Pragma Annotations understood by dart2js

Pragmas for general use

Unsafe pragmas for general use

These pragmas are available for use in third-party code but are potentially unsafe. The use of these pragmas is discouraged unless the developer fully understands potential repercussions.

Pragmas for internal use

These pragmas can cause unsound behavior if used incorrectly and therefore are only allowed within the core SDK libraries.

Detailed descriptions

Annotations related to function inlining

Function (method) inlining is a compiler optimization where a call to a function is replaced with the body of the function. To perform function inlining, the compiler needs to determine that the call site calls exactly one function, the target. This is trivial for top-level methods, static methods and constructors. For calls to instance methods, the compiler does an analysis of the possible types of the receiver and uses that to reduce the set of potential targets. If there is a single target, it can potentially be inlined.

Not all functions can be inlined. For example, a recursive function cannot be expanded by inlining indefinitely. dart2js will not inline functions complex control flow, such as methods with exception handling (try-catch-finally) or many return or throw exit points.

We say a function is a viable inlining candidate when it is the single target and it is possible to perform the inlining.

One benefit of inlining is that the execution cost of performing the call is avoided, which can be a substantial part of the total cost of the call when the body of the callee is simple. Copying instructions from the callee into the caller can create more opportunities for optimization, for example, it becomes possible to recognize and remove repeated operations.

The compiler automatically makes a decision whether or not to inline a function or method based on heuristics. One heuristic is to inline if the inlined code is likely to be smaller that the call, as this results in a smaller and faster program. Another heuristic is to inline even if the code is likely to be slightly larger when the call is in a loop, as loops here is a chance that some of the code can be hoisted out of the loop.

The annotations described below allow the developer to override the default decisions. They should be used sparingly since it is likely that over time manual overrides will become increasingly out of date and mismatched with the evolving capabilities of the compiler.

Requesting a function be inlined

@pragma('dart2js:tryInline')

@pragma('dart2js:prefer-inline') // Alias for the above annotation.

This annotation may be placed on a function or method.

The compiler will inline the annotated function wherever it is a viable inlining candidate.

Requesting a function never be inlined

@pragma('dart2js:noInline')

@pragma('dart2js:never-inline') // Alias for the above annotation.

This annotation may be placed on a function or method to prevent the function from being inlined.

Disabling inlining

@pragma('dart2js:disable-inlining')

This annotation may be placed on a function or method.

Function inlining is disabled at call sites within the annotated function. Inlining is disabled even when the call site has a viable inlining candidate that is annotated with @pragma('dart2js:prefer-inline').

Annotations related to run-time checks

The Dart language and runtime libraries mandate checks in various places. Checks result in some kind of Error exception being thrown. If a program has a high degree of test coverage, the developer might have some confidence that the checks will never fail. If this is the case, the checks can be disabled via command line options or annotations. Annotations override the command line settings.

Trusting (i.e. disabling) checks can lead to a smaller and faster program. The cost is highly confusing unspecified behavior in place of the Errors that would otherwise have been thrown. The unspecified behavior is not necessarily consistent between runs and includes the program execution reaching statements that are 'impossible' to reach and variables being assigned values of an 'impossible' type.

Casts

@pragma('dart2js:as:check')
@pragma('dart2js:as:trust')

These annotations may be placed on a function or method to control whether as casts in the body of the function are checked.

One use of dart2js:as:trust is to construct an unsafeCast method.

@pragma('dart2js:prefer-inline')
@pragma('dart2js:as:trust')
T unsafeCast<T>(Object? o) => o as T;

The prefer-inline pragma ensures that the function is inlined, removing the cost of the call and passing the type parameter T, and the as:trust pragma removes the code that does the check.

Downcasts

@pragma('dart2js:downcast:check')
@pragma('dart2js:downcast:trust')

These annotations may be placed on a function or method to control whether implicit downcasts in the body of the function are checked.

This is similar to the dart2js:as:check and dart2js:as:trust pragmas except it applies to implicit downcasts. Implicit downcasts are as checks that are inserted to cast from dynamic.

The unsafeCast method described above could also be written by trusting implicit downcasts.

