Re: birthpoints in rtl.

[Richard Sandiford <rsandifo at nildram dot co dot uk>]

"Steven Bosscher" <stevenb.gcc@gmail.com> writes:
> On Tue, Mar 4, 2008 at 8:47 PM, Richard Sandiford
> <rsandifo@nildram.co.uk> wrote:
>> "Steven Bosscher" <stevenb.gcc@gmail.com> 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