digitalmars.D - My Language Feature Requests
- Craig Black (45/45) Dec 22 2007 So now that the const thing seems like it might be settled, I would like...
- Craig Black (2/4) Dec 22 2007 Another option would be to only allow fixed pointers to point to the hea...
- Craig Black (3/7) Dec 22 2007 When I said "heap", I meant the GC heap of course.
- Christopher Wright (46/88) Dec 22 2007 Don't see the point of this. You'd map a single old value to a single
- Craig Black (43/128) Dec 22 2007 I'm not exactly sure what you are talking about, but you mention computa...
- Craig Black (4/5) Dec 22 2007 Sorry ... I use C++ a lot at work. Should read:
- Christopher Wright (9/123) Dec 23 2007 It requires you to store a struct by reference. Thus, performance hit.
- Craig Black (6/14) Dec 23 2007 No it doesn't. Structs will be able to be allocated on the stack, witho...
- Christopher Wright (30/50) Dec 23 2007 Slicing problem:
- Craig Black (23/39) Dec 23 2007 Right, but that's not a problem if you disallow polymorphism for stack
- Christopher Wright (42/91) Dec 23 2007 Ideally you'd determine whether your polymorphic struct has inheritors
- Craig Black (22/86) Dec 23 2007 That's the approach for both C++ structs and classes. But this decision...
- Frits van Bommel (11/18) Dec 23 2007 Since "fixedness" as proposed would be a compile-time property, and you
- Christopher Wright (11/30) Dec 23 2007 Yes, I thought of that. Currently, however, offTi isn't populated. Just
- Christopher Wright (15/39) Dec 23 2007 You are just making sure that the garbage collector is handling all
- Craig Black (12/36) Dec 23 2007 It has nothing to do with the garbage collector run-time stuff. It is
- Craig Black (4/8) Dec 23 2007 I hereby detract this statement. These run-time checks could be optiona...
- Christopher Wright (20/64) Dec 23 2007 The point of overloading new and delete is to work around the garbage
- Craig Black (7/23) Dec 23 2007 I was not proposing that anyone rely on "manually created objects withou...
So now that the const thing seems like it might be settled, I would like to put my vote in as to what features should be included next. I can only think of 2 features that would be high on my priority list. My votes go for the following language features that would enhance the performance of D. 1) Adding better support for structs including ctors/dtors, inheritance, copy semantics, etc. This would allow for more efficient data structures that perform heap allocation without relying on the GC. Making applications GC-lite is fastest path to high-performance D applications. The big reason I want this is that I would then be able to write an efficient array template that does not rely on GC. Since I use arrays so much, this would provide a huge performance improvement for me. 2) Adding language features that would allow for a moving GC. A modern, moving GC would also be a huge performance win. I think we would have a safety problem if we currently implemented a moving GC. Languages that have moving GC greatly restrict what can be done with pointers. We need to provide a syntax that will allow pointers to be used when memory is explicitly managed, but disallow pointers for GC memory. So, here's one idea for making D more safe for moving GC. a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope. b) Allow new and delete to work with structs, and allocate them on the malloc heap. I would still want to be able to override new and delete for structs, specifically to be able to use nedmalloc. Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance. fixed keyword. In D, it could work like this: a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap. b) Pointer arithmetic would be disallowed for fixed pointers. c) A fixed pointer will mark the corresponding GC object as "pinned" so that the GC knows not to move the object. d) When the fixed pointer is changed or deallocated, it will unpin the object, and pin any new object that it refers to. The fixed pointer will have to know whether or not it points to GC memory so that it doesn't pin non-GC objects. Using the first idea, we can determine at compile time whether a pointer points to the heap or not. Yes, this would be a big change, but not as big as const IMO. I feel if any feature warrants breaking some code, it would be high-performance GC. But maybe someone else can find a solution that doesn't break compatibility. Thoughts? -Craig
Dec 22 2007
Using the first idea, we can determine at compile time whether a pointer points to the heap or not.Another option would be to only allow fixed pointers to point to the heap. It might simplify the implementation.
Dec 22 2007
"Craig Black" <craigblack2 cox.net> wrote in message news:fkk79p$226d$1 digitalmars.com...When I said "heap", I meant the GC heap of course.Using the first idea, we can determine at compile time whether a pointer points to the heap or not.Another option would be to only allow fixed pointers to point to the heap. It might simplify the implementation.
