The CPU you compile on will probably not be the CPU your program runs on. If you know what CPU you're running on I guess you can just make assumptions about what features that chip supports, but it's much safer to use something like this to verify at runtime that you support avx or whatever features you need.

Even for same architecture compilation, the host's feature set is rarely that of the targets. If you're distributing binaries, you generally want to target a safe, old minimum baseline to maximize user bases--for x86-64, resorting to just sse2 is safe [1] and often sufficient [2]. Most compile farms and developer machines are often newer hardware.

[1] The x86-64 ABI actually requires SSE2 extensions to work correctly.

[2] SSE2 added double-precision and vectorized integer support to the SSE registers, the former allowing you to replace x87 FPU usage for floating point (unless you need long double, which is extremely rare). The newer SSE sets generally add only specialized operations that are unlikely to show up in autovectorization anyways, and the wider instruction set of AVX is less useful for performance in the "it might be useful" autovectorization scenario. They are useful for specific known hot regions of code, and in distributing binaries, variants of these are constructed for different levels of feature sets and are dynamically selected based on the user's actual hardware.

To add on the other comments, not only the machine where you compile is not the same as the one where you run your software, but consider the case where you want a software to run on as many CPUs as possible while still making use of advanced features where supported.

You'd tell the compiler "target a machine with <basic instruction set>", so it's not allowed to use advanced features like FMA because it can't assume they're supported. With this library, you'd check at run-time if FMA is supported, and then change functions accordingly.

Comment from the post:

> Just in case anyone is tempted, please do not write code that assumes the host on which it is compiled will always be the host on which it will be executed.

Only if you are compiling and running on the same machine, no?