Indeed, in the code below
asyncWork() never returned. That's because a copy by-value of the variable go was used in the closure.
void main()
{
bool go;
void asyncWork()
{
while (!go)
{
//busy wait
}
}
Threading.Thread t = new Threading.Thread(&asyncWork);
t.Start();
go = true;
}
I am trying to avoid generating unverifiable code in this compiler project: IL does not allow managed pointers as fields in a class; ILASM accepts unmanaged pointers but that yields unverifiable code. My first instinct for closures was to use a copy for all the referenced variables, but as observed by my reader that approach did not work for multi-threaded programs.
One way to solve the problem is to use unmanaged pointers in the closure; under this implementation the example above runs correctly. There may be at least another solution: wrap each referenced variable into an object, and have both the nested function and the surrounding context share the object reference; I found it to convoluted and pursued the unmanaged pointers route instead.
This is how the generated IL looks for the D code in the example:
.module 'example'
.custom instance void [mscorlib]System.Security.UnverifiableCodeAttribute::.ctor() = ( 01 00 00 00 )
//--------------------------------------------------------------
// main program
//--------------------------------------------------------------
.method public hidebysig static void _Dmain ()
{
.entrypoint
.maxstack 3
.locals init (
[0] bool pinned 'go',
[1] class [mscorlib]System.Threading.Thread 't',
[2] class example.main.closure1 '$closure3'
)
newobj instance void example.main.closure1::.ctor()
stloc.s 2 // '$closure3'
ldloc.2 // '$closure3'
ldloca 0 // 'go'
stfld bool* 'example.main.closure1'::go2
ldloc.2 // '$closure3'
dup
ldvirtftn instance void example.main.closure1::'asyncWork' ()
newobj instance void class [mscorlib]System.Threading.ThreadStart::.ctor(object, native int)
newobj instance void [mscorlib]System.Threading.Thread::.ctor (ThreadStart)
stloc.s 1 // 't'
ldloc.1 // 't'
callvirt instance void [mscorlib]System.Threading.Thread::'Start' ()
ldc.i4 1
stloc.s 0 // 'go'
ret
}
.class private auto example.main.closure1 extends [dnetlib]core.Object
{
.method public virtual newslot hidebysig instance void 'asyncWork' ()
{
.maxstack 2
L1_example:
ldarg.0
ldfld bool* 'example.main.closure1'::go2
ldind.i1
ldc.i4 0
beq L1_example
ret
}
.field public bool* go2
// default ctor, compiler-generated
.method public hidebysig instance void .ctor()
{
ldarg.0
call instance void [dnetlib]core.Object::.ctor()
ret
}
} // end of example.main.closure1
Now there is only one more small problem to address: synchronization between threads...
.
