[dart2wasm] Pass arg counts from JS to Dart as i32

The JS code we generate for converting a Dart function to a function
callable from JS currently looks like this:

      _194: f => finalizeWrapper(f, function(x0,x1) {
          return dartInstance.exports._194(f,arguments.length,x0,x1)
      }),

`_194` called by this JS code is an export:

    (func $_194 (;296;) (export "_194")
        (param $callback (;0;) anyref)
        (param $argumentsLength (;1;) f64)
        (param $x1 (;2;) externref)
        (param $x2 (;3;) externref)
        (result externref)
        ...)

Here the `double` parameter current causing redundant boxing and slow
`BoxedDouble` operations, because it's compared against an `int` values
when checking whether the right number of arguments is passed to the
Dart function: (in the body of `$_194`, unoptimized)

    local.get $argumentsLength
    local.set $var5
    i32.const 75
    local.get $var5
    struct.new $BoxedDouble
    global.get $global664
    call $BoxedDouble.>=
    ...

We could compare it against double values instead of int to improve
this, but a better way is to make `argumentsLength` an `i32`, as the
argument will always be a small integer (smi), which can be passed
without allocation, so that's what we do in this CL.

The same code with this CL: (unoptimized)

    (func $_194 (;294;) (export "_194")
        (param $callback (;0;) anyref)
        (param $argumentsLengthWasmI32 (;1;) i32)
        (param $x1 (;2;) externref)
        (param $x2 (;3;) externref)
        (result externref)
      ...
      local.get $argumentsLengthWasmI32
      i64.extend_i32_s
      local.set $argumentsLength
      local.get $argumentsLength
      i64.const 2
      i64.ge_s
      ...)

Change-Id: Ib9d4d625bea896e8b680e7e5bffc02c19db0d3cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/423740
Reviewed-by: Srujan Gaddam <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/js/callback_specializer.dart

@@ -61,7 +61,7 @@ class CallbackSpecializer {
   /// Creates a callback trampoline for the given [function].
   ///
   /// This callback trampoline expects a Dart callback as its first argument,
-  /// then an integer value(double type) indicating the number of arguments
+  /// then an integer value (as `WasmI32`) indicating the number of arguments
   /// passed, then a "cast closure" if needed, followed by all of the arguments
   /// to the Dart callback as JS objects. Depending on [boxExternRef], the
   /// trampoline will `dartifyRaw` or box all incoming JS objects and then cast
@@ -81,8 +81,9 @@ class CallbackSpecializer {
     // needed.
     final callbackVariable = VariableDeclaration('callback',
         type: _util.nonNullableObjectType, isSynthesized: true);
-    final argumentsLength = VariableDeclaration('argumentsLength',
-        type: _util.coreTypes.doubleNonNullableRawType, isSynthesized: true);
+    final argumentsLengthWasmI32 = VariableDeclaration('argumentsLengthWasmI32',
+        type: InterfaceType(_util.wasmI32Class, Nullability.nonNullable),
+        isSynthesized: true);
     final castClosure = VariableDeclaration('castClosure',
         type: _util.nonNullableObjectType, isSynthesized: true);
 
@@ -104,6 +105,24 @@ class CallbackSpecializer {
     // explicitly passed by the user, and then we dispatch to a Dart function
     // with the right number of arguments.
     List<Statement> body = [];
+
+    // Convert `WasmI32` argument to Dart `int`.
+    final argumentsLength = VariableDeclaration('argumentsLength',
+        type: _util.coreTypes.intNonNullableRawType,
+        isSynthesized: true,
+        initializer: InstanceInvocation(
+            InstanceAccessKind.Instance,
+            VariableGet(
+              argumentsLengthWasmI32,
+            ),
+            Name('toIntSigned'),
+            Arguments([]),
+            interfaceTarget: _util.wasmI32ToIntSigned,
+            functionType:
+                _util.wasmI32ToIntSigned.computeSignatureOrFunctionType()));
+
+    body.add(argumentsLength);
+
     if (castClosureArguments.isNotEmpty) {
       // Call the cast closure, but only if the arity is okay. In the case where
       // the arity is not sufficient, we end up coercing `undefined` to `null`,
@@ -133,8 +152,7 @@ class CallbackSpecializer {
         i >= function.requiredParameterCount;
         i--) {
       body.add(IfStatement(
-          _util.variableCheckConstant(
-              argumentsLength, DoubleConstant(i.toDouble())),
+          _util.variableCheckConstant(argumentsLength, IntConstant(i)),
           _generateDispatchCase(
               function, callbackVariable, positionalParameters, i,
               boxExternRef: boxExternRef),
@@ -151,7 +169,7 @@ class CallbackSpecializer {
     body.add(ExpressionStatement(Throw(StringConcatenation([
       StringLiteral('Too few arguments passed. '
           'Expected ${function.requiredParameterCount} or more, got '),
-      invokeMethod(argumentsLength, _util.numToIntTarget),
+      VariableGet(argumentsLength),
       StringLiteral(' instead.')
     ]))));
     Statement functionTrampolineBody = Block(body);
@@ -167,7 +185,7 @@ class CallbackSpecializer {
         FunctionNode(functionTrampolineBody,
             positionalParameters: [
               callbackVariable,
-              argumentsLength,
+              argumentsLengthWasmI32,
               if (castClosureArguments.isNotEmpty) castClosure,
               ...positionalParameters
             ],

pkg/dart2wasm/lib/js/util.dart

@@ -33,6 +33,8 @@ class CoreTypesUtil {
   final Procedure wrapDartFunctionTarget;
   final Procedure exportWasmFunctionTarget;
   final Member wasmExternRefNullRef;
+  final Class wasmI32Class;
+  final Procedure wasmI32ToIntSigned;
 
   // Dart value to JS converters.
   final Procedure toJSBoolean;
@@ -124,6 +126,9 @@ class CoreTypesUtil {
         wasmArrayClass = coreTypes.index.getClass('dart:_wasm', 'WasmArray'),
         wasmArrayRefClass =
             coreTypes.index.getClass('dart:_wasm', 'WasmArrayRef'),
+        wasmI32Class = coreTypes.index.getClass('dart:_wasm', 'WasmI32'),
+        wasmI32ToIntSigned = coreTypes.index
+            .getProcedure('dart:_wasm', 'WasmI32', 'toIntSigned'),
         wrapDartFunctionTarget = coreTypes.index
             .getTopLevelProcedure('dart:_js_helper', '_wrapDartFunction'),
         exportWasmFunctionTarget = coreTypes.index