add Placement New

[WalterBright]

This implements Placement New, described in https://github.com/WalterBright/documents/blob/master/placementnew.md

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#17057"

[WalterBright]

I got it to work for some basic cases. Next comes extending it to more complex ones.

[WalterBright]

Placement new for structs seem to be working now!

[Connor-GH]

If one desires to use classes without the GC, such as in BetterC, it's just awkward to use emplace

If i'm reading this right, this will allow classes in betterC, using a kind of "allocator argument" like in $OTHER_LANGUAGES? Once upon a time I made my own internal fork of dmd that forced betterC on all D code but of course that didn't last long because it couldn't compile Phobos.

(I should clarify, my usecase is making a kernel with betterC and so far I have had to use a hacky mixin and alias this in order to use inheritance. Also, will this unlock the door for interfaces?)

[WalterBright]

Given:

T* newObject = new(allocateMem(T.sizeof)) T;

how does this look:

T* newObject = new(mallocate!T()) T;

? Where:

ref T mallocate(T)()
{
    return *cast(T*)malloc(T.sizeof);
}

Not bad! Only one dirty cast for all the new's, no &, and the user needn't use a sizeof.

[TurkeyMan]

1. I think that's the most insane permutation yet
2. I feel like you've dismissed everything I wrote :/

[TurkeyMan]

This is what a function called mallocate(T)() is expected to do:

void[] mallocate(size_t);

ref T mallocate(T)()
{
    return *new(mallocate(T.sizeof)) T;
}

No casts. It's completely @safe.

No function that returns a T should return an invalid/uninitialised T ever.

[TurkeyMan]

My whole point is, there's an opportunity here to eliminate UB, invalid memory references, and unsafe casts associated with lifetime management from the language specification entirely... and that's a HUGE deal. If you haven't understood the point of what I'm suggesting, then we need to get on a call and talk.

[WalterBright]

We both want the same thing. But realistically, there's no way to not have UB when allocating and initializing memory. That's why at some point it's going to have to not be verifiably safe by the compiler. As for void initialized objects,

S s = void;

we already have that in the language, and it is supported. But only in system code. Casting an array of void into a real object is always going to be system code. I appreciate your efforts to make this as safe as practical. But I also want the union case to be easy - this also applies to option types and sum types. And I want things to be correct without adding runtime checks. So here goes, I verified that it works:

import core.stdc.stdlib;

struct S {    int i = 1, j = 4, k = 9; }

ref void[T.sizeof] mallocate(T)() {
    return *(cast(void[T.sizeof]*) malloc(T.sizeof));
}

void main() {
    S* ps = new(mallocate!S()) S;
    assert(ps.i == 1);
    assert(ps.j == 4);
    assert(ps.k == 9);
}

I changed your proposal of a slice void[] to a static array void[sizeof(T)], so the compiler knows its size at compile time. But does it blend? Let's check the code generated:

000c:   B8 01 00 00 00           mov       EAX,1
0011:   48 89 45 F8              mov       -8[RBP],RAX
0015:   E8 00 00 00 00           call      mallocate
001a:   48 89 C3                 mov       RBX,RAX              ; RBX gets pointer to storage
001d:   48 8B 45 F8              mov       RAX,-8[RBP]
0021:   89 03                    mov       [RBX],EAX            ; .i = 1
0023:   C7 43 04 04 00 00 00     mov       dword ptr 4[RBX],4   ; .j = 4;
002a:   C7 43 08 09 00 00 00     mov       dword ptr 8[RBX],9   ; .k = 9

Efficient, compact, and no monkey business.

[TurkeyMan]

return malloc(T.sizeof)[0 .. T.sizeof] should be sufficient; you don't need that weird thunk.

This said though; you keep using malloc() as your experiment, but nobody in D calls malloc without wrapping it in a function that just returns a void[] instead of void*; all the memory allocators, (including Andrei's std API), all established long ago that we return void[] from allocation functions... this is why I'm trying to make it native compatible with that type.