Dec 22 2007
Craig Black wrote:2) Adding language features that would allow for a moving GC. A modern, moving GC would also be a huge performance win. I think we would have a safety problem if we currently implemented a moving GC. Languages that have moving GC greatly restrict what can be done with pointers. We need to provide a syntax that will allow pointers to be used when memory is explicitly managed, but disallow pointers for GC memory. So, here's one idea for making D more safe for moving GC. a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.Don't see the point of this. You'd map a single old value to a single new value...or map an old range to a new one. You're changing one equality check and one assignment to two comparisons and an addition. And this is when you're looking through the entire address space of the program.b) Allow new and delete to work with structs, and allocate them on the malloc heap. I would still want to be able to override new and delete for structs, specifically to be able to use nedmalloc.This can allow polymorphism for structs, actually, but it is a bit of a performance hit.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.Ugly. What do you do for taking the address of a class variable? Well, okay, you have to take the address of the reference; you can't take the address of the variable directly. The current method is ugly and undefined behavior: *cast(void**)&obj; And you can assume that all pointers that point to that region of memory have to be moved. The problem is granularity. class Foo { Foo next; size_t i, j, k, l, m, n, o, p; } Here, the current regime would mark *Foo as hasPointers. If i, j, k, l, m, n, o, or p just happened to look like a pointer, they'd be changed. You'd need to find where each object begins, then you'd need to go through the offset type info to see which elements are really pointers. Since you're running the garbage collector, that's doable, if the offset type info is currently available (I think it wasn't, last I checked, but I don't really recall).the fixed keyword. In D, it could work like this: a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap. b) Pointer arithmetic would be disallowed for fixed pointers.Why? fixed float* four_floats = std.gc.malloc(4 * float.sizeof); fixed float* float_one = four_floats; fixed float* float_two = four_floats + 1; fixed float* float_three = four_floats + 2; fixed float* float_four = four_floats + 3; Seems fine to me. You might go beyond the allocated space, but that's already undefined behavior.c) A fixed pointer will mark the corresponding GC object as "pinned" so that the GC knows not to move the object. d) When the fixed pointer is changed or deallocated, it will unpin the object, and pin any new object that it refers to.While there is a fixed reference to the GC object, it is pinned. If that reference is rebound to another GC object, the original object is unpinned and the new one is pinned. How to mark these is a difficult problem. On a 64-bit machine, I'd say you just use the most significant bit as a flag; you're not going to use petabytes of address space.The fixed pointer will have to know whether or not it points to GC memory so that it doesn't pin non-GC objects. Using the first idea, we can determine at compile time whether a pointer points to the heap or not.The fixed pointer will just stand there shouting "I am a fixed pointer! Look on me and despair!" And the garbage collector will look where it's pointing; if it is pointing at GC memory, the garbage collector will indeed look on it and despair. Otherwise, it will ignore the fixedness.Yes, this would be a big change, but not as big as const IMO. I feel if any feature warrants breaking some code, it would be high-performance GC. But maybe someone else can find a solution that doesn't break compatibility. Thoughts? -Craig
Dec 22 2007
"Christopher Wright" <dhasenan gmail.com> wrote in message news:fkkm0i$2oa9$1 digitalmars.com...Craig Black wrote:I'm not exactly sure what you are talking about, but you mention computation performed at run-time. The concept here is that it will be a compile-time restriction. The reason to disallow new and delete is to ensure that all instances of a class not instantiated using "scope" will be GC objects. This gives the compiler the information necessary to enforce pointer assignment restrictions at compile-time.2) Adding language features that would allow for a moving GC. A modern, moving GC would also be a huge performance win. I think we would have a safety problem if we currently implemented a moving GC. Languages that have moving GC greatly restrict what can be done with pointers. We need to provide a syntax that will allow pointers to be used when memory is explicitly managed, but disallow pointers for GC memory. So, here's one idea for making D more safe for moving GC. a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.Don't see the point of this. You'd map a single old value to a single new value...or map an old range to a new one. You're changing one equality check and one assignment to two comparisons and an addition. And this is when you're looking through the entire address space of the program.Yes, polymorphism for structs could be allowed. I don't