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Postings from June 2001
mad thought for the day: I was thinking back to the earlier discusions on opcode dispatch, and the fact that some people thought that a big switch was as good as, or possibly faster than a dispatch table. Which led me to think... should we abandon vtables (ie arrays of fn pointers indexed by op), and just have a single hander function per type which has the op as an arg? Then each type is free to handle things as it likes - from having its own private dispatch table, to having a big switch statement, to having just a single "if (IS_MODIFY_OP(op)) .. else ...." statement for a copy-on-write wrapper type. The big problem would be handling the variable number of args, but this could be solved by passing a singe pointer to a block of args - ie extending PDD 2's idea of the key array, to include the other args as well. This might be a win (easy for lightweight wrapper types like copy-on-write to pass the same args to the real type below), or a loss (lots of time wasted marshalling args). One of the main jobs of the optimiser might end up being to maintain a scrathpad area of the regiser file for use as by the arg pointer, in such as way that it needs to be modified as little as possible between opcodes. I think the biggest advantage to having a single handler function is that it becomes very easy to create lightweight wrapper types without having to construct a whopping big vtable each time. For example: readonly type: if its a read op, pass on to underlying type, otherwise carp(). copy-on-write: if its a write op, call clone(); then pass on to underlying type. for $i (@a): $i might be set to a simple iterator type that passes it's args plus the current index to the underlying array object. But there are probably lots of disadvantages too that simply havent occured to me... Dave.Thread Next