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From: daniel@mojave.stanford.EDU (Daniel Weise)
Newsgroups: comp.lang.scheme
Subject: Order of evaluation (was Re: evaluating () should be an error
Message-ID: <9103271633.AA26563@mojave.Stanford.EDU>
Date: 27 Mar 91 16:33:06 GMT
Article-I.D.: mojave.9103271633.AA26563
References: <1991Mar26.224805.23381@cs.ubc.ca>
Sender: tytso@athena.mit.edu (Theodore Y. Ts'o)
Organization: The Internet
Lines: 40


   As an example, suppose that we want to evaluate

      (baz (foo 1) 2 3 (+ 4 (bar 5)))

   on a machine which has four available registers. If the language insists
   we evaluate from left to right, then about the best we can do is

      load r0,1
      call foo   ;; Assume args are passed in lowest numbered regs.
      load r1,2
      load r2,3
      load r3,4
      store r0,temp ;; Register spill. 
      load r0,5
      call bar
      add  r0,4  ;; Very smart compiler notices it can cheat here. 
      load r4,r0 ;; Assume result is returned in r0.
      load r0,temp
      call baz

   If, on the other hand, the compiler is free to evaluate arguments in any
   order, it can generate

      load r0,5
      call bar
      load r3,r0
      load r0,1
      call foo
      load r1,2
      load r2,3
      call baz

Um, you are restricting your compiler far too much.  If left to right
order of evaluation were mandated, the compiler would merely have to
give the illusion of doing things left to right.  It doesn't have to
do them left to right if the programmer can't tell.  In this case,
the loads of constants could be performed evaluating the other arguments.

Daniel