"Steven Bosscher" <[EMAIL PROTECTED]> writes: > On Tue, Mar 4, 2008 at 8:47 PM, Richard Sandiford > <[EMAIL PROTECTED]> wrote: >> "Steven Bosscher" <[EMAIL PROTECTED]> writes: >> Going back to something discussed upthread: would you expect to use this >> for hard regs as well as pseudos? No-op moves aren't necessarily supported >> for all hard registers. E.g. MIPS doesn't have patterns for LO <- LO, >> even though LO is a normal non-fixed register. You can also have hard >> registers that only appear in fixed patterns, such as condition code REGs. > > Yes, for hard registers you can't use this. Another example is the > loop counter register on ia64, or the flags register on i386. > > You should have a look at the history of RTL SSA for hard registers > (http://gcc.gnu.org/ml/gcc-patches/2000-07/msg01285.html and thread). > They used to put selected hard regs into SSA form. Lessons learned: > don't do that. I think the same applies to FUD chains for hard > registers.
Yeah, I remember the RTL-SSA stuff. And for avoidance of doubt, I wasn't advocating explicit hard-reg moves. I was trying to figure out whether you were. Clearly you weren't. ;) >> Kenny said that pseudos-only was better than nothing, and I can't >> disagree with that. But one of the nice things about the on-the-side >> idea is that you have none of these problems. There should be nothing >> special about hard regs. > > Uh, hard registers *are* special. And, also quite important, what > would you *do* with FUD chains for hard registers? We don't optimize > many things with hard registers, usually just because it's harder to > do than for pseudos. I don't think FUD chains would change that. I don't see why hard registers are special as far as FUD chains go. We have DU chains for hard regs, so why not FUDs too? Richard