@pragma('dart2js:prefer-inline')
@pragma('dart2js:downcast:trust')
T unsafeCast<T>(dynamic o) => o; // implicit downcast `as T`.

Trusting implicit downcasts is part of the -O3 and -O4 optimization level command line options. dart2js:downcast:check can be used to enable checking of implicit downcasts in a method when it would otherwise be trusted due to the command line options.

Late checks

Late checks - checking whether a late variable has been initialized - occur on all late variables. The checks on late instance variables (i.e. late fields) can be controlled via the following annotations.

@pragma('dart2js:late:check')
@pragma('dart2js:late:trust')

These annotations may be placed on the declaration of a late field, class, or library. When placed on a class, the annotation applies to all late fields of the class. When placed on a library, the annotation applies to all late fields of all classes in the library. dart2js:late annotations are scoped: when there are multiple annotations, the one nearest the late field wins.

In the future this annotation might be extended to apply to late local variables, static variables, and top-level variables.

Stack starts at throw

In order to generate smaller code, throw expressions are usually implemented by calling a helper method to capture the stack trace and perform the actual throw. This adds one (or occasionally two) frames to the stack trace showing the helper functions. It is possible for the stack trace to be captured directly in the method containing the throw expression. This takes a little extra code, so is opt-in via the following annotation.

@pragma('dart2js:stack-starts-at-throw')

This annotation can be placed on a method, class or library.

Annotations related to compiler optimizations

Allow CSE

EXPERIMENTAL: This annotation may be removed without notice.

This annotation may be placed on a method or getter. If, after some optimization, there are two calls to the same annotated method or getter, and the calls have the same input values, and the first call will always have happened by the time control flow reaches the second call, the compiler may choose to remove the second call and use the value returned by the first call instead.

@pragma('dart2js:allow-cse')

This annotation is intended for an external getter or method that always refers to the same thing.

Allow DCE

EXPERIMENTAL: This annotated may be removed without notice.

This annotation may be placed on a method or getter. If the result of the call to the method or getter is unused (or becomes unused due to other optimizations), the compiler may choose to remove the call.

@pragma('dart2js:allow-dce')

This annotation is intended for and external getter or method that makes no observable change.

Annotations related to deferred library loading

Load priority

This is not fully implemented. The annotation exists but has no effect.

@pragma('dart2js:load-priority', option)

Provides a string argument option forwarded to the dartDeferredLibraryLoader as the loadPriority argument.

These annotations may be placed on the import specification to change the priority for all calls to prefix.loadLibrary().

The annotation may also be placed closer to the loadLibrary() call. When placed on a method, the annotation affects all calls to prefix.loadLibrary() inside the method.

When placed on a local variable, the annotation affects all calls to prefix.loadLibrary() in the initializer of the local variable. In the following example, only prefix2 is loaded with high priority because of the annotation on the variable called "_":

    await prefix1.loadLibrary();
    @pragma('dart2js:load-priority', 'someArg')
    final _ = await prefix2.loadLibrary();
    await prefix3.loadLibrary();

dart2js:load-priority annotations are scoped: when there are multiple annotations, the one on the nearest element enclosing the call to loadLibrary() is in effect.

Declaring a static weak reference intrinsic method

@pragma('weak-tearoff-reference')
T Function()? weakRef<T>(T Function()? x) => x;

Declares a special static method weakRef which can be used to create weak references to tearoffs of static methods. Weak reference declaration should be a static method taking one positional required argument. Its return type should be nullable and should match argument type. It should be either external or return its argument (for backwards compatibility).

Compiler replaces weakRef(foo) expression with either foo if method foo() is used and retained during tree shaking, or null if foo() is only used through weak references. Target foo should be a constant tearoff of a static method without arguments.

Declaring an external effect method

@pragma('external-effect')
external void effect(Object? o);

Declares a special static external method effect which the compiler will treat as live code when performing any analysis of the program. For example, a type referenced from this call that would otherwise be tree-shaken will no longer be tree-shaken. To reduce code size the call itself (and its arguments) are dropped in the compiled output though.