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. Matt -- "Computer Science is merely the post-Turing Decline of Formal Systems Theory." -???