know why you think that would be a performance hit. C++ structs and classes allow polymorphism, but do not take any performance hit or memory overhead when polymorphism is not used. If polymorphism is used, it doesn't affect the performance of non-polymorphic functions, and only requires a pointer to be stored in each object in order to reference the vtable. Maybe you think I am implying that ALL structs will be allocated on the malloc heap. No, no, no. I am suggesting that a struct could be allocated on the heap or on the stack. How would the syntax look? Structs allocated on the stack would retain the same syntax. The ones allocated on the heap would be allocated with the new operator. These could be referenced using pointers, or maybe some form of reference type. But the reference types would need to be explicitly declared like. struct A { A(int x) {} } A a = A(1); // stack allocation A *a = new A(1); // possible syntax for heap allocation A &a = new A(1); // another possible syntax, I'm sure there are other ideas.b) Allow new and delete to work with structs, and allocate them on the malloc heap. I would still want to be able to override new and delete for structs, specifically to be able to use nedmalloc.This can allow polymorphism for structs, actually, but it is a bit of a performance hit.I'm not sure you understand what I'm proposing. What you are talking about is run-time information used by the garbage collecter. I'm talking about a compile-time restriction. No checking anything at run-time, and so no performance hit. Maybe the confusion stems from the fact that I didn't describe in detail how this would work. That's because I haven't thought it through yet. But I'm confident that there is a good way this restriction could enforced at compile-time.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.Ugly. What do you do for taking the address of a class variable? Well, okay, you have to take the address of the reference; you can't take the address of the variable directly. The current method is ugly and undefined behavior: *cast(void**)&obj; And you can assume that all pointers that point to that region of memory have to be moved. The problem is granularity. class Foo { Foo next; size_t i, j, k, l, m, n, o, p; } Here, the current regime would mark *Foo as hasPointers. If i, j, k, l, m, n, o, or p just happened to look like a pointer, they'd be changed. You'd need to find where each object begins, then you'd need to go through the offset type info to see which elements are really pointers. Since you're running the garbage collector, that's doable, if the offset type info is currently available (I think it wasn't, last I checked, but I don't really recall).Ok, point taken. Pointer arithmetic might be useful. I'm just trying to make it as safe as possible, and maybe disallowing this is going too far. However, your above example could be implemented without pointer arithmetic using a static array.the fixed keyword. In D, it could work like this: a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap. b) Pointer arithmetic would be disallowed for fixed pointers.Why? fixed float* four_floats = std.gc.malloc(4 * float.sizeof); fixed float* float_one = four_floats; fixed float* float_two = four_floats + 1; fixed float* float_three = four_floats + 2; fixed float* float_four = four_floats + 3; Seems fine to me. You might go beyond the allocated space, but that's already undefined behavior.Right. Pointer is the wrong word. Sorry.c) A fixed pointer will mark the corresponding GC object as "pinned" so that the GC knows not to move the object. d) When the fixed pointer is changed or deallocated, it will unpin the object, and pin any new object that it refers to.While there is a fixed reference to the GC object, it is pinned. If that reference is rebound to another GC object, the original object is unpinned and the new one is pinned.How to mark these is a difficult problem. On a 64-bit machine, I'd say you just use the most significant bit as a flag; you're not going to use petabytes of address space.I'm not sure what the best way would be because I don't know a lot of details about D's GC.Yes, that will work, but requires a run-time check (and a branch). The run-time overhead for what you propose might end up being trivial, but I think it could be done at compile-time.The fixed pointer will have to know whether or not it points to GC memory so that it doesn't pin non-GC objects. Using the first idea, we can determine at compile time whether a pointer points to the heap or not.The fixed pointer will just stand there shouting "I am a fixed pointer! Look on me and despair!" And the garbage collector will look where it's pointing; if it is pointing at GC memory, the garbage collector will indeed look on it and despair. Otherwise, it will ignore the fixedness.Yes, this would be a big change, but not as big as const IMO. I feel if any feature warrants breaking some code, it would be high-performance GC. But maybe someone else can find a solution that doesn't break compatibility. Thoughts? -Craig
Dec 22 2007
struct A { A(int x) {} }Sorry ... I use C++ a lot at work. Should read: struct A { this(int x) {} } (Further, this code is based on the hypothesis that we may get struct ctors.)
