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digitalmars.D.learn - Any chance to avoid monitor field in my class?

reply "Yuriy" <yuriy.glukhov gmail.com> writes:
Hello, is there a way of reducing size of an empty class to just 
vtbl? I tried to declare it as extern(C++) which works, but has a 
nasty side effect of limited mangling.
May 07 2014
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Yuriy:

 Hello, is there a way of reducing size of an empty class to 
 just vtbl? I tried to declare it as extern(C++) which works, 
 but has a nasty side effect of limited mangling.
extern(C++) classes is only for interfacing with C++ code. It's not to write regular D code. Bye, bearophile
May 07 2014
prev sibling next sibling parent "Rene Zwanenburg" <renezwanenburg gmail.com> writes:
On Wednesday, 7 May 2014 at 14:44:57 UTC, Yuriy wrote:
 Hello, is there a way of reducing size of an empty class to 
 just vtbl? I tried to declare it as extern(C++) which works, 
 but has a nasty side effect of limited mangling.
May I ask what your use case is? Perhaps there's another solution to the problem.
May 07 2014
prev sibling next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Wed, 07 May 2014 10:44:55 -0400, Yuriy <yuriy.glukhov gmail.com> wrote:

 Hello, is there a way of reducing size of an empty class to just vtbl? I  
 tried to declare it as extern(C++) which works, but has a nasty side  
 effect of limited mangling.
The de-facto minimum size of a class is 16 bytes, due to the minimum block size of the heap. 8 bytes vtbl pointer on 64-bit systems would still allocate into 16-byte blocks. -Steve
May 08 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Thursday, 8 May 2014 at 14:57:37 UTC, Steven Schveighoffer 
wrote:
 The de-facto minimum size of a class is 16 bytes, due to the 
 minimum block size of the heap.

 8 bytes vtbl pointer on 64-bit systems would still allocate 
 into 16-byte blocks.

 -Steve
Yes, but still the question remains open for non-empty classes (e.g. want to use a 64bit useful payload), and for _emplacing_ any classes anywhere (e.g. on stack). Afaiu, there's no solution except for declaring extern(C++) (yes, i know, it's a hack), and it will not work, if a class is templated on something which can not be cpp-mangled. So the question is: is there any reason why this is not possible? I mean, maybe this question was closed long before. Also, do shared classes actually require monitors?
May 08 2014
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 08 May 2014 13:21:07 -0400, Yuriy <yuriy.glukhov gmail.com> wrote:

 On Thursday, 8 May 2014 at 14:57:37 UTC, Steven Schveighoffer wrote:
 The de-facto minimum size of a class is 16 bytes, due to the minimum  
 block size of the heap.

 8 bytes vtbl pointer on 64-bit systems would still allocate into  
 16-byte blocks.

 -Steve
Yes, but still the question remains open for non-empty classes (e.g. want to use a 64bit useful payload), and for _emplacing_ any classes anywhere (e.g. on stack).
To what end? What are you trying to save?
 Afaiu, there's no solution except for declaring extern(C++) (yes, i  
 know, it's a hack), and it will not work, if a class is templated on  
 something which can not be cpp-mangled. So the question is: is there any  
 reason why this is not possible? I mean, maybe this question was closed  
 long before.
It would not be derived from Object, which has the field. In other words, this would crash: synchronized(cast(Object)obj) { ... }
 Also, do shared classes actually require monitors?
Perhaps you meant unshared classes? No, they don't, but a monitor is only allocated on demand, so you don't have to worry about it. However, note that it is perfectly acceptable to cast to and from shared as long as you guarantee the uniqueness of the reference. If the monitor is not present, this can cause problems. -Steve
May 08 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Thursday, 8 May 2014 at 17:49:01 UTC, Steven Schveighoffer 
wrote:
 To what end? What are you trying to save?
I'm trying to reimplement std.variant in a nice OOP way, that supports CTFE, zero-size and a minimal amount of void*-casts. For that i'm using my VariantPayload(T) class, which i want to be as small as possible, as this is supposed to be an utility class which you never know how will be used.
 It would not be derived from Object, which has the field. In 
 other words, this would crash:

