The subset of SIMD that translates is so narrow that it is almost useless in real code. That’s the problem. Additionally, it is invalid for the compiler to redesign your data structures.

The ability of compilers to use SIMD is no better than it was 20 years ago. Total stagnation for all practical purposes, and the reason is that compilers fundamentally lack the context and freedom to rewrite the code architecture appropriately for SIMD optimization (even if they could).

I think a point people miss is that highly optimized SIMD code is almost never about parallelizing loops. It is about deeply probing and manipulating arbitrarily complex data structures in parallel that involve collections of unrelated types. SIMD allows you to do this, and it is fantastically efficient, but a compiler has no conceivable way of understanding this.

For this reason, SIMD optimization has often been classified as an AI-Complete problem in computer science.