Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!snorkelwacker!bloom-beacon!eru!luth!sunic!enea!sommar
From: sommar@enea.se (Erland Sommarskog)
Newsgroups: comp.lang.misc
Subject: Re: Common subexpression optimization
Message-ID: <767@enea.se>
Date: 11 Feb 90 11:38:53 GMT
References: <14229@lambda.UUCP> <2844@pkmab.se >
Organization: Enea Data AB, Sweden
Lines: 49

Kristoffer Eriksson (ske@pkmab.se) writes:
 >Jim Giles (jlg@lambda.UUCP) writes:
 >>      int a[200][2];                    int a[200][2]; int *p; int *q;
 >>      ...                               ...
 >>      ...                               p = &a; q = p+1;
 >>      for (i=0;i<200;i++){              for (i=0;i<200;i++){
 >>        a[i][0] = expr1(i);               *p = expr1(i);
 >>        a[i][1] = expr2(i);               *q = expr2(i);
 >>      };                                  p += 2; q += 2;
 >>                                        };
 >>
 >>The first is clearly more readible than the second.  ...
 >
 >Not that I necessarily agree with the first poster about always using
 >pointers for array loops, but why would you use *two* pointers? It
 >would be much more "natural" and less obscure this way:
 >
 >int a[200][2]; int *p;            
 >p = &a[0][0];			/* Start fromthe first int of a. */
 >for (i = 0; i < 200; i++) {
 >    *(p++) = expr1(i);
 >    *(p++) = expr2(i);
 >};

OK, I don't speak C, maybe this is a standard approach once you
know the language, but I find Kristoffer's proposal to be at
least as obscure as Jim Giles' right-hand example. I fail to
see any problems with the left-hand fragment which is the obvious
way to write it. The other two look as the programmer's been
training for that obsfucated (or whatever the word is) C contest.

 >or this way:
 >
 >int a[200][2];  /* Array of 200 arrays of two int. */
 >int (*p)[2];	/* Pointer to array of two int. */
 >p = a;		/* Start with the first array of two int of a. */
 >for (i = 0; i < 200; i++) {
 >    (*p)[0] = expr1(i);
 >    (*p)[1] = expr2(i);
 >    p++;	/* Note automatic scaling. */
 >}

Slightly more understandable, but still obscure. It seems to assume
that the dimension are stored in a certain order. Although this
probably is defined by the language, it still seems unnecessary
to rely on such a fact. Anyway, the human reader have to think
a while before he can ensure that the fragment is correct.
-- 
Erland Sommarskog - ENEA Data, Stockholm - sommar@enea.se
Unix is a virus.