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From: philip@pescadero.Stanford.EDU (Philip Machanick)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Increment (Was Re: Pascal deficiency)?
Message-ID: <1990Dec18.004838.5623@Neon.Stanford.EDU>
Date: 18 Dec 90 00:48:38 GMT
References: <1990Dec17.160242.5095@phri.nyu.edu> <1990Dec17.172613.7941@cs.umn.edu> <ewright.661465844@convex.convex.com> <1990Dec18.001753.3756@Neon.Stanford.EDU>
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In article <1990Dec18.001753.3756@Neon.Stanford.EDU>, kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes:
|> Well, since we ARE in a Mac group, lets just look at the code MPW C generates
|> for just such constructs:
|> 
|> 	i = i+1;
|> 	MOVE.L     i,D2
|> 	ADDQ.L     #$1,D2
|> 	MOVE.L     D2,i
|> 
|> 	i++;
|> 	MOVE.L     i,D2
|> 	ADDQ.L     #$1,i
|> 
|> 	++i;
|> 	ADDQ.L     #$1,i
|> 
|> Behold.  The 68K does, indeed, have an instruction to increment variables in
|> memory.

Interesting - but remember looking at "toy" examples doesn't tell you much.
In "real" code, where performance really matters, I would hope the compiler
would have loaded the variable into a register for as long as possbile.
Still, it's bad that the compiler doesn't pick up i=i+small constant as a
special case - it would be even worse if the Pascal compiler also did this
since (the original point) you have no option of asking for i++.

Many of the programmer-directed "optimizations" in C, like register
variables, ought to be unnecessary with a modern optimizing compiler.
-- 
Philip Machanick
philip@pescadero.stanford.edu