I think what a SIMD library does, above all else, is get the programmer to write code in a way that can be directly translated into SIMD instructions. A big is

I think what a SIMD library does, above all else, is get the programmer to write code in a way that can be directly translated into SIMD instructions. A big issue that compilers have to contend with is that they aren't allowed (unless you enable ffast_math) to rearrange floating point operations. Putting an add or a multiply in the wrong place can spoil SIMD optimizations that the compiler could otherwise pull off. But the problem is as you state. For people that really care about that sort of thing, they are likely going to have the exact SIMD sequence they want to execute in mind anyways. That leaves you with a definition that is doomed to be both not low level enough and too low level. I think what this is useful for is a fallback description of the desired SIMD operations. It won't be ideal on non-targeted platforms, but it will be something.