SWE TLM
on the Dart team
I am
a DREAMER
on the Dart team
Disclaimer
Nobody have seen or reviewed this talk. The content of this talk is aspirational and does not necessarily reflect plans of the Dart or Flutter teams.
LEGO® System in Play
- The toy has to be compact in its dimensions without limiting the free expression of imagination.
- It has to be reasonably priced.
- It has to be simple and durable and yet offer unlimited variety.
- It has to be suitable for children of all ages and for both boys and girls.
- It has to be classic in its presentation, i.e. a classic among toys, needing no renewal.
- It has to be easily distributed.
Dart SDK
is a construction built out of smaller pieces
Why bother?
// Which one is faster?
// Classic indexed loop.
for (var i = 0; i < list.length; i++) {
final e = list[i];
}
// or this Dart 3.0 beauty?
for (var (i, e) in list.indexed) {
// ...
}
// Which one is faster? (On native)
// Classic indexed loop.
for (var i = 0; i < list.length; i++) {
final e = list[i];
}
// or this Dart 3.0 beauty?
for (var (i, e) in list.indexed) {
// ...
}
Easy to measure
$ dart compile exe -o loops loops.dart
$ ./loops
$ flutter run --release -t lib/loops.dart
but we are not here for easy
How does it work?
import 'dart:isolate';
Isolate.spawnUri('my-aot-snapshot.aot');
#include "dart_api.h"
Dart_Initialize(...); // Init runtime system
isolate = Dart_CreateIsolateGroup(
// ...
"main", // isolate name
snapshot_data,
snapshot_code,
// ...
);
Practice time
Dart SDK uses GN + Ninja
but today I am using Bazel
$ bazel build :gen_kernel \
:platform.dill \
:gen_snapshot \
:run_aot
$ bazel-bin/gen_kernel --aot \
--platform bazel-bin/platform.dill \
-o /tmp/loops.dill \
dart/loops.dart
$ bazel-bin/gen_snapshot \
--snapshot-kind=app-aot-assembly \
--snapshot=/tmp/loops.S \
/tmp/loops.dill
$ gcc -shared -o /tmp/loops.dylib /tmp/loops.S
$ bazel-bin/run_aot /tmp/loops.dylib
$ bazel-bin/run_aot /tmp/loops.dylib
ClassicLoop(RunTime): 63.10348942208462 us.
ForInIndexedLoop(RunTime): 819.2325 us.
AOT compiler can generate DWARF debug info
$ bazel-bin/gen_snapshot \
--dwarf-stack-traces \
--resolve-dwarf-paths \
...
AOT compiler can print its IL
$ bazel-bin/gen_snapshot \
--print-flow-graph-optimized \
--print-flow-graph-filter=... \
--disassemble-optimized \
...
Problem #1
Not enough inlining ⇒ not enough specialization
B1:
v2 ← Parameter(0)
v5 ← LoadField(v2 . list)
v63 ← StaticCall( IndexedIterable. v65, v5)
v7 ← StaticCall( get:iterator v63)
v49 ← LoadField(v7 . _source)
v89 ← LoadField(v49 . _iterable)
// ...
B3:
v98 ← DispatchTableCall( Iterable.elementAt, v89, v94)
Solution #1
Use @pragma('vm:prefer-inline')
// In sdk/lib/internal/iterable.dart
class IndexedIterable<T> extends Iterable<(int, T)> {
// ...
@pragma('vm:prefer-inline')
factory IndexedIterable(Iterable<T> source, int start) {
// ...
}
// ...
@pragma('vm:prefer-inline')
Iterator<(int, T)> get iterator => // ...
}
$ bazel-bin/run_aot /tmp/loops.dylib
ClassicLoop(RunTime): 62.95430732508757 us.
ForInIndexedLoop(RunTime): 249.33367062990135 us
ForInIndexedLoop improved by 3x
Problem #2
Compiler did not inline int.operator&
B32:
v20 ← StaticCall( & v242, v292, recognized_kind = Integer_bitAnd)
Solution #2
Compiler got confused about the type of e
for (var (i, int e) in list.indexed) {
// ...
}
Solution #2
Fix the compiler!
