I also prefer fixed width. At least in C++, all of the padding, alignment, etc is automagically codegen-ed for the register type in my use cases, so the overhead is approximately zero. All the complexity and cost is in specializing for the capabilities of the underlying SIMD ISA, not the width.

The benefit of fixed width is that optimal data structure and algorithm design on various microarchitectures is dependent on explicitly knowing the register width. SIMD widths aren’t not perfectly substitutable in practice, there is more at play than stride size. You can also do things like explicitly combine separate logic streams in a single SIMD instruction based on knowing the word layout. Compilers don’t do this work in 2025.

The argument for vector width agnostic code seems predicated on the proverbial “sufficiently advanced compiler”. I will likely retire from the industry before such a compiler actually exists. Like fusion power, it has been ten years away my entire life.