carters@ajpo.sei.cmu.edu (Scott Carter) (03/21/91)
In article <98063@tut.cis.ohio-state.edu> Joe Hollingsworth <holly@cis.ohio-state.edu> writes: >In article <jls.669368339@rutabaga> jls@rutabaga.Rational.COM (Jim Showalter) writes: >>[ use built-in checking via CONSTRAINT_ERROR rather than explicit test ] >>Note that in the second case the procedure is faster, since it doesn't >>have to do the check first. > >I figured I'd see this argument, but I don't buy it. > >1) Refering back to Kernighan & Plauger's "The Elements of Programming >Style," I'll quote another rule: > >"Write clearly - don't sacrifice clarity for 'efficiency'." " - at first, until you find the real cpu burners [if they are localized]. Recode them for efficiency (document it!), so you have more slop to make most of the code clearer and more maintainable." POP is an example of a system utility for which the tradeoffs push more for efficiency than in most of the code (maybe). > > >2) Ok, so you think that the second version of Pop is faster >because it is not doing the test. Well there may not be an explicit >test, but there sure has to be an implicit one. The compiler has >to generate code to test to see if a constraint error needs to be >raised, i.e. there is run time checking going on here. And you >sure can't use the SUPRESS pragma to get rid of it. Don't be so sure about this, or assume that the check has the anywhere near the same cost as the explicit test. A constraint test for < 0 is a single instruction in some machines (e.g. TBNDU on the 88000, or on any machine which has a trap on bit instruction), and further that instruction has more slop as far as where in the canonical order it must go compared to the explicit test. A really snazzy compiler which is inlining Is_empty() might reduce the explicit check cost down to about three cycles (and introduce a block boundary), but we just aren't there yet. > >Joe >holly@cis.ohio-state.edu Scott Carter - McDonnell Douglas Electronic Systems Company carter%csvax.decnet@mdcgwy.mdc.com (preferred and faster) - or - carters@ajpo.sei.cmu.edu (714)-896-3097 The opinions expressed herein are solely those of the author, and are not necessarily those of McDonnell Douglas.