And sorry, let me be more clear; yes I understand there will be unsafe escape hatches (union for instance, that's unsafe by definition), and some low-level coercions are still available and useful, but what I'm trying to achieve is where the common case (allocate and initialise) have no casts, no weird shit, so that flow can actually be completely safe.

I thought about static array briefly too. It seems workable, but it's got some ergonomic problems. You can't pass fixed arrays easily, because you need a different overload of a function for every array size, which means this will always be wrapped in something that; receives a dynamic array, does a bounds check, and then slices the appropriately bounded static array, then pass to a call like you show above.

The nice thing about a fixed array is that you can elide runtime bounds checking, which is nice, but I think the ergonomic problem carries more weight. Memory allocation is NEVER a high-frequency operation; I can't imagine any situation where I wouldn't tolerate a runtime bounds check in the void[].

Perhaps you'd consider accepting BOTH fixed array and also dynamic array; if a dynamic array was supplied, perform a runtime bounds check, static array, static bounds check?

I want to re-enforce though that my proposal is designed to work very nicely if we also make destroy return the buffer to the user (for recycling or returning to the allocator). The whole point is that in idiomatic and safe code, typed variables will never be handled at any time when they are in an invalid state. I really like that the type is only introduced to the program as a result of new; that is, the moment of its birth, and then you supply it to delete or destroy and it's gone; you get the void buffer back as a result which is the only thing you need to handle to perform a recycle, or just hand that memory back to the allocator. Your final contact with the typed value is supplying it as argument to delete.

[WalterBright]

return malloc(T.sizeof)[0 .. T.sizeof] should be sufficient; you don't need that weird thunk.

It doesn't work because it returns a void[], and its size cannot be checked at compile time. The static array works, I tried it and looked at the assembler.

Memory allocation is NEVER a high-frequency operation

Placement new has a major use case for unions, option types, and sum types, and these can be very high frequency. See CTFE, which I sped up quite a bit by using emplace on a stack allocated union. I've also written a Javascript interpreter - the operand stack is a critical high-frequency piece of code, and the operands get overwritten like any stack. As I've shown, the static array method generates optimal code, so the user doesn't need to hack around it.

we return void[] from allocation functions

Not a problem, it's still castable to a pointer to a static array, since the length is known in advance.

You can't pass fixed arrays easily, because you need a different overload of a function for every array size

I haven't implemented placement new for variable sized arrays yet. The proposed solution uses a template to take care of placement new expressions for types of fixed size. I don't think it is a big deal, it's just a few instructions.

Perhaps you'd consider accepting BOTH fixed array and also dynamic array; if a dynamic array was supplied, perform a runtime bounds check, static array, static bounds check?

I expect a different implementation will be required for new'ing variable length arrays, meaning a void[].

The whole point is that in idiomatic and safe code, typed variables will never be handled at any time when they are in an invalid state.

Placement new is never going to be safe. Prefixing a destroy() means that void-initialized objects will also be destroyed, and that's UB for sure.

[TurkeyMan]

return malloc(T.sizeof)[0 .. T.sizeof] should be sufficient; you don't need that weird thunk.

It doesn't work because it returns a void[], and its size cannot be checked at compile time. The static array works, I tried it and looked at the assembler.

No, your code you wrote:

ref void[T.sizeof] mallocate(T)() {
    return *(cast(void[T.sizeof]*) malloc(T.sizeof)); // <-- this is gross and blind cast/thunk is unsafe
}

You don't need to write that. Just write what I suggested instead; you don't need the thunk:

ref void[T.sizeof] mallocate(T)() {
    return malloc(T.sizeof)[0 .. T.sizeof]; // <-- this is not gross and it is safe
}

we return void[] from allocation functions

Not a problem, it's still castable to a pointer to a static array, since the length is known in advance.

Don't cast, just slice it to the proper length.

The whole point is that in idiomatic and safe code, typed variables will never be handled at any time when they are in an invalid state.

Placement new is never going to be safe.