Dec 22 2007
Craig Black wrote:"Christopher Wright" <dhasenan gmail.com> wrote in message news:fkkm0i$2oa9$1 digitalmars.com...I misplaced the text and am now feeling stupid.Craig Black wrote:I'm not exactly sure what you are talking about, but you mention computation performed at run-time. The concept here is that it will be a compile-time restriction. The reason to disallow new and delete is to ensure that all instances of a class not instantiated using "scope" will be GC objects. This gives the compiler the information necessary to enforce pointer assignment restrictions at compile-time.2) Adding language features that would allow for a moving GC. A modern, moving GC would also be a huge performance win. I think we would have a safety problem if we currently implemented a moving GC. Languages that have moving GC greatly restrict what can be done with pointers. We need to provide a syntax that will allow pointers to be used when memory is explicitly managed, but disallow pointers for GC memory. So, here's one idea for making D more safe for moving GC. a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.Don't see the point of this. You'd map a single old value to a single new value...or map an old range to a new one. You're changing one equality check and one assignment to two comparisons and an addition. And this is when you're looking through the entire address space of the program.It requires you to store a struct by reference. Thus, performance hit.Yes, polymorphism for structs could be allowed. I don't know why you think that would be a performance hit. C++ structs and classes allow polymorphism, but do not take any performance hit or memory overhead when polymorphism is not used. If polymorphism is used, it doesn't affect the performance of non-polymorphic functions, and only requires a pointer to be stored in each object in order to reference the vtable.b) Allow new and delete to work with structs, and allocate them on the malloc heap. I would still want to be able to override new and delete for structs, specifically to be able to use nedmalloc.This can allow polymorphism for structs, actually, but it is a bit of a performance hit.Maybe you think I am implying that ALL structs will be allocated on the malloc heap. No, no, no. I am suggesting that a struct could be allocated on the heap or on the stack. How would the syntax look? Structs allocated on the stack would retain the same syntax. The ones allocated on the heap would be allocated with the new operator. These could be referenced using pointers, or maybe some form of reference type. But the reference types would need to be explicitly declared like. struct A { A(int x) {} } A a = A(1); // stack allocation A *a = new A(1); // possible syntax for heap allocation A &a = new A(1); // another possible syntax, I'm sure there are other ideas.Okay, I swapped that section of text with the previous one that was out of place.I'm not sure you understand what I'm proposing. What you are talking about is run-time information used by the garbage collecter. I'm talking about a compile-time restriction. No checking anything at run-time, and so no performance hit. Maybe the confusion stems from the fact that I didn't describe in detail how this would work. That's because I haven't thought it through yet. But I'm confident that there is a good way this restriction could enforced at compile-time.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.Ugly. What do you do for taking the address of a class variable? Well, okay, you have to take the address of the reference; you can't take the address of the variable directly. The current method is ugly and undefined behavior: *cast(void**)&obj; And you can assume that all pointers that point to that region of memory have to be moved. The problem is granularity. class Foo { Foo next; size_t i, j, k, l, m, n, o, p; } Here, the current regime would mark *Foo as hasPointers. If i, j, k, l, m, n, o, or p just happened to look like a pointer, they'd be changed. You'd need to find where each object begins, then you'd need to go through the offset type info to see which elements are really pointers. Since you're running the garbage collector, that's doable, if the offset type info is currently available (I think it wasn't, last I checked, but I don't really recall).I'm not so sure. You'd have to make it undefined behavior to assign a non-fixed address to a fixed pointer. The reverse is fine, of course. Since class references are pointers, you'd have to have the fixed storage class apply to them as well. Any reference type, really.Yes, that will work, but requires a run-time check (and a branch). The run-time overhead for what you propose might end up being trivial, but I think it could be done at compile-time.The fixed pointer will have to know whether or not it points to GC memory so that it doesn't pin non-GC objects. Using the first idea, we can determine at compile time whether a pointer points to the heap or not.The fixed pointer will just stand there shouting "I am a fixed pointer! Look on me and despair!" And the garbage collector will look where it's pointing; if it is pointing at GC memory, the garbage collector will indeed look on it and despair. Otherwise, it will ignore the fixedness.
Dec 23 2007
It requires you to store a struct by reference. Thus, performance hit.No it doesn't. Structs will be able to be allocated on the stack, without any referencing. As an OPTION, you will be able to store a struct by reference. C++ does this very same thing and it is very efficient.Yes and all class fields would be fixed as well, unless the class object was instantiated using scope. This means that when you take the address of them, it results in a fixed pointer.Yes, that will work, but requires a run-time check (and a branch). The run-time overhead for what you propose might end up being trivial, but I think it could be done at compile-time.I'm not so sure. You'd have to make it undefined behavior to assign a non-fixed address to a fixed pointer. The reverse is fine, of course. Since class references are pointers, you'd have to have the fixed storage class apply to them as well. Any reference type, really.