- Force inline recognized arithmetic methods
- Fix a bug in the code that was supposed to figure out that
LoadIndexed(...)from aList<int>returnsint
$ bazel-bin/run_aot /tmp/loops.dylib
ClassicLoop(RunTime): 62.71734375 us.
ForInIndexedLoop(RunTime): 109.20825648258652 us.
ForInIndexedLoop improved by 2.5x
Problem #3
Unfused and redundant comparison
v44 ← RelationalOp(>=, v42, v263)
Branch if StrictCompare(===, v44, v10) goto (37, 18)
B37:
Branch if RelationalOp(>=, v256, v235) goto (19, 3)
B3:
Branch if StrictCompare(===, v44, v10) goto (32, 11)
Solution #3
Fix the compiler!
- Propagate
x === trueandx === falseinto blocks which comparison dominates
$ bazel-bin/run_aot /tmp/loops.dylib
ClassicLoop(RunTime): 62.81853125 us.
ForInIndexedLoop(RunTime): 93.52465990369782 us.
ForInIndexedLoop improved by 15%
Problem #4
Left over iteration variable, redundant comparison and a bounds check
final l = list.length;
for (var i = 0, j = -1;
++j >= 0 && i < l;
i++) {
if (i >= l) throw OutOfBoundsError();
final e = list[i];
result ^= i & e;
}
Solution #4 (TODO)
Need to fix issues with range analysis
Much bigger project than other fixes
What else
could we do?
struct HelloWorldView: View {
@State private var name: String = ""
var body: some View {
VStack(alignment: .leading) {
TextField("Your name?", text: $name)
Text("Hello, \(name)")
}
}
}
struct HelloWorldView: View {
@State private var name: String = ""
var body: some View {
VStack(alignment: .leading) {
TextField("Your name?", text: $name)
Text("\(askDart(name))")
}
}
}
func askDart(_ input : String) -> String {
// ???
}
func askDart(_ input : String) -> String {
let args : [Dart_Handle] = [
Dart_NewStringFromCString(input)
]
let result =
args.withUnsafeBufferPointer { args in
return Dart_Invoke(
Dart_RootLibrary(),
Dart_NewStringFromCString("askDart"),
1,
args
)
}
// ...
}
😨 way too complicated
dart:ffi?
Accessing any C ABI code from Dart is simple
import 'dart:ffi';
typedef CString = Pointer<Utf8>;
@Native<CString Function(CString input)>()
external CString askSwift(CString input);
What if it was just as simple to export?
// hello.dart
@ffi.Export()
String askDart(String input) {
return 'Dart says: Hi, $input';
}
// HelloWorldView.swift
import Hello
// ...
Text("\(Hello.askDart(input: name))")
How???
// dart-sdk/pkg/vm/lib/transformations/ffi/export.dart
void transformLibraries(...) {
// ...
mainLibrary.addProcedure(Procedure(
Name('#ffiExports'),
ProcedureKind.Method,
FunctionNode(
ReturnStatement(StaticInvocation(
allocateExports,
Arguments([
ListLiteral(elements,
typeArgument: transformer.pointerVoidType),
ConstantExpression(
(callocField.initializer as ConstantExpression).constant),
]),
)),
returnType: coreTypes.intNonNullableRawType),
isStatic: true,
fileUri: mainLibrary.fileUri,
));
}
// Injected in Dart
CString _exported#askDart(CString input) =>
askDart(input.toDartString()).toNativeUtf8();
@pragma('vm:ffi:exports-list', ...)
@pragma('vm:entry-point')
int #ffiExports() => ffi._allocateExports([
Pointer<...>.fromFunction(_exported#askDart)
]);
// In C
struct hello_Exports {
char* (*askDart)(const char*);
char* (*makeView)();
};
char* hello_askDart(const char* input) {
return dart::embedder::
Exports<hello_Exports>()->askDart(input);
}
IMAGINE
// In Swift
DynamicView(factory: Hello.makeView)
// In Dart
import 'swiftui.dart' as swiftui;
@ffi.Export()
swiftui.View makeView() => swiftui.VStack([
swiftui.Text('Hello, World!'),
]).toSwift();