It does fine if you actually write the correct code :)

https://godbolt.org/z/qpLeWP

There is nothing incorrect about the code I posted - clang will compile it to a single read + bswap just fine. And you don't need to go that far back for your code to not be recognized - GCC 4.9 will produce individual loads and shifts for that too.

The point is that you can't rely on compilers consistently to recognize complex patterns.

Incorrect was too strong but it's a weird pattern to do this with sums. The OR pattern is what is used pretty much everywhere and conveys the intention of the code much more clearly.

And even if the compiler doesn't always optimize ideally my original point still stands. Delegating this to the compiler instead of trying to manually call swap instructions is a much better way of going about it.