 synchronized(cast(Object)obj) { ... }
Wouldn't cast(Object) return null here, so that synchronized will throw or assert or smth? I see no reason for a crash.
 Perhaps you meant unshared classes? No, they don't, but a 
 monitor is only allocated on demand, so you don't have to worry 
 about it.
Errm.. I'm not sure i understand the subject correctly, but according to Alexandrescu's book, a class declared as shared does not require synchronized() over it. I mean, it manages it's synchronization inside itself, and it's user just has to trust it. And if so, why ever synchronizing() on it?
May 08 2014
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 08 May 2014 14:17:42 -0400, Yuriy <yuriy.glukhov gmail.com> wrote:

 On Thursday, 8 May 2014 at 17:49:01 UTC, Steven Schveighoffer wrote:
 To what end? What are you trying to save?
I'm trying to reimplement std.variant in a nice OOP way, that supports CTFE, zero-size and a minimal amount of void*-casts. For that i'm using my VariantPayload(T) class, which i want to be as small as possible, as this is supposed to be an utility class which you never know how will be used.
Well, I don't think I know enough to judge whether what you are doing is worthwhile... But my question more was about where do you plan to put so many of these objects that you will save a significant amount of bytes, aside from the heap (which already uses 16-byte blocks).
 It would not be derived from Object, which has the field. In other  
 words, this would crash:

 synchronized(cast(Object)obj) { ... }
Wouldn't cast(Object) return null here, so that synchronized will throw or assert or smth? I see no reason for a crash.
Then what is this object? All D objects derive from Object.
 Perhaps you meant unshared classes? No, they don't, but a monitor is  
 only allocated on demand, so you don't have to worry about it.
Errm.. I'm not sure i understand the subject correctly, but according to Alexandrescu's book, a class declared as shared does not require synchronized() over it. I mean, it manages it's synchronization inside itself, and it's user just has to trust it. And if so, why ever synchronizing() on it?
The meaning of shared is not well defined. Even TDPL is outdated on this. The idea in the book is that shared types would use memory barriers to ensure correct ordering of access, and correct data access. But it does not prevent races for multiple threads, you still need synchronized. Unshared objects, on the other hand, should not ever need synchronization tools, since only one thread has access! -Steve
May 08 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
 But my question more was about where do you plan to put so many 
 of these objects that you will save a significant amount of 
 bytes, aside from the heap (which already uses 16-byte blocks).
Hm.. Stack/emplace, arrays, n-dimensional arrays? :) Besides, if we're talking of D as a system language to replace C++ and to scratch everything out of a silicon wafer (also think of embedded platforms here), it's crucial for me to be able to control such things. From my experience, in a 5000-class project you would have about 20 classes that need to be synchronized on. Moreover, mutex synchronization is not in fashion nowadays, as we tend to use transitional synchronization. And so my 4980 classes will contain an extra field i don't use. What?? =)
 It would not be derived from Object, which has the field. In 
 other words, this would crash:
Those are your words.
 Then what is this object? All D objects derive from Object.
Those are your words also =)
 The meaning of shared is not well defined. Even TDPL is 
 outdated on this.

 The idea in the book is that shared types would use memory 
 barriers to ensure correct ordering of access, and correct data 
 access. But it does not prevent races for multiple threads, you 
 still need synchronized.
Yes, i understand that. By implementing a shared class, you're on your own with syncing, but also you tell the user, that your class doesn't need to be synchronized on. Right?
 Unshared objects, on the other hand, should not ever need 
 synchronization tools, since only one thread has access!
Here's two use-cases. class A {} shared class B {} // Somewhere in code { shared A sharedA; // This would need synchronized() on access. A unsharedA; // This would not. But since, the class is defined as unshared, we still will have __monitor in it, and that is good, since we can cast between unshared A and shared A. B b; shared B sharedB; // Here in both cases we know, that we will never need to sync on b or sharedB, as both of those are "thread safe" (it's not our business, how they do it, but they kinda are). So do we need this __monitor, which will never be used actually? }
May 08 2014
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 08 May 2014 15:47:46 -0400, Yuriy <yuriy.glukhov gmail.com> wrote:

 But my question more was about where do you plan to put so many of  
 these objects that you will save a significant amount of bytes, aside  
 from the heap (which already uses 16-byte blocks).
Hm.. Stack/emplace,
How many of these? In order to justify saving 8 bytes per instance, you have have a lot. I don't see emplacing thousands or tens of thousands of objects on the stack.
 arrays, n-dimensional arrays?
Arrays of objects are stored as arrays of object references, with each one pointing at a separate block on the heap.
 :) Besides, if we're talking of D as a system language to replace C++  
 and to scratch everything out of a silicon wafer (also think of embedded  
 platforms here), it's crucial for me to be able to control such things.  
 From my experience, in a 5000-class project you would have about 20  
 classes that need to be synchronized on. Moreover, mutex synchronization  
 is not in fashion nowadays, as we tend to use transitional  
 synchronization. And so my 4980 classes will contain an extra field i  
 don't use. What?? =)
In D, class is not used for such things, struct is.
 It would not be derived from Object, which has the field. In other  
 words, this would crash:
Those are your words.
I'm assuming you want D classes, but without the monitor object. D classes derive from Object.
 Then what is this object? All D objects derive from Object.
Those are your words also =)
"Any chance to avoid monitor field in my class?" Those are your words. What is it that you want?
 The meaning of shared is not well defined. Even TDPL is outdated on  
 this.

 The idea in the book is that shared types would use memory barriers to  
 ensure correct ordering of access, and correct data access. But it does  
 not prevent races for multiple threads, you still need synchronized.
Yes, i understand that. By implementing a shared class, you're on your own with syncing, but also you tell the user, that your class doesn't need to be synchronized on. Right?
A defined shared class I think is supposed to imply that all its methods are shared (meaning the 'this' pointer must be shared). It does not imply that they are thread safe.
 Unshared objects, on the other hand, should not ever need  
 synchronization tools, since only one thread has access!
Here's two use-cases. class A {} shared class B {} // Somewhere in code { shared A sharedA; // This would need synchronized() on access. A unsharedA; // This would not. But since, the class is defined as unshared, we still will have __monitor in it, and that is good, since we can cast between unshared A and shared A. B b; shared B sharedB; // Here in both cases we know, that we will never need to sync on b or sharedB, as both of those are "thread safe" (it's not our business, how they do it, but they kinda are). So do we need this __monitor, which will never be used actually? }
shared != thread safe. You still need to synchronize -Steve
May 08 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
 How many of these? In order to justify saving 8 bytes per 
 instance, you have have a lot. I don't see emplacing thousands 
 or tens of thousands of objects on the stack.
Ok, i guess i have to agree with you. But. Why are you protecting __monitors so eagerly? :)
 Arrays of objects are stored as arrays of object references, 
 with each one pointing at a separate block on the heap.
Or again you can emplace them in the heap, so that they occupy a continuous chunk.
 In D, class is not used for such things, struct is.
But classes have vtbls which is an ultimate feature for me, and moreover, it works in ctfe, while "reinventing" vtbls for ctfe might be a challenging task.
 I'm assuming you want D classes, but without the monitor 
 object. D classes derive from Object.
 "Any chance to avoid monitor field in my class?" Those are your 
 words. What is it that you want?
Thats right. I was saying that extern(C++) almost suits me except for it's mangling. And you said that extern(C++) classes are not derived from Object? But still such objects would have RTTI which will forbid casting to Object, wouldn't they?
 shared != thread safe. You still need to synchronize
Ok. So shared class is not a reason to omit __monitor field, as it still can be used, am i right here?
May 08 2014
next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Yuriy:

