Leopold Toetsch <[EMAIL PROTECTED]> writes: > Piers Cawley <[EMAIL PROTECTED]> wrote: >> Leopold Toetsch <[EMAIL PROTECTED]> writes: > >>> We don't have a problem WRT register preservation, the problem arises >>> due to register re-using. > >> 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"? > > This would make quite a fine explanation in the docs, except that's a > bit unclear about "stuff *it*". The waterbuffalo is concerned of > preserved *temporary* variables too.
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. Condition 2 is something of a bugger if you have big compilation units, but register allocation is always going to be a pain when there are big compilation units around.