I don't see why you think this... it's definitely safe. The example code you wrote above is already safe, ie:

struct S {    int i = 1, j = 4, k = 9; }

ref void[T.sizeof] mallocate(T)() {
    return malloc(T.sizeof)[0 .. T.sizeof]; // no casts, nothing nasty
}

void main() {
    S* ps = new(mallocate!S()) S;
    assert(ps.i == 1);
    assert(ps.j == 4);
    assert(ps.k == 9);
}

What's unsafe about this? It's completely fine by my reading.

Prefixing a destroy() means that void-initialized objects will also be destroyed, and that's UB for sure.

I don't know what prefixing a destroy means, but you're scaring me... nothing weird please! I'm satisfied at this point that we're going the right direction here!

[TurkeyMan]

This is the case we recently discussed... but what are all the other cases?
I thought we finally agreed on raw buffer initialisation using sized void buffers?

[WalterBright]

Whether the void array buffers or used or a variable is up to the user. I left it in because it makes it most convenient when dealing with unions / optional types / sum types. The implementation doesn't care what type it is, only that it is an lvalue of sufficient size.

I didn't add tests for other cases, as they would be redundant. A tutorial should include them, however.

[TurkeyMan]

I thought we ejected this pattern into space? Please, just no.

[WalterBright]

I responded to this in the other comment.

[Connor-GH]

This is code straight ripped from my kernel:

module memory;
import traits;
import kalloc : kmalloc, kfree;

T *d_new(T, Args...)(auto ref Args args) {
	import core.lifetime : emplace, forward;
	T *mem = cast(T *)kmalloc(T.sizeof);
	return mem.emplace(forward!args);
}
void d_delete(T)(T *data) {
	if (data is null)
		return;
	kfree(data);
}

I believe placement new would replace at least the d_new, but what about the d_delete? delete is deprecated, and in a betterC environment, I don't have access to classes and interfaces to even make an interface to the GC. In other words, I have no way of being able to use keyword new and keyword delete.

[WalterBright]

ref void[T.sizeof] mallocate(T)() {
    return malloc(T.sizeof)[0 .. T.sizeof]; // <-- this is not gross and it is safe
}

you're right, that is better. Thanks!

What's unsafe about this?

It's equivalent to this:

struct T { int* a,b,c; }

void foo(T);

@safe void test(void[24] a) {
    foo(cast(T)a); // Error: cast from `void[24]` to `T` not allowed in safe code
}

I.e. the compiler cannot tell what it's stepping on, as it does not know what the provenance of the void[] is.

[TurkeyMan]

Okay, so, what I'm suggesting by new accepting void[](and only that), is that it DOES know; it can assume (written in the spec even), that the argument it receives is raw memory, and any argument that is not raw memory is an invalid argument. It can be confident that given raw memory, it has agency to construct a new value, and it is safe to do so.

This assumption and assertion only works well when the function is specified to accept void[].
This assumption gets awkward and tenuous if new accepts a ref T, because there's nothing stopping the user from passing a live or valid T, or distinguishing it from an uninitialised T; and new can't know. But if new only accepts void[], then it knows it received raw memory, and the only way a user could pass something risky is with an explicit cast in user code, at which point the user has accepted an unsafe operation in support of their optimisation.

If the user has responsibility for the unsafe cast, then the operation itself is safe... this is quite specifically why I reject the idea that new would receive a T so vehemently; it fundamentally undermines the opportunity to make this whole thing safe, and wrecks lifetime analysis.

[WalterBright]

D allows conversion of a type to *void in @safe code, but not the other way around. Passing a void[] through new to convert it to an object is not checkably safe.

@System code is not necessarily unsafe, it just means its up to the user to ensure its safety as the compiler cannot do it. Memory allocation and initialization is always going to be uncheckable unless the language controls the whole process - like what gc new does and like what stack variables do.

If the user has responsibility for the unsafe cast, then the operation itself is safe

Sure, but it cannot be marked @safe.

Not accepting T as an argument means the user will have to cast it to void*, which will look ugly. Casting to void* is a safe operation, so it becomes an ugly construct with no added value.

