On 2014-07-11 19:07, Jonathan Marler wrote:
I'm not sure how AST macros would assist in thread safety the way that
this feature would. Maybe you could elaborate?
Looking at the first example:
@thread:main
int mainThreadGlobal;
@thread:main
int main(string[] args)
{
// Start the worker thread at some point
}
@thread:worker
void workerLoop()
{
// do some work , cannot access mainThreadGlobal
}
This would be implemented as a declaration macro [1], something like this:
macro thread (Context, Ast!(string) name, Declaration decl)
{
if (decl.isVariable)
decl.attributes ~= Thread(name);
else if (decl.isCallable)
{
foreach (var ; decl.accessedVariables)
{
if (auto attr = var.getAttribute!(Thread))
if (attr.name != name)
context.compiler.error("Cannot access variable with
thread name " ~ attr.name ~ " from callable with thread name " ~ name);
}
}
return decl;
}
Usage:
@thread("main") int mainThreadGlobal;
@thread("worker") void workerLoop ();
To explain a little more, when you put a @thread:name or @sync(object)
attribute on something, the compiler will guarantee that no safe D code
will ever use that code or data unless it is either on the given thread
or can guarantee at compile time that it has synchronized on the given
object.
You mentioned making the variable thread local. So if I'm
understanding, you're saying just make it a regular global variable.
However, the point is that if you tell the compiler that it can only be
accessed by a single thread then it doesn't need to be thread local.
Real global variables are preferred over thread local for
performance/memory reasons. Their address is known at compile time and
you don't need to allocate a new instance for every thread. The only
reason for thread local variables is to alleviate problems with
multithreaded applications, but using an attribute like this would allow
someone to have the benefit of a real global variable without exposing
it to other threads fixing the synchronization issue.
Makes sense.
[1] http://wiki.dlang.org/DIP50#Declaration_macros
--
/Jacob Carlborg
