C#•3mo ago

Arrays of floats in stack

I'm creating a structure to represent a 4x4 matrix and want it to be, at least, allocated entirelly on the stack. I also need to be able to get the array pointer in a unsafe context. There is some way to store a array-like value like this on the stack instead of the heap?

48 Replies

Anton•3mo ago

There are fixed-size arrays and stackalloc ref structs is also a thing you can put a span as a field in a ref struct to get the pointer, use fixed to make a struct that is the object on the stack and not a reference to this object, the only way are fixed-size arrays or listing the amount of fields you need manually which people actually do more often for this like a vector field for each row or a float field for each cell

Jimmacle•3mo ago

why do you want it on the stack? stackalloc and ref structs are only allowed on the stack and would limit how you can use the data if you need more, then you can create a struct with sequential layout with 16 fields in it which is how every math library i've seen does it

Cattywampus•3mo ago

that's what InlineArray is for

[InlineArray(16)]
public struct MyMatrix4x4
{
    private float _data; 
}

//then you can do this
var mat = new MyMatrix4x4();

//accessing the indexer
mat[0] = 0f;

//or do thing what Span<T> can do
Span<float> span = mat;
ReadOnlySpan<float> span1 = mat;

//OR pass them directly as Span with the signature of Span<T> or ReadOnlySpan<T>
MyMethodPassAsSpan(mat);

//It's king of the hill if you're working with vectorization
//by simply unsafe loading stuff
Vector128.LoadUnsafe(ref _data, 0);
Vector128.LoadUnsafe(ref _data, 4);
Vector128.LoadUnsafe(ref _data, 8);
Vector128.LoadUnsafe(ref _data, 12);

//those 0, 4, 8, 12 are the offsets

//Storing is easy peasy if you're working with vectorization via StoreUnsafe()

Vector128.LoadUnsafe(ref _data, 0)
.StoreUnsafe(ref _data);

[InlineArray(16)]
public struct MyMatrix4x4
{
    private float _data; 
}

//then you can do this
var mat = new MyMatrix4x4();

//accessing the indexer
mat[0] = 0f;

//or do thing what Span<T> can do
Span<float> span = mat;
ReadOnlySpan<float> span1 = mat;

//OR pass them directly as Span with the signature of Span<T> or ReadOnlySpan<T>
MyMethodPassAsSpan(mat);

//It's king of the hill if you're working with vectorization
//by simply unsafe loading stuff
Vector128.LoadUnsafe(ref _data, 0);
Vector128.LoadUnsafe(ref _data, 4);
Vector128.LoadUnsafe(ref _data, 8);
Vector128.LoadUnsafe(ref _data, 12);

//those 0, 4, 8, 12 are the offsets

//Storing is easy peasy if you're working with vectorization via StoreUnsafe()

Vector128.LoadUnsafe(ref _data, 0)
.StoreUnsafe(ref _data);

another neat thing about InlineArray is that passing it by ref will be slower if you've a large enough matrices, instead you pass them directly as raw Span<T> or ROS<T> and it will be faster than by ref passing manually, and as you said for pointer stuff, you can just do this

var mat = new MyMatrix4x4();
Span<float> span = mat;

//for best practices, always use Marshal.GetReference()
//you can ofcourse point to the indexer directly if you would, like :
// e.g: ref var tref = ref mat[0];
ref var tref = ref MemoryMarshal.GetReference(mat);

//then do whatever

var mat = new MyMatrix4x4();
Span<float> span = mat;

//for best practices, always use Marshal.GetReference()
//you can ofcourse point to the indexer directly if you would, like :
// e.g: ref var tref = ref mat[0];
ref var tref = ref MemoryMarshal.GetReference(mat);

//then do whatever

then you can offset it with Unsafe.Add() OR just by slicing them as span 🙂

var mat = new MyMatrix4x4();
Span<float> span = mat;

ref var tref = ref MemoryMarshal.GetReference(mat.Slice(4));

var mat = new MyMatrix4x4();
Span<float> span = mat;

ref var tref = ref MemoryMarshal.GetReference(mat.Slice(4));

easy peasy with inlinearray the main issue with large sturcts has always been defensive copies, if you somehow rolling your own fat custom matrices (NOT the inlinearray way I shown above) make sure you tag each of modifying method with readonly e.g : public readonly void DoSomethingThatWillMutateTheMatrix(T foo, T foo1) including all your getters, yes you're reading it right, getters like so

public readonly float Row0 => row0;

public readonly float Row0 => row0;

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Arrays of floats in stack

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