 But. Why are you protecting __monitors so eagerly? :)
Also take a look at the Rust language, that avoids some of your problems :-) Bye, bearophile
May 08 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
 Also take a look at the Rust language, that avoids some of your 
 problems :-)
Done already =). Rust is great, but I like D, and i strongly believe it's the next big language. If only it could allow a bit more tweaks ;)
May 08 2014
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Yuriy:

 but I like D, and i strongly believe it's the next big language.
Oh, good. Do you want to briefly explain why? :) Bye, bearophile
May 09 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
Imho, offtop, also i'm a C++/Obj-C guy and that might partially 
explain my preferences, but here are some more reasons:
1. I like the concept of CT-reflection and CTFE a lot. This makes 
metaprogramming extremely powerful without any RT overheads. It 
brings a lot more control to what goes to RT. I guess D still 
needs to shrink it's runtime a bit more, and __monitors is just 
another example of that.
2. It's extremely easy for C++/C#/Java/Objc-C developers to 
switch to D without loosing any bit of their productivity, but 
gaining lots of possibilities, that can be used in future. And 
C++/C#/Java/Obj-C is the majority of the world now. Even PHP 
developers should think of D one day =).
3. That's the most arguable, but D's syntax and semantics looks 
much cleaner and uniform to me than Rust's.
May 09 2014
parent reply "flamencofantasy" <flamencofantasy gmail.com> writes:
One thing I hate about C# (which is what I use professionally) is
the sync block index in every single class instance. Why not have
the developer decide when he needs a Monitor and manually use
it?! I am disappointed D took the same route.
May 09 2014
next sibling parent "John Colvin" <john.loughran.colvin gmail.com> writes:
On Friday, 9 May 2014 at 14:56:21 UTC, flamencofantasy wrote:
 One thing I hate about C# (which is what I use professionally) 
 is
 the sync block index in every single class instance. Why not 
 have
 the developer decide when he needs a Monitor and manually use
 it?! I am disappointed D took the same route.
If it can be changed without breaking existing code, you might be able to convince people to make it somehow optional or elided when unnecessary.
May 09 2014
prev sibling parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
flamencofantasy, thanx for that! Where do we vote here? =)
May 09 2014
parent "Yuriy" <yuriy.glukhov gmail.com> writes:
Moving further discussion to here: 
http://forum.dlang.org/thread/xpliectmvwrwthamquke forum.dlang.org
May 13 2014
prev sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 08 May 2014 17:05:56 -0400, Yuriy <yuriy.glukhov gmail.com> wrote:

 How many of these? In order to justify saving 8 bytes per instance, you  
 have have a lot. I don't see emplacing thousands or tens of thousands  
 of objects on the stack.
Ok, i guess i have to agree with you. But. Why are you protecting __monitors so eagerly? :)
I'm not, I'm trying to help you justify the path your taking :) Because where it's currently leading is somewhere that D doesn't support. This means in order to support it, you have to maintain a parallel compiler, or somehow convince the compiler writers to add such support. Neither of these burdens is small.
 Arrays of objects are stored as arrays of object references, with each  
 one pointing at a separate block on the heap.
Or again you can emplace them in the heap, so that they occupy a continuous chunk.
This is not a good idea. The dtors of classes in the GC is stored per block, not per chunk of a block.
 In D, class is not used for such things, struct is.
But classes have vtbls which is an ultimate feature for me, and moreover, it works in ctfe, while "reinventing" vtbls for ctfe might be a challenging task.
Removing the monitor could also prove quite challenging. I don't doubt your reasons, but then again, you have what you have right now in D. Asking for more, you have to provide it, or convince others to. If it's the latter, you need to make a very very strong case.
 I'm assuming you want D classes, but without the monitor object. D  
 classes derive from Object.
 "Any chance to avoid monitor field in my class?" Those are your words.  
 What is it that you want?
Thats right. I was saying that extern(C++) almost suits me except for it's mangling. And you said that extern(C++) classes are not derived from Object? But still such objects would have RTTI which will forbid casting to Object, wouldn't they?
extern(C++) objects are not considered D objects. A D object can implement a C++ interface, but once you get to the C++ interface, you cannot go back. This sounds to me very different from your goal. -Steve
May 08 2014
parent "Yuriy" <yuriy.glukhov gmail.com> writes:
 I don't doubt your reasons, but then again, you have what you 
 have right now in D. Asking for more, you have to provide it, 
 or convince others to. If it's the latter, you need to make a 
 very very strong case.
I want to provide it, but before i do, i want to know if there were any decisions made earlier, that would render my work useless. I mean, i have to know all possible cons for not having __monitor in an instance. What i suggest is the following: - Object does not have any __monitor field by default. - One can add a __monitor object to his class. - Offset to monitor is stored in TypeInfo. -1 if doesn't exist. - synchronized() inspects typeInfo. If an object has monitor, then it is used. Otherwise, the monitor is allocated/looked up in a global hash-table from object pointer to monitor. This way we could achieve performance in terms of both speed and memory. Also old code would not break. Some additional optimizations might always include a __monitor field to a class, if compiler can prove, that this class is being synchronized on.
May 08 2014
prev sibling next sibling parent "John Colvin" <john.loughran.colvin gmail.com> writes:
On Wednesday, 7 May 2014 at 14:44:57 UTC, Yuriy wrote:
 Hello, is there a way of reducing size of an empty class to 
 just vtbl? I tried to declare it as extern(C++) which works, 
 but has a nasty side effect of limited mangling.
Just a general FYI: Classes are relatively heavyweight in D. struct-based approaches are often favoured, as can be seen across the more heavily developed parts of phobos.
May 08 2014
prev sibling parent reply "Daniel Murphy" <yebbliesnospam gmail.com> writes:
"Yuriy"  wrote in message news:uflaemdlxvavfmvkbudq forum.dlang.org... 

 Hello, is there a way of reducing size of an empty class to just 
 vtbl? I tried to declare it as extern(C++) which works, but has a 
 nasty side effect of limited mangling.
What exactly is the mangling problem with extern(C++) classes?
May 13 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Tuesday, 13 May 2014 at 17:09:01 UTC, Daniel Murphy wrote:
 What exactly is the mangling problem with extern(C++) classes?
Can't use D arrays (and strings) as function argument types. Can't use D array types as template arguments. extern (C++) MyClass(T) { } MyClass!string a; // Mangling error
May 13 2014
next sibling parent reply "Daniel Murphy" <yebblies nospamgmail.com> writes:
On Tuesday, 13 May 2014 at 17:41:42 UTC, Yuriy wrote:
 On Tuesday, 13 May 2014 at 17:09:01 UTC, Daniel Murphy wrote:
 What exactly is the mangling problem with extern(C++) classes?
Can't use D arrays (and strings) as function argument types. Can't use D array types as template arguments. extern (C++) MyClass(T) { } MyClass!string a; // Mangling error
I'm not getting any errors with the development head. What os/compiler version?
May 14 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Wednesday, 14 May 2014 at 08:47:38 UTC, Daniel Murphy wrote:
 I'm not getting any errors with the development head.  What 
 os/compiler version?
Hm, now that's strange. Building with latest public version seems to work. However, development head is doing the following: $ ./test.d Error: ICE: Unsupported type string Assertion failed: (0), function visit, file cppmangle.c, line 440. I'm using MacOS 10.9.2. The test.d is: http://dpaste.dzfl.pl/2aa4ca932be1
May 14 2014
parent reply "Daniel Murphy" <yebbliesnospam gmail.com> writes:
"Yuriy"  wrote in message news:rfirqtgbparjbqxwtppo forum.dlang.org...

