Sean O'Rourke wrote: > From: Sean O'Rourke [mailto:[EMAIL PROTECTED] > Sent: Thursday, August 21, 2003 1:56 PM > To: Tom Locke > Cc: [EMAIL PROTECTED] > Subject: Re: Registers vs. Stack > > > "Tom Locke" <[EMAIL PROTECTED]> writes: > > p.s. (and at the risk of being controversial :) Why did Miguel de > > Icaza say Parrot was "based on religion"? Was it realted to this > > issue? Why is he wrong? > > IIRC it is -- his take is that stack VM code provides useful > information about variable lifetimes, which can help the JIT produce > good code. If everything's jitted, then his contention is that > bytecode opcount doesn't matter that much. > > I'm not going to stick my neck out and offer a technical opinion, but > I'll just note that religion has been the driving force behind some > great things in the past -- witness Europe's cathedrals ;). Oh, > wait... we're supposed to be "the bazaar".
More specifically, if IMCC spills a register into the frame or a lexical pad or who-knows-where, it's hard for the JIT compiler to "unspill" that value. Not having a register file in the bytecode leaves register spill up to the JIT compiler, which knows more about the native platform. For instance, IA-64 (Itanium) has a native register file large enough to dedicate one native register to each parrot string register, integer register, and PMC register--and still have plenty left over. It'd be hard for the hypothetical IA-64 JIT to fully utilize that register file. (OTOH, it would be hard to write even a C function which is complex enough to fully utilize that particular register file without giving the optimizer a seizure.) By contrast, x86 has nowhere near enough native GP registers for the 96 total IRs, SRs, and PRs--the x86 JIT is likely spilling registers left and right. (OTOH, C code is spilling left and right, too.) The ideal stack-based JIT might be able to more easily emit better code for both architectures. Stack-based operators are an unnatural paradigm for most assembly-level programmers, though. Easier to give a register a name than to remember how deep on the stack such-and-such value is. What else to recommend them? Well--parrot's pretty married to them by now, for one. -- Gordon Henriksen IT Manager ICLUBcentral Inc. [EMAIL PROTECTED]
