Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: () ignored in some expressions
Message-ID: <1990Apr14.235432.6340@utzoo.uucp>
Organization: U of Toronto Zoology
References: <48079@lanl.gov> <16414@haddock.ima.isc.com> <1990Apr10.181833.5453@utzoo.uucp> <819@s6.Morgan.COM>
Date: Sat, 14 Apr 90 23:54:32 GMT

In article <819@s6.Morgan.COM> amull@Morgan.COM (Andrew P. Mullhaupt) writes:
>> ... only associative and commutative operators were
>> covered by the old rule, and subtraction is neither.
>
>Well what you say is true, but his example is still important when 
>recast into the form:
>
>	Big * (Big + x)

The "*" operator neither associates nor commutes with "+", so this is
*still* irrelevant to the discussion at hand.  Some compilers will try
to use the *distributive* property to optimize this expression, but that
has always been an optimization that would -- if integer overflow produces
noticeable effects, rare in old compilers -- break the rules.  There has
never been a rule allowing the compiler to use distributivity in the way
that use of associativity and commutativity was explicitly allowed.

>It is quite important to have this sorted out by compile time, since
>run time checks on all integer arithmetic are very expensive. 

This depends on the machine and the compiler.  On some it's essentially
free.
-- 
With features like this,      |     Henry Spencer at U of Toronto Zoology
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