On Fri, 08 Jun 2012 15:30:26 -0400, Artur Skawina <[email protected]>
wrote:
On 06/08/12 19:13, Steven Schveighoffer wrote:
On Fri, 08 Jun 2012 10:57:15 -0400, Artur Skawina <[email protected]>
wrote:
On 06/08/12 06:03, Steven Schveighoffer wrote:
I agree that the type should be shared(int), but the type should not
transfer to function calls or auto, it should be sticky to the
particular variable. Only references should be sticky typed.
The problem with this is that it should be symmetrical, IOW the
conversion
from non-shared to shared would also have to be (implicitly) allowed.
A type that converts to both would be better, even if harder to
implement.
It should be allowed (and is today).
Hmm. I think it shouldn't be. This is how it is today:
shared Atomic!int ai;
shared Atomic!(void*) ap;
void f(Atomic!int i) {} // Atomic() struct template temporarily made
unshared for this test.
void fp(Atomic!(void*) i) {}
void main() {
f(ai);
fp(ap);
}
Error: function f (Atomic!(int) i) is not callable using argument
types (shared(Atomic!(int)))
Error: cannot implicitly convert expression (ai) of type
shared(Atomic!(int)) to Atomic!(int)
Error: function fp (Atomic!(void*) i) is not callable using argument
types (shared(Atomic!(void*)))
Error: cannot implicitly convert expression (ap) of type
shared(Atomic!(void*)) to Atomic!(void*)
This is a bug (a regression, actually).
I tested this simple code:
struct S
{
int i;
}
void main()
{
shared(S) s;
S s2 = s;
}
which fails on 2.057-2.059, but passes on 2.056
Looking at the changelog, looks like there were some changes related to
shared and inout in 2.057. Those probably had a hand in this.
I'll file a bug to document this regression.
It seems to work for built-in value types, which i didn't even realize,
because the thought of
using them with 'shared' never crossed my mind. I don't really see why
those should be treated
differently from user defined types, which should not allow implicit
shared<>unshared conversions.
They shouldn't be treated differently, everything should implicitly
convert. Here is why:
shared means "shared". If you make a copy of the value, that's your
private copy, it's not shared! So there is no reason to automatically
mark it as shared (you can explicitly mark it as shared if you want, but I
contend this shouldn't be valid on stack data).
Now, a shared *Reference* needs to keep the referred data as shared. So
for instance, shared(int) * should *not* implicitly cast to int *.
It's the same rules for immutable. You can assign an immutable int to an
int no problem, because you aren't affecting the original's type, and the
two are not referencing the same data.
I am talking about stripping head-shared, so shared(int *)
automatically converts to shared(int)* when used as an rvalue.
Where would the 'shared(int*) type come from? IOW, given 'shared struct
S { int i; } S s;'
what would the type of '&s.i' be? In your model; because right now it is
'shared(int)*'.
struct S
{
int *i;
}
void main()
{
shared(S) s;
auto x = s.i;
pragma(msg, typeof(x).stringof); // prints shared(int *)
}
Right, I was thinking shared structs do not make sense, since I don't
think shared members do not make sense. Either a whole struct/class
is shared or it is not. Because you can only put classes on the
heap, shared makes sense as an attribute for a class.
But then again, it might make sense to say "this struct is only ever
shared, so it should be required to go on the heap". I like your
idea later about identifying shared struct types that should use
synchronization.
Of course shared structs make sense, it's what allows implementing any
non-trivial shared type.
static Atomic!int counter;
inside a function is perfectly fine. And, as somebody already mentioned
in this thread, omitting 'static' should cause a build failure; right
now it is accepted, even when written as
shared Atomic!int counter;
The problem? 'shared' is silently dropped. Move the counter from a
struct
into a function after realizing it's only accessed from one place,
forget
to add 'static' - and the result will compile w/o even a warning.
The difference is that static is not a type constructor.
The problem is that 'shared' is lost, resulting in an incorrect program.
When you explicitly declare something as shared the compiler better treat
it as such, or fail to compile it; silently changing the meaning is never
acceptable.
later:
That was misleading; "shared" isn't actually lost, but as the variable is
placed on the stack it becomes effectively thread local, which can be
very
unintuitive. But i can't think of an easy way to prevent this mistake,
while
still allowing shared data to be placed on the stack. And the latter can
be useful sometimes...
I don't think it's unintuitive at all. shared *is* lost because it's *no
longer shared*. It makes perfect sense to me.
I also don't think it is a mistake. I frequently use the pattern of
capturing the current state of a shared variable to work with locally
within a function. Normally, in C or C++, there is no type difference
between shared and unshared data, so it's just an int in both cases.
However, while I'm working with my local copy, I don't want it changing,
and it shouldn't be.
A mistake is to mark it shared, because then I can send it to another
thread possibly inadvertently.
e.g.:
shared int x; // typeof(x) == int
This could be made illegal, but if it is accepted then it should retain
its type.
void foo(shared int *n){...}
foo(&x); // compiler error? huh?
I think this is a no-go. Shared has to be statically disallowed for
local variables.
It's a possibility. Except _static_ local variables, those must work.
static is different, because they are not local, they are global. Again,
this comes down to a storage class vs. a type constructor.
All that is different is that the symbol is local, it's still put in the
global segment.
If 'shared(VT)' implicitly converts to VT, then
auto myY = x.y; // typeof(myY) == shared(int)
would still be fine.
No, because then &myY yields a reference to shared data on the stack,
which is what I think should be disallowed.
The only problem with shared data on the stack i can think of is
portability.
But this is something that can be decided at a much later time, it
wouldn't
be used much in practice anyway.
It's the same problem as taking addresses of stack variables. It's
allowed, and can be valid in some cases, but it will cost you dearly if
you get it wrong. You are better off allocating on the heap, or using
library constructs that know what they are doing.
But I'm not sure allowing these implicit conversions is a good idea.
At least not yet. :)
Implicit conversions to and from shared already are valid. i.e. int x
= sharedInt; is valid code.
yes, but see above. shared(BVT)->BVT and shared(P*)->shared(P)* are
allowed,
and i don't think the latter is necessarily sound. Yes, the current
shared
model practically requires this, but i don't think raw access to shared
data
is the best approach.
It's not raw access, as soon as you create an rvalue, it's no longer
aliased to the shared data. shared(P)* is it's own copy of the pointer.
In other words, it's no longer shared, so shared should be stripped.
However, what it *points* to is still shared, and still maintains the
shard attribute.
Making shared storage illegal on the stack is somewhat orthogonal to
this. While I can see where having shared stack data is useful, it's
completely incorrect to forward shared attributes on copies of data.
But it's so hard to guarantee that the stack variable storage does not go
away, especially when you have now thrown it out to another thread, which
may or may not tell you when it's done with it, that I think it should be
made illegal.
At the very least, it should be illegal in @safe code.
Note that inside synchronized() statements such conversions would be
fine.
I think you are not understanding the storage aspect.
I'm talking about changing the types of expressions, such that the
expression type is always the tail-shared version. In fact, simply
using a shared piece of data as an rvalue will convert it into a
tail-shared version.
Could you provide an example? Because I'm not sure what problem this is
supposed
to solve. Eg. what is "a shared piece of data" and where does it come
from?
If the above replies haven't responded enough, I will elaborate, let me
know (responded while reading, I probably should have read the whole post
first ;)
-Steve
