The compiler supports generating a source map alongside a binary using the
By default, specifying the
--sourceMap option will create a source mapping section pointing to the source map with a relative path, defaulting to
myModule being the name of the respective binary. This works when instantiating a module with WebAssembly.instantiateStreaming because the VM can obtain the absolute URL the source map is relative to from the provided
Response object, but does not work if a module is instantiated from a buffer or otherwise without a path context.
Where relative source maps cannot be used, for example if WebAssembly.instantiate is used to instantiate a module from a binary buffer, it is also possible to specify an absolute path to the source map using
--debug compiler option.
By default, the compiler will preserve any
assert(expression)s in a module, leading to an
abort if one of the respective expectations failed. These assertions can be disabled with the
--noAssert compiler option, though, essentially replacing them with
nops, doing nothing. Doing so can lead to smaller binaries once sufficiently confident that no assertions will be hit anyway, but also introduces the risk that a module explodes for no longer asserted reasons.
As mentioned above, assertions require that an implementation of the
abort interface is present, which by default is imported as
abort from within the
env module, handling aborts externally. This can be overridden by specifying a custom abort handler through
--use abort=index/myAbort (here: a function named
myAbort in the
index file) or the abort interface can be disabled completely (just trapping with an
--use abort=. The signature of the abort function, if overridden, is:
function abort(message: string | null,fileName: string | null,lineNumber: u32,columnNumber: u32): void;
The standard library provides a relatively basic
trace utility function that is imported from the host and understood by the loader. For example
trace("HERE", 2, value, otherValue);
will, by default, call the
trace function imported from the
env module with a string message and two arbitrary values that can be anything convertible to an
f64. The loader for example will log the output to console. Similar to overriding abort, the implementation can be overridden using the
--use compiler option. Signature of the trace function is:
function trace(message: string,n: i32 = 0, // number of given parameters a0 to a4a0?: f64,a1?: f64,a2?: f64,a3?: f64,a4?: f64): void;
One thing to note here is that calling
trace in top-level statements can lead to situations where memory is accessed during instantiation, hence not being able to read the message without taking the respective precautions.
Making Web Assembly Even Faster: Debugging Web Assembly Performance with AssemblyScript and a Gameboy Emulator (Aaron Turner, March 2018)