 On Wednesday, 14 May 2014 at 08:47:38 UTC, Daniel Murphy wrote:
 I'm not getting any errors with the development head.  What os/compiler 
 version?
Hm, now that's strange. Building with latest public version seems to work. However, development head is doing the following:
Never mind I can reproduce the bug with master, I probably ran 'dmd test.d' instead of './dmd test.d' after building dmd. This version seems to compile - the new manger can't handle extern(C++) functions with D arrays as arguments or return types. extern(C++) class A(T) { extern(D): string hi() { return "asdf"; } } void main() { A!string a; } Only the subset of extern(C++) required to interface with actual C++ code has been tested at all, so using it with D-only types is going to be fairly unpleasant.
May 15 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Thursday, 15 May 2014 at 11:51:38 UTC, Daniel Murphy wrote:
 This version seems to compile - the new manger can't handle 
 extern(C++) functions with D arrays as arguments or return 
 types.
Ok, i can understand that, but what about this one: http://dpaste.dzfl.pl/6a9961e32e6d It doesn't use d arrays in function interfaces. Should it work?
May 15 2014
parent "Daniel Murphy" <yebbliesnospam gmail.com> writes:
"Yuriy"  wrote in message news:klosrzuxwmvilupzzuvm forum.dlang.org... 

 Ok, i can understand that, but what about this one:
 http://dpaste.dzfl.pl/6a9961e32e6d
 It doesn't use d arrays in function interfaces. Should it work?
Similar problem, D arrays cannot be mangled correctly with C++ mangling. This compiles: extern(C++) interface I { extern(D): int hi(); } extern(C++) class A(T) : I { extern(D): override int hi() { return 0; } } void main() { A!string a; }
May 16 2014
prev sibling parent reply "Dejan Lekic" <dejan.lekic gmail.com> writes:
On Tuesday, 13 May 2014 at 17:41:42 UTC, Yuriy wrote:
 On Tuesday, 13 May 2014 at 17:09:01 UTC, Daniel Murphy wrote:
 What exactly is the mangling problem with extern(C++) classes?
Can't use D arrays (and strings) as function argument types. Can't use D array types as template arguments. extern (C++) MyClass(T) { } MyClass!string a; // Mangling error
that should not compile at all. Perhaps you thought extern(C++) interface MyClass(T) ?
May 14 2014
parent reply "Yuriy" <yuriy.glukhov gmail.com> writes:
On Wednesday, 14 May 2014 at 10:21:00 UTC, Dejan Lekic wrote:
 that should not compile at all. Perhaps you thought extern(C++) 
 interface MyClass(T) ?
Ok, how about this one? http://dpaste.dzfl.pl/04655ff6ddfd It doesn't compile either.
May 14 2014
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 05/14/2014 04:15 AM, Yuriy wrote:
 On Wednesday, 14 May 2014 at 10:21:00 UTC, Dejan Lekic wrote:
 that should not compile at all. Perhaps you thought extern(C++)
 interface MyClass(T) ?
Ok, how about this one? http://dpaste.dzfl.pl/04655ff6ddfd It doesn't compile either.
extern(C++) interface I { string hi(); } extern(C++) class A(T) : I { override string hi() { return "asdf"; } } void main() { A!string a; } /d208/f532.d(9): Error: class f532.A!(string).A cannot create C++ classes /d208/f532.d(19): Error: template instance f532.A!(string) error instantiating Yeah, that is a documented limitation when using C++ code. C++ templates cannot be used in D. "C++ Templates" here: http://dlang.org/cpp_interface.html Ali
May 14 2014
parent "Yuriy" <yuriy.glukhov gmail.com> writes:
Ali, i think that paragraph is talking about another case, which 
is not my case. I'm not trying to use C++ templates, nor to 
export a D template to C++. Besides, i guess that D template, 
implementing a C++ interface is perfectly valid, regardless it's 
template arguments, since it is instantiated on the D side 
anyway. Also i guess that such types as D arrays should not 
support mangling when used as function arguments/rettype, they 
are not compatible with C++ anyway, but when they become part of 
a class template, they should mangle somehow. IMHO.

On Wednesday, 14 May 2014 at 14:08:52 UTC, Ali Çehreli wrote:
 Yeah, that is a documented limitation when using C++ code. C++ 
 templates cannot be used in D. "C++ Templates" here:

   http://dlang.org/cpp_interface.html

 Ali
May 14 2014