[gclist] Re: PPC & GC, or GC and threads

David Chase chase@world.std.com
Mon, 26 Jan 1998 13:20:57 -0500

At 09:39 AM 1/26/98 -0800, Hans Boehm wrote:
>There is every reason to expect that with an implementation that's integrated
>into the compiler even the 2% number could be significantly lower.  After all,
>empirically nearly all optimized C code is GC-safe, even on PPC.

>You can combine all of this with a safe-point notion and reduce the cost
>further.  But I suspect that by the time you have an integrated implementation
>you're talking about reducing the slowdown from 1% to 0.5%.

The difficulty is in convincing someone to take the time to
design and implement such a compiler.  One odd problem with
conservative collection is that its robustness makes it
difficult to test for compiler cooperation; if an alleged
cooperative compiler failed to cooperate in certain instances,
how much would you need to test it to be sure of this?
Given a choice between the blunt club described in the 1996
PLDI paper, and a compiler claimed to be cooperative-by-
design, I might well give up a bit of performance to get
the cooperation guaranteed by the blunt club.

This somewhat different for compacting collectors, especially
if you unmap the old space after it has been evacuated.

David Chase