金属结构中未使用的属性会损害性能(Do unused properties in a metal struct hurt performance)

金属结构中未使用的属性会损害性能(Do unused properties in a metal struct hurt performance)

假设我在.metal文件中有以下结构：

struct VertexOut{ float4 position [[position]]; float2 a; float2 b; };

此结构是我的片段着色器的[[ stage-in ]]输入。 现在我的一些着色器使用b而有些则不使用。 这是否会伤害性能，这意味着我应该为不需要它的着色器创建另一个没有b结构吗？

Suppose I have the following struct in a .metal file:

struct VertexOut{ float4 position [[position]]; float2 a; float2 b; };

This struct is the [[ stage-in ]] input for my fragment shaders. Now some of my shaders use b and some do not. Is this hurting performance at all, meaning should I create another struct without b for the shaders that do not need it?

最满意答案

答案取决于您的使用情况。 将会有内存带宽性能上升，但它会相当小。 如果使用此着色器输出全屏像素并且带宽有限，则删除不需要的float2 可能会有所帮助。

但是，在更改着色器时通常会出现性能损失，因此您可能无需更改片段着色器就可以获得性能优势，这很可能超过任何损失。

我要做的一个建议是将a和b改为float4 a_b并使用a_b.xy和a_b.zw访问它。 寄存器对齐到16个字节，因此这将节省空间。 以这个例子：

struct X { float4 position; float2 a; float2 b; }; struct Y { float4 position; float2 a; }; struct Z { float4 position; float4 a_b; };

在上面的例子中，X将使用48个字节（每个寄存器有3个16字节的寄存器），而Y或Z只有32个字节（有2个寄存器）。 所以使用Z可能会给你两全其美。

The answer depends on your usage. There will be a memory bandwidth performance hit but it will be fairly small. If you were outputting a full screen of pixels using this shader and were bandwidth limited, removing the unneeded float2 may help.

However, there is often a performance hit in changing shaders, so you are possibly getting a performance benefit by not needed to change fragment shaders which quite possibly outweighs any losses.

One suggestion I would make is to some change a and b to be float4 a_b and access it with a_b.xy and a_b.zw. Registers are aligned to 16 bytes, so this will save you space. Take this example:

struct X { float4 position; float2 a; float2 b; }; struct Y { float4 position; float2 a; }; struct Z { float4 position; float4 a_b; };

In the above case, X will use 48 bytes (there are 3 registers of 16 bytes each), whereas Y or Z is only 32 bytes (there are 2 registers). So using Z would potentially give you the best of both worlds.