On Wed, Sep 11, 2019 at 3:15 AM William Tambe <tambewilliam@xxxxxxxxx> wrote: > > What is the role of unspec or unspec_volatile in a machine description pattern ? They permit describing operations that otherwise cannot be described in RTL, or for which there is no advantage to describing in RTL. For example access to the TLS base pointer on x86, which is an offset from a segment register. There is no reason to describe the segment registers in RTL just to support this single instruction which always looks the same. Ian