Taking a T argument is useful for unions, option types, and sum types. Although you don't use them, they are popular constructs (and I use them in D and would likely use them even more with placement new!).

Note there's an inconsistency in the compiler - in @safe code, a T cannot be cast to a void[T.sizeof], but an &T can be cast to void*. https://issues.dlang.org/show_bug.cgi?id=24866

The ref T semantics was chosen because:

1. under the hood a simple pointer is passed - this is the most efficient code
2. the size of the object pointed to by the pointer is known at compile time and can be checked at compile time

Such efficiency matters a great deal with unions, option types, and sum types. We have to be competitive.

[TurkeyMan]

I don't see how you get from ref T to void[] (or void[N]) though without unsafe casts, even considering the implicit cast from T* to void*. You need to write something like (&lvalue)[0..1]; taking the pointer is safe, but slicing a pointer to an array is not (pointer doesn't know the length); so the implicit cast to void array is not where it gets caught; in this expression, it's the blind slice on the T*.

Not accepting T as an argument means the user will have to cast it to void*, which will look ugly. Casting to void* is a safe operation, so it becomes an ugly construct with no added value.

I don't think it looks ugly, it looks appropriate. It's visible to the reader that you're taking the buffer from under a value; which is what you are doing. I think the clarity of that operation holds huge value.

And again, you're arguing from niche-cases; the default case that people will type doesn't encounter any of this; the 2 cases you've demonstrated that do are your union (unsafe by definition; special handling is completely appropriate), and a void-init stack variable; which is fairly uncommon; in almost all circumstances you just let the declaration init the value... but a void-init stack variable is also unsafe, and requires handling a typed value before it's alive. Maybe the programmers in the safe-future will see a shift to placing buffers on the stack rather than uninitialised lvalues.

I think it's fundamentally bad for the language at the lowest level to promote or require handling invalid-but-already-typed objects. If the variable hasn't been initialised yet, it's memory, so it's void[].

Talking about option types and sum types is a red-herring; I use option types and sum types too, but they are a tool in the toolbox. You don't write placement new ever while using those objects; they have strong API associated with initialisation and assignment. Everything's internal, nobody will handle placement new expressions when interacting with those objects.

Optimise for what people actually do in their code, which is allocate memory and then initialise it.

Note there's an inconsistency in the compiler - in @safe code, a T cannot be cast to a void[T.sizeof], but an &T can be cast to void*. https://issues.dlang.org/show_bug.cgi?id=24866

Is that an inconsistency? That's specifically what I'm taking advantage of.
It's never occurred to me to close that gap that way... I quite like that how it is.

The ref T semantics was chosen because:

They weren't "chosen"; you just trivially decided it with no warning or discussion, against the strongest advice I can possibly muster.

1. under the hood a simple pointer is passed - this is the most efficient code
2. the size of the object pointed to by the pointer is known at compile time and can be checked at compile time

void[N] can be passed by ref, and the size is known at compile time.

Such efficiency matters a great deal with unions, option types, and sum types. We have to be competitive.

Complete red herring; I'm not suggesting anything that would change the codegen in any way whatsoever; I'm interested in getting the expression right; clear, self-explanatory, and relatively foolproof. Nothing I'm discussing changes a single thing about the codegen.

I have proposed the OPTION to accept void[] in ADDITION to void[N], which would causes a runtime bound check to be emit, but if that doesn't land, I'm happy with void[N] only; the user will have to slice to N length, and cause the runtime bounds check at the users slice expression rather than the new expression.

[WalterBright]

I have proposed the OPTION to accept void[] in ADDITION to void[N]

I avoided that because if T is a void[] then the compiler will have to introduce a special case to have T not be the target but what it refers to being the target.

Talking about option types and sum types is a red-herring

I use emplace 206 times in the compiler front end. I use a more primitive method extensively in the optimizer and back end. It's also used extensively in Phobos.

We're just going in circles now repeating ourselves.

