Dan~
On Tue, 30 Nov 2004 09:49:54 -0500, Dan Sugalski <[EMAIL PROTECTED]> wrote: > At 9:36 AM -0500 11/30/04, Matt Fowles wrote: > > > >Dan~ > > > > > >On Tue, 30 Nov 2004 08:28:35 -0500, Dan Sugalski <[EMAIL PROTECTED]> wrote: > >> At 1:45 AM -0800 11/29/04, Jeff Clites wrote: > >> > >> > >> >On Nov 28, 2004, at 2:48 AM, Piers Cawley wrote: > >> > > >> >>I just thought of a heuristic that might help with register > >> >>preservation: > >> >> > >> >>A variable/register should be preserved over a function call if > >>either of the > >> >>following is true: > >> >> > >> >>1. The variable is referred to again (lexically) after the function has > >> >> returned. > >> >>2. The variable is used as the argument of a function call within the > >> >> current compilation unit. > >> > > >> >That doesn't solve it, though you'd think it would. Here's the > >> >counter-example: > >> > > >> > x = 1 > >> > foo() > >> > print x > >> > y = 2 > >> > return y > >> > > >> >You'd think that x and y could use the same memory location > >> >(register, variable--whatever), since ostensibly their lifetimes > >> >don't overlap. But continuation re-invocation can cause foo() to > >> >return multiple times, and each time it should print "1", but it > >> >won't if x and y use the same "slot" (it would print "2" each time > >> >after the first). In truth, their lifetimes do overlap, due to the > >> >hidden (potential) loops created by continuations. > >> > >> Except... we've already declared that return continuations are > >> special, and preserve the registers in the 16-31 range. So when we > >> return from foo, regardless of how or how many times, the pointer to > >> x's PMC will be in a register if it was in there before the call to > >> foo, if it's in the preserved range. So in this case there's no > >> problem. Things'll look like: > >> > >> x = 1 # new P16, .Integer; P16 = 1 # P16 has pointer value 0x04 > >> foo() # foo invocation > >> print x # P16 still has pointer value 0x04 > >> y = 2 # new P16, .Integer; P16 = 2 # P16 now has pointer value 0x08 > >> return y # Passes back 0x08 > >> > >> With more or less clarity. > > > >I think that the concern is for the circumstance where foo() promotes > >it return continuation to a full continuation. Then, that guarantee > >is no longer provided (I think), and repeated invocation could leave y > >in P16 rather than x. > > Nope. The guarantee's still there. Promotion will force things up the > call chain to get marked as un-recyclable, but registers still get > restored on invocation. In that case, I am confused. When does the guarantee NOT apply? Thanks, Matt -- "Computer Science is merely the post-Turing Decline of Formal Systems Theory." -???
