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From: levy@ttrdc.UUCP
Newsgroups: comp.lang.c
Subject: Re: Disregarding parentheses in C
Message-ID: <1670@ttrdc.UUCP>
Date: Sat, 25-Apr-87 23:57:36 EDT
Article-I.D.: ttrdc.1670
Posted: Sat Apr 25 23:57:36 1987
Date-Received: Thu, 30-Apr-87 05:30:09 EDT
References: <7019@brl-adm.ARPA> <1504@umn-cs.UUCP>
Organization: AT&T, Skokie, IL
Lines: 22
Summary: WATCH OUT FOR SIDE EFFECTS AND FUNCTION CALLS

In article <1504@umn-cs.UUCP>, herndon@umn-cs.UUCP (Robert Herndon) writes:
<   Often-times, I also use C as a "machine independent assembly
< code".  When I do so, I often define macros for performing
< various operations, e.g.,
< #define ABS(x) ((x) < 0 ? -(x) : (x))
< and again, hope the C compiler doesn't literally evaluate x
< repeatedly if it's a macro or expression that can be rearranged.
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Just being able to be rearranged isn't enough!  It can be perfectly rearrange-
able and still have side effects.  Note that if MACRO_1 is called
multiple times by MACRO_2, and MACRO_1 contains something with side effects
like x++ or getchar() then you had jolly well realize that the side-effects
in MACRO_1 WILL get evaluated as many times as it was called, in an undefined
order.  Even if there are function calls in MACRO_1 which do not change the
state of the computation, the compiler doesn't know to optimize the multiple
calls to that function into one call, which is wasteful of CPU time.
-- 
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