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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.lang.c
Subject: Re: Portable C vs Efficient C or "Cost of Portability"
Message-ID: <7915@utzoo.UUCP>
Date: Thu, 16-Apr-87 11:21:03 EST
Article-I.D.: utzoo.7915
Posted: Thu Apr 16 11:21:03 1987
Date-Received: Thu, 16-Apr-87 11:21:03 EST
References: <213@pyuxe.UUCP>
Organization: U of Toronto Zoology
Lines: 36
Keywords: portability, performance

>   If one writes portable C, does it have to be less efficient 
>   than non-portable C ?

It depends on what kind of "non-portable C" you are thinking about.

C does let you get moderately intimate with the machine if you really want
to.  On the pdp11, with a compiler that didn't do global optimization, it
was possible for someone familiar with the machine to write