Dec 23 2007
Craig Black wrote:Slicing problem: struct A { int i, j; } struct B : A { long k; } A foo (A a) { static assert (a.sizeof == 8); assert (a.sizeof == 8); return a; } B b; b.k = 14; assert (b.sizeof == 16); b = foo(b); assert (b.k == 14); // FAIL Polymorphic structs *have* to be reference types, unless you determine stack layout at runtime. And not only that, you have to modify stack layout after you've created a stack frame. The only saving grace is that you won't have to do that for a stack frame higher than the current one.It requires you to store a struct by reference. Thus, performance hit.No it doesn't. Structs will be able to be allocated on the stack, without any referencing. As an OPTION, you will be able to store a struct by reference. C++ does this very same thing and it is very efficient.You're saying: class Foo { int i; } Foo f = new Foo(); int* i_ptr = &f.i; That would be a compile error? f is not fixed; I don't care if the bits in i_ptr change, or the bits in the reference f. Why should I? Just because I took the address of f.i and stored it in an unfixed pointer, the garbage collector, which has full authority to change the pointer I just got, can't move *f? Why?Yes and all class fields would be fixed as well, unless the class object was instantiated using scope. This means that when you take the address of them, it results in a fixed pointer.Yes, that will work, but requires a run-time check (and a branch). The run-time overhead for what you propose might end up being trivial, but I think it could be done at compile-time.I'm not so sure. You'd have to make it undefined behavior to assign a non-fixed address to a fixed pointer. The reverse is fine, of course. Since class references are pointers, you'd have to have the fixed storage class apply to them as well. Any reference type, really.
Dec 23 2007
Polymorphic structs *have* to be reference types, unless you determine stack layout at runtime. And not only that, you have to modify stack layout after you've created a stack frame. The only saving grace is that you won't have to do that for a stack frame higher than the current one.Right, but that's not a problem if you disallow polymorphism for stack objects. This is what C++ does and it works very well. Rather than generating a run-time assertion, your code would simply not compile. If you want polymorphism then you have to instantiate then you would have to instantiate the struct on the heap. struct A { int i, j; } struct B : A { long k; } A foo (A a) { return a; } B b; b = foo(b); // compile error: instance of struct B can't be implicitly converted to an instance of struct A Anyway, this is all moot anyway, because I've thought of an easier solution. Pointers can be checked at run-time to determine if they address the GC heap. This check could be removed when compiling in release mode, so there will be no performance degradation. So there's no need to dissallow new and delete for classes and we don't need struct polymorphism.You're saying: class Foo { int i; } Foo f = new Foo(); int* i_ptr = &f.i; That would be a compile error? f is not fixed; I don't care if the bits in i_ptr change, or the bits in the reference f. Why should I? Just because I took the address of f.i and stored it in an unfixed pointer, the garbage collector, which has full authority to change the pointer I just got, can't move *f? Why?I'm not really sure what you are asking. If the GC moves the relocates f, then i_ptr no longer points the appropriate location. Isn't that obvious? Are you suggesting that the GC relocate i_ptr as well? No GC I know of relocates raw pointers, so there's probably a good technical reason why they don't. I'm not a GC expert though. -Craig
Dec 23 2007
Craig Black wrote:Ideally you'd determine whether your polymorphic struct has inheritors or base classes and, if so, put it on the heap, else put it on the stack. This is why it'd be better to keep structs as they are, but have by-value classes.Polymorphic structs *have* to be reference types, unless you determine stack layout at runtime. And not only that, you have to modify stack layout after you've created a stack frame. The only saving grace is that you won't have to do that for a stack frame higher than the current one.Right, but that's not a problem if you disallow polymorphism for stack objects. This is what C++ does and it works very well. Rather than generating a run-time assertion, your code would simply not compile. If you want polymorphism then you have to instantiate then you would have to instantiate the struct on the heap.struct A { int i, j; } struct B : A { long k; } A foo (A a) { return a; } B b; b = foo(b); // compile error: instance of struct B can't be implicitly converted to an instance of struct A Anyway, this is all moot anyway, because I've thought of an easier solution. Pointers can be checked at run-time to determine if they address the GC heap. This check could be removed when compiling in release mode, so there will be no performance degradation.That's the current system, and it's basically what I've been saying all this time. I guess I was unclear. But you can't remove the check in release mode: static import std.c.stdlib; static import std.gc; void main () { // gc memory auto o = new Object(); o = null; // not gc memory void* ptr = std.c.stdlib.malloc(128); // gc memory (and memory leak) ptr = null; ptr = std.gc.malloc(512); // At this point, no reference to o, so it's deleted. // The first malloc'd memory still exists and can't ever be // collected. ptr = std.gc.malloc(8); // Now the gc collected the previous 512-byte buffer; of course, // the 128-byte buffer still exists. }So there's no need to dissallow new and delete for classes and we don't need struct polymorphism.Well, we don't need any kind of polymorphism, but quite separate from the rest of the requests, struct polymorphism would be useful. Though I wouldn't refer to them as structs if they're polymorphic, since