Explaining the different types in Metal and SIMD

Question

When working with Metal, I find there\'s a bewildering number of types and it\'s not always clear to me which type I should be using in which context.

In Apple\'s Metal

Answer 1

The types with vector_ and matrix_ prefixes have been deprecated in favor of those with the simd_ prefix, so the general guidance (using float4 as an example) would be:

• In C code, use the simd_float4 type. (You have to include the prefix unless you provide your own typedef, since C doesn't have namespaces.)
• Same for Objective-C.
• In C++ code, use the simd::float4 type, which you can shorten to float4 by using namespace simd;.
• Same for Objective-C++.
• In Metal code, use the float4 type, since float4 is a fundamental type in the Metal Shading Language [1].
• In Swift code, use the float4 type, since the simd_ types are typealiased to shorter names.
• Update: In Swift 5, float4 and related types have been deprecated in favor of SIMD4 and related types.

These types are all fundamentally equivalent, and all have the same size and alignment characteristics so you can use them across languages. That is, in fact, one of the design goals of the simd framework.

I'll leave a discussion of packed types to another day, since you didn't ask.

[1] Metal is an unusual case since it defines float4 in the global namespace, then imports it into the metal namespace, which is also exported as the simd namespace. It additionally aliases float4 as vector_float4. So, you can use any of the above names for this vector type (except simd_float4). Prefer float4.