Whoops, thanks for the correction. My main experience with register variables is with the global kind -- QEMU used to use one to hold a CPU environment pointer,

Whoops, thanks for the correction. My main experience with register variables is with the global kind -- QEMU used to use one to hold a CPU environment pointer, and we had to refactor to get rid of it in order to support clang. (By that point in time the register global was more of a legacy bit of design rather than something that was actively useful for performance or convenience, so it was a useful piece of cleanup to do anyway.)