Leopold Toetsch <[EMAIL PROTECTED]> writes: > Piers Cawley <[EMAIL PROTECTED]> wrote: >> Okay, I'm confused, I thought that the whole point of a caller saves, >> continuation passing regime was that the caller only saves what it's >> interested in using after the function returns. > > We don't have a problem WRT register preservation, the problem arises > due to register re-using. > >> ... Exactly *where* that >> return happens, and whether it happens more than once, is completely >> irrelevant from the point of view of the caller. > > The return can only happen, where the normal function call would have > returned, but anyway. > >> ... ISTM that the register >> allocator should work on the principle that anything it didn't save >> before it made the call will be toast afterwards. > > Yes. But - please remember your example "Fun with nondeterministic searches". > Here's the relevant piece of code from main: > > arr1=[1,3,5] > arr2=[1,5,9] > x = choose(arr1) > y = choose(arr2) > $P0 = find_lex "fail" > $P0() > > You know, both "choose" calls capture the continuation and backtrack via > "fail" (basically). But the register allocator isn't aware of that. The > control flow graph (CFG) is linear top down, with new basic blocks > starting after each function call. "arr2" is obviously used around a > call and allocated in the preserved (non-volatile) register area. This > works fine. > > Now the register allocator assigns a register to "$P0". It finds the > register that "arr2" had usable, because in a linear CFG, there's no way > that "arr2" might be used again. So that register is considered being > available. Now if $P0 happens to get the register that "arr2" had, > backtracking through the call to "fail()" obviously fails, as "arr2" is > now the Closure PMC. And that was exactly the case.
Ah! [a light goes on over Piers's head]. > >> Or am I missing something fundamental? > > I don't know ;) I was. Hmm... bugger. So, unless we make the register allocator solve the halting problem, the rule becomes "If you're playing silly beggars with continuations and you're expecting to get at something in a 'surprising' way, stuff it in a lexical or we guarantee that you will be anally violated by an enraged waterbuffalo that's just sick to death of non-determinism"?