you really can't put them on the stack, and so they have to be reference types with value semantics.Because no language besides D allows you to take the address of an class member and has native garbage collection, and D doesn't have a moving collector. There's the Boehm collector for C++, but that's not a moving those blocks you don't have a garbage collector running. And...that's it. Maybe we'll see something revolutionary with Objective-C 2.0, but probably not, and it's not here yet. You're already moving raw pointers, so you may as well move all of them. Otherwise you're eliminating a decently general use case or causing random segfaults or losing a lot of efficiency in memory layout (which will make future collections quicker, too) for pretty much nothing.You're saying: class Foo { int i; } Foo f = new Foo(); int* i_ptr = &f.i; That would be a compile error? f is not fixed; I don't care if the bits in i_ptr change, or the bits in the reference f. Why should I? Just because I took the address of f.i and stored it in an unfixed pointer, the garbage collector, which has full authority to change the pointer I just got, can't move *f? Why?I'm not really sure what you are asking. If the GC moves the relocates f, then i_ptr no longer points the appropriate location. Isn't that obvious? Are you suggesting that the GC relocate i_ptr as well? No GC I know of relocates raw pointers, so there's probably a good technical reason why they don't. I'm not a GC expert though.
Dec 23 2007
"Christopher Wright" <dhasenan gmail.com> wrote in message news:fkmoj4$k0u$1 digitalmars.com...Craig Black wrote:That's the approach for both C++ structs and classes. But this decision is made by the programmer, not the compiler. I don't see how the compiler could do this, since it is impossible to have knowledge of subclasses that exist in an external library. The compiler would have to revert to a worst case, and put everything on the heap.Ideally you'd determine whether your polymorphic struct has inheritors or base classes and, if so, put it on the heap, else put it on the stack.Polymorphic structs *have* to be reference types, unless you determine stack layout at runtime. And not only that, you have to modify stack layout after you've created a stack frame. The only saving grace is that you won't have to do that for a stack frame higher than the current one.Right, but that's not a problem if you disallow polymorphism for stack objects. This is what C++ does and it works very well. Rather than generating a run-time assertion, your code would simply not compile. If you want polymorphism then you have to instantiate then you would have to instantiate the struct on the heap.This is why it'd be better to keep structs as they are, but have by-value classes.By-value classes is just another way to do the same thing, but inferior IMO.Jeez. It's like we've been speaking two different languages the whole time. I'm not talking about turning off the garbage collector in release mode. I'm talking about run-time checks that prohibit raw pointers from pointing to the GC heap. The same thing you said should be "undefined behavior". Thus, it would be undefined behavior in release mode, but in debug mode there would be a check. Like array bounds-checking.Anyway, this is all moot anyway, because I've thought of an easier solution. Pointers can be checked at run-time to determine if they address the GC heap. This check could be removed when compiling in release mode, so there will be no performance degradation.That's the current system, and it's basically what I've been saying all this time. I guess I was unclear. But you can't remove the check in release mode:Sorry, but I don't see the novelty of "reference types with value semantics". What would it be useful for? The reason I am pushing improvements to structs is that I know it will allow for more versatile aggregate types that aren't allocated on the heap. It's important that they are not allocated on the heap because that is more efficient. From my perspective, your proposal does nothing for performance, since there is still a heap allocation.So there's no need to dissallow new and delete for classes and we don't need struct polymorphism.Well, we don't need any kind of polymorphism, but quite separate from the rest of the requests, struct polymorphism would be useful. Though I wouldn't refer to them as structs if they're polymorphic, since you really can't put them on the stack, and so they have to be reference types with value semantics.Heck, you may be right about this. Like I said I'm no GC expert.Because no language besides D allows you to take the address of an class member and has native garbage collection, and D doesn't have a moving collector. There's the Boehm collector for C++, but that's not a moving those blocks you don't have a garbage collector running. And...that's it. Maybe we'll see something revolutionary with Objective-C 2.0, but probably not, and it's not here yet. You're already moving raw pointers, so you may as well move all of them. Otherwise you're eliminating a decently general use case or causing random segfaults or losing a lot of efficiency in memory layout (which will make future collections quicker, too) for pretty much nothing.You're saying: class Foo { int i; } Foo f = new Foo(); int* i_ptr = &f.i; That would be a compile error? f is not fixed; I don't care if the bits in i_ptr change, or the bits in the reference f. Why should I? Just because I took the address of f.i and stored it in an unfixed pointer, the garbage collector, which has full authority to change the pointer I just got, can't move *f? Why?I'm not really sure what you are asking. If the GC moves the relocates f, then i_ptr no longer points the appropriate location. Isn't that obvious? Are you suggesting that the GC relocate i_ptr as well? No GC I know of relocates raw pointers, so there's probably a good technical reason why they don't. I'm not a GC expert though.
