打包数据类型与普通数据类型之间的区别 [英] Difference between packed vs normal data type

问题描述

在《金属》中，packed_float4和float4有什么区别?

In Metal what is the difference between a packed_float4 and a float4?

推荐答案

此信息来自 float4的对齐方式为16个字节.这意味着这种类型的内存地址(例如0x12345670)将被16整除(又名最后一个十六进制数字为0).

float4 has an alignment of 16 bytes. This means that the memory address of such a type (e.g. 0x12345670) will be divisible by 16 (aka the last hexadecimal digit is 0).

packed_float4的对齐方式为4 bytes.地址的最后一位是0，4，8或c

packed_float4 on the other hand has an alignment of 4 bytes. Last digit of the address will be 0, 4, 8 or c

这在创建自定义结构时很重要.假设您想要一个具有2个普通float s和1个float4/packed_float4的结构:

This does matter when you create custom structs. Say you want a struct with 2 normal floats and 1 float4/packed_float4:

struct A{
    float x, y;
    float4 z;
}

struct B{
    float x, y;
    packed_float4 z;
}

对于A:float4的对齐方式必须为16，并且由于float4必须在常规float之后，因此8字节之间的空白空间应为y和z.这是A在内存中的样子:

For A: The alignment of float4 has to be 16 and since float4 has to be after the normal floats, there is going to be 8 bytes of empty space between y and z. Here is what A looks like in memory:

 Address | 0x200 | 0x204 | 0x208 | 0x20c | 0x210 | 0x214 | 0x218 | 0x21c |
 Content |   x   |   y   |   -   |   -   |   z1  |   z2  |   z3  |   z4  |
                                             ^Has to be 16 byte aligned

对于B:packed_float4的对齐方式是4，与float相同，因此在任何情况下都可以紧随float之后:

For B: Alignment of packed_float4 is 4, the same as float, so it can follow right after the floats in any case:

 Address | 0x200 | 0x204 | 0x208 | 0x20c | 0x210 | 0x214 |
 Content |   x   |   y   |   z1  |   z2  |   z3  |   z4  |

如您所见，A占用32个字节，而B只使用24个字节.当您拥有这些结构的数组时，A将为每个元素占用8个更多的字节.因此，为了传递大量数据，首选后者.

As you can see, A takes up 32 bytes whereas B only uses 24 bytes. When you have an array of those structs, A will take up 8 more bytes for every element. So for passing around a lot of data, the latter is preferred.

根本需要float4的原因是因为GPU无法处理4字节对齐的packed_float4 s，因此您将无法在着色器中返回packed_float4.我认为这是因为性能.

The reason you need float4 at all is because the GPU can't handle 4 byte aligned packed_float4s, you won't be able to return packed_float4 in a shader. This is because of performance I assume.

最后一件事:当您声明结构的Swift版本时:

One last thing: When you declare the Swift version of a struct:

struct S {
    let x, y: Float
    let z : (Float, Float, Float, Float)
}

此结构在Metal中等于B，而在不是 A中等于.元组就像packed_floatN.

This struct will be equal to B in Metal and not A. A tuple is like a packed_floatN.

所有这些同样适用于其他向量类型，例如packed_float3，packed_short2等.

All of this also applies to other vector types such as packed_float3, packed_short2, ect.

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