The good news is that it works just fine for your use case. You can get rid of emplace now! I'm a bit less lucky, as we're still using an ancient version of D for the bootstrap compiler, grr grr.

you just trivially decided it with no warning or discussion

This is a bit unfair. If Bob writes a proposal, he makes numerous decisions. People then ask for a reference implementation. Bob implements it, discovering that more decisions have to be made. Then the discussion happens. That's normally how things work.

[TurkeyMan]

I have proposed the OPTION to accept void[] in ADDITION to void[N]

I avoided that because if T is a void[] then the compiler will have to introduce a special case to have T not be the target but what it refers to being the target.

I don't follow. I don't see how there's any room for special cases here?

Talking about option types and sum types is a red-herring

I use emplace 206 times in the compiler front end. I use a more primitive method extensively in the optimizer and back end. It's also used extensively in Phobos.

How is it used? If you're doing low-level/unsafe hack-ey stuff (like void init, and your unions), then just wrap a principled new in a function you like. The language should present the clearest and most fool-proof spec possible.

The principle is as simple and clear as day: before the new there is no T, after the new, then T has come to exist. new has no business accepting any typed variable as input; confusion and user-error are guaranteed to follow. The input is "memory", which is most appropriately typed as void.

If you want to do some hacky stuff, then just wrap it in a function that does your preferred casts or type puns in whatever way you like; the world is your oyster. Don't wreck the spec for your personal convenience.

If I'm wrong; the spec can be relaxed, but it can never be tightened...

We're just going in circles now repeating ourselves.

It's because I consistently feel casually dismissed, and I always seem to feel that the only way I can be heard eventually is to repeat myself over and over. I'm having flashbacks of that time we needed to mangle C++ symbols with a namespace; that really did my head in!

I don't know how to say this without sounding arrogant, but I know about this stuff better than most, and I am completely confident my spec on rvalue semantics, and placement new/delete (which are both related tools), will lead this aspect of the language to a really great place that people will be excited about. I wish you'd trust me enough to try my proposals verbatim before perverting them. Let them show their merit before casually dismissing them or butchering them into something I'm no longer excited about... you've spent a few days thinking about this stuff, but I've been baking for years or decades, and I also know the landscape; all these semantics are things I've been using for a very long time. I've worked on a broad and varied range of projects and written millions of lines of code in projects where these tools and patterns are present. I know what they're for, where they lead, and how they scale.

We must have gone round in circles on the rvalue conversation no less than 10 times (possibly more!), but we got there eventually... so I'm not sure going around in circles isn't eventually productive.

The good news is that it works just fine for your use case. You can get rid of emplace now! I'm a bit less lucky, as we're still using an ancient version of D for the bootstrap compiler, grr grr.

This is a different matter. 'Fine' is not the benchmark we should be trying to achieve when finally correcting some of the most fundamental aspects of the D spec. This is a once-in-a-lifetime opportunity to get this stuff really right... and from my point of view, there is nothing in the language that needs work more important than these 2 things.

you just trivially decided it with no warning or discussion

This is a bit unfair. If Bob writes a proposal, he makes numerous decisions. People then ask for a reference implementation. Bob implements it, discovering that more decisions have to be made. Then the discussion happens. That's normally how things work.

I don't understand. I see that the other way around. From my point of view; this is my proposal... I'm glad you liked it and ran with it, but there was a lot more to it that I couldn't easily share in the initial conversation we had.
Should I have perceived that it was forked from my initial proposal and I was cut out of it? I'll admit that I was somewhat offended to note my name's nowhere to be seen near the DIP on this. That feels a bit unfair... but perhaps that's the signal where I should have recognised that it's no longer my design.

[TurkeyMan]

I have proposed the OPTION to accept void[] in ADDITION to void[N]

I avoided that because if T is a void[] then the compiler will have to introduce a special case to have T not be the target but what it refers to being the target.

I don't follow. I don't see how there's any room for special cases here?

I realised what you meant here... this situation naturally occurs when you want to new an array though, so the case still has to be handled?
Why do you see this case as a problem?

[WalterBright]

I am catching up after 4 days without internet or power. Seattle is a technologically advanced city with a primitive electric grid.

