Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!mailrus!ames!amdahl!pyramid!prls!mips!mash From: mash@mips.COM (John Mashey) Newsgroups: comp.arch Subject: Re: RISC machines and scoreboarding Message-ID: <2477@winchester.mips.COM> Date: 30 Jun 88 08:31:43 GMT References: <1082@nud.UUCP> <2438@winchester.mips.COM> <1098@nud.UUCP> <2465@winchester.mips.COM> <318@mucmot.UUCP> Reply-To: mash@winchester.UUCP (John Mashey) Organization: MIPS Computer Systems, Sunnyvale, CA Lines: 36 In article <318@mucmot.UUCP> ron@mucmot.UUCP (Ron Voss) writes: >In article <2465@winchester.mips.COM>, mash@mips.COM (John Mashey) writes: >> c) Figuring that a RISC compiler should do optimization, but worrying >> that this feature might be buggy, is like worrying about the safety >> of flying in a 747 and bringing your own seat-belt because you >> don't really trust Boeing to remember to include them :-) >I have observed a few cases where hours were wasted searching for a >programming bug, only to find that the CISC (68020) compiler's optimizer >is buggy (so one turns on the optimizer only for a release, since it >slows compilations anyway, thoughouly test again after turning on, etc.). >Are there really *good* reasons to put more trust in RISC optimizers? >Methinks the metaphor a bit overdone. Sorry, let me try again. I wasn't trying to say optimizers were easy or safe at all: they're not, in fact, they are like building a 747. The point was, in fact, that if you're doing an optimizer (building a 747), THAT's what you worry about, because it's hard. You don't worry about getting nops after loads ("this feature") (seatbelts), because if a somebody can't get THAT right, they have no business building optimizers (747s). [No disrespect to Boeing, I just picked the biggest plane I've flown in.] 1) I have marginally more trust in RISC optimizers than in CISC ones, on general principles [i.e., at least some parts are simpler.] 2) What I mostly trust/distrust is the optimizer's author(s); I'd much rather have an expert's CISC optimizer than a turkey's RISC one. (Fortunately, I get to use a super-expert's RISC optimizer. :-) -- -john mashey DISCLAIMER:UUCP: {ames,decwrl,prls,pyramid}!mips!mash OR mash@mips.com DDD: 408-991-0253 or 408-720-1700, x253 USPS: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086