Dec 23 2007
Christopher Wright wrote:While there is a fixed reference to the GC object, it is pinned. If that reference is rebound to another GC object, the original object is unpinned and the new one is pinned. How to mark these is a difficult problem. On a 64-bit machine, I'd say you just use the most significant bit as a flag; you're not going to use petabytes of address space.Since "fixedness" as proposed would be a compile-time property, and you already need metadata to find pointers to implement a moving GC, such a flag could be in that metadata instead of in the pointer itself. (The OffsetTypeInfo could say "there's a pointer at offset 8, of type Object, and it's fixed") If run-time pinning is used instead (where whether the GC cell pointed to by a pointer is pinned is not known at compile time), it could be a simple (synchronized) counter that starts out at 0 for each memory cell, that's incremented when pinned and decremented when unpinned. The GC is then only allowed to move cells whose counter is 0.
Dec 23 2007
Frits van Bommel wrote:Christopher Wright wrote:Yes, I thought of that. Currently, however, offTi isn't populated. Just like the Interface* that's supposed to be the first element of each interface's vtbl pointer. It would be useful if it existed, but no cheese.While there is a fixed reference to the GC object, it is pinned. If that reference is rebound to another GC object, the original object is unpinned and the new one is pinned. How to mark these is a difficult problem. On a 64-bit machine, I'd say you just use the most significant bit as a flag; you're not going to use petabytes of address space.Since "fixedness" as proposed would be a compile-time property, and you already need metadata to find pointers to implement a moving GC, such a flag could be in that metadata instead of in the pointer itself. (The OffsetTypeInfo could say "there's a pointer at offset 8, of type Object, and it's fixed")If run-time pinning is used instead (where whether the GC cell pointed to by a pointer is pinned is not known at compile time), it could be a simple (synchronized) counter that starts out at 0 for each memory cell, that's incremented when pinned and decremented when unpinned. The GC is then only allowed to move cells whose counter is 0.You would do both. During a collection, you mark each block to see if it's referenced, and mark it again if it's got a fixed reference. Then you collect every section that's not referenced and optionally move the sections that aren't marked as pinned to a more advantageous layout. If you are proposing a compile-time garbage collector, one that determines when to delete an object using static analysis, I will be quite impressed if you come up with an implementation.
Dec 23 2007
This is to fix the stuff I botched with my other reply. Craig Black wrote:a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.You are just making sure that the garbage collector is handling all memory that is associated with objects. I don't see a point to this. The collector won't try to move memory that it doesn't control. You could do bad things with overloading new/delete, but those are hardly unique situations.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.That's not necessary, since you can map a source range to a destination range. It would be a simplifying assumption that improves performance, by changing two comparisons and an addition for each pointer (plus one subtraction per move) to one comparison and one assignment for each pointer. But you're going through a large amount of memory, so that's not a serious concern, I think.a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap.It'd be undefined behavior to do otherwise. But safe as long as no collections happen before you use the pointer.The fixed pointer will have to know whether or not it points to GC memory so that it doesn't pin non-GC objects. Using the first idea, we can determine at compile time whether a pointer points to the heap or not. Yes, this would be a big change, but not as big as const IMO. I feel if any feature warrants breaking some code, it would be high-performance GC. But maybe someone else can find a solution that doesn't break compatibility. Thoughts? -Craig
Dec 23 2007
"Christopher Wright" <dhasenan gmail.com> wrote in message news:fklr1g$1uat$1 digitalmars.com...This is to fix the stuff I botched with my other reply. Craig Black wrote:It has nothing to do with the garbage collector run-time stuff. It is giving the compiler more information so that compile-time checks can be done.a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.You are just making sure that the garbage collector is handling all memory that is associated with objects. I don't see a point to this. The collector won't try to move memory that it doesn't control.You could do bad things with overloading new/delete, but those are hardly unique situations.Granted. There are so many ways to mess things up with pointers. It's hard to make a systems language "safe". I guess my approach would be to make it "safer".Unless I am missing something, this would require a run-time check for each pointer assignment or pointer arithmetic operation. Personally, I would make every effort to avoid this overhead. Pointers should be lightweight and fast.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.That's not necessary, since you can map a source range to a destination range. It would be a simplifying assumption that improves performance, by changing two comparisons and an addition for each pointer (plus one subtraction per move) to one comparison and one assignment for each pointer. But you're going through a large amount of memory, so that's not a serious concern, I think.a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap.It'd be undefined behavior to do otherwise. But safe as long as no collections happen before you use the pointer.