I consistently feel casually dismissed

I'm sorry you got that impression. I've provided a rationale for each decision.

Should I have perceived that it was forked from my initial proposal and I was cut out of it? I'll admit that I was somewhat offended to note my name's nowhere to be seen near the DIP on this. That feels a bit unfair... but perhaps that's the signal where I should have recognised that it's no longer my design.

You did indeed propose new(ptr) T(...) in email along with a couple examples.

The first line of the first post of the DIP in the n.g. on Oct 30: "Based on a suggestion by Manu Evans:"
https://www.digitalmars.com/d/archives/digitalmars/dip/development/First_Draft_Placement_New_Expression_421.html#N421
I updated the DIP Nov 18: https://github.com/WalterBright/documents/blob/master/placementnew.md

The dip is about passing an lvalue rather than a pointer.

perverting ... butchering

It's a good thing we're friends, Manu!

If you want things verbatim, write the DIP the way you want it. I wrote it the way I thought would work best. I've never seen an idea that survived into a specification without modification, like the replacement of void* with void[]. I've also never seen a specification that survived into an implementation without modification. (I rewrote the DIP after this implementation.)

You've wanted 3 things, and I want them too:

1. __rvalue
2. placement new
3. extended alias

The first two now have implementations and will work for your use cases. The other aspect of placement new works for my use cases. We should be both happy about this.

but there was a lot more to it that I couldn't easily share in the initial conversation we had.

I didn't see the more, other than what I mentioned earlier. I know placement new from C++, having implemented it. I also implemented that in an earlier version of D: https://dlang.org/deprecate.html#Class%20allocators%20and%20deallocators and the implementation code for it is still present (and I made some use of it!).

From my point of view; this is my proposal...

Ok, but you've also made it clear it is not what you proposed :-)

The hinge of our disagreement is placement new cannot be made @safe and trying to make it safe with an unsafe cast doesn't fix it. The implementation won't allow its use in @safe code.

[TurkeyMan]

The hinge of our disagreement is placement new cannot be made @safe and trying to make it safe with an unsafe cast doesn't fix it. The implementation won't allow its use in @safe code.

Not at all, the hinge of our disagreement is that accepting a T as input is absolutely unacceptable.
Accepting a T by ref is palpably worse, because it further hides the appearance that you're handling memory at all (no pointer in sight) and looks exactly like passing a value.

By writing:

T x;
new(x) T;

There's nothing you could do to make it look more like a value of T is the input to the function, which couldn't be further from the truth; the ONLY valid input is raw uninitialised memory. The only language we have to handle that concept is void.

I think if you were going to design this API, it's not possible to make the API worse than that; it communicates exactly the wrong thing, and I don't think you could inspire misunderstanding and user error more confidently if you tried.

Before new, there IS NO T... I'll die on that hill.
The language should not have any spec that encourages handling invalid values, and I reckon it will also complicate future lifetime analysis.

Yes I know you can write T x = void and then you want to hand that to placement new; I absolutely want to see the coersion from T to void[N] right in my face, so that there can be no mistake. Anyone that encounters that line of code will stop and ask themselves if the value they're coercing has definitely been destroyed prior, or assure it's uninitialised.

[nordlow]

Could the current semantics of C++'s " Placement new" described in https://en.cppreference.com/w/cpp/language/new give insights on what type(s) that should be allowed to be passed as argument to new()?

[TurkeyMan]

C++ accepts void*, and it does not perform any bounds checking.

[TurkeyMan]

The trouble with the function apparently accepting T values, is that people will supply T values! They will get their new object out the other side like they expect, and (depending on the implementation of the constructor/destructor) there's a high chance their code will appear to just work. There's nothing in sight to suggest they might be making a mistake, and chances are they are just silently leaking memory or something like that.

[TurkeyMan]

Oh no!

void f(int* p) @nogc
{
    new(*p) int;
}

error : cannot use `new` in `@nogc` function `main.f`

Seems that placement new is not @nogc, probably hangover from new?
The expression needs to infer the attributes of the constructor it calls...