Dec 23 2007
Unless I am missing something, this would require a run-time check for each pointer assignment or pointer arithmetic operation. Personally, I would make every effort to avoid this overhead. Pointers should be lightweight and fast.I hereby detract this statement. These run-time checks could be optional. They could be like asserts or bounds-checking that is removed in release mode. Then all that compile-time stuff I mentioned before would be unnecessary I think.
Dec 23 2007
Craig Black wrote:"Christopher Wright" <dhasenan gmail.com> wrote in message news:fklr1g$1uat$1 digitalmars.com...The point of overloading new and delete is to work around the garbage collector. It's not smart enough, it doesn't have the knowledge about my specific problem, so I'm going to fix the problem myself. The most common situation is, I want to manually allocate the memory for the variable, and I don't want the garbage collector to know about this object.This is to fix the stuff I botched with my other reply. Craig Black wrote:It has nothing to do with the garbage collector run-time stuff. It is giving the compiler more information so that compile-time checks can be done.a) Disallow overloading new and delete for classes, and make classes strictly for GC, perhaps with an exception for classes instantiated on the stack using scope.You are just making sure that the garbage collector is handling all memory that is associated with objects. I don't see a point to this. The collector won't try to move memory that it doesn't control.I don't see that. I mean, if D didn't have arrays, you couldn't ever get an array bounds error; if it didn't have pointers, you would have trouble segfaulting; but those are too useful. I've manually created objects without using the new operator or the constructor. It's ugly. It's error-prone. Overloading new is safer, when you just want to control how the memory is allocated. (I couldn't avoid it because I didn't want to use a constructor.)You could do bad things with overloading new/delete, but those are hardly unique situations.Granted. There are so many ways to mess things up with pointers. It's hard to make a systems language "safe". I guess my approach would be to make it "safer".Undefined behavior means there are no checks preventing it, but bad things can happen if you do it, so be careful, and it isn't Walter's fault if it explodes in your face. The point is, it might be a useful thing, in which case you wouldn't want to disallow it. But either way, checking it is too expensive, so calling it undefined behavior should suffice.Unless I am missing something, this would require a run-time check for each pointer assignment or pointer arithmetic operation.Then the compiler could disallow taking the address of a class field, since we know the resulting pointer would pointer to the GC heap. Note that this would be a compile-time check, and so would not degrade run-time performance.That's not necessary, since you can map a source range to a destination range. It would be a simplifying assumption that improves performance, by changing two comparisons and an addition for each pointer (plus one subtraction per move) to one comparison and one assignment for each pointer. But you're going through a large amount of memory, so that's not a serious concern, I think.a) Preceding a pointer declaration with fixed would allow that pointer to take the address in the GC heap.It'd be undefined behavior to do otherwise. But safe as long as no collections happen before you use the pointer.
Dec 23 2007
I was not proposing that anyone rely on "manually created objects without using the new operator or the constructor". I was proposing that the capability to allocate an object on the malloc heap would be moved to structs, so that structs behaved like C++ aggregate types. However, I realize now that this is no longer necessary, because a run-time check that enforces pointer restrictions could be removed in release mode.Granted. There are so many ways to mess things up with pointers. It's hard to make a systems language "safe". I guess my approach would be to make it "safer".I don't see that. I mean, if D didn't have arrays, you couldn't ever get an array bounds error; if it didn't have pointers, you would have trouble segfaulting; but those are too useful. I've manually created objects without using the new operator or the constructor. It's ugly. It's error-prone. Overloading new is safer, when you just want to control how the memory is allocated. (I couldn't avoid it because I didn't want to use a constructor.)Undefined behavior means there are no checks preventing it, but bad things can happen if you do it, so be careful, and it isn't Walter's fault if it explodes in your face. The point is, it might be a useful thing, in which case you wouldn't want to disallow it. But either way, checking it is too expensive, so calling it undefined behavior should suffice.Again the run-time check could be removed in release mode, so no harm done.
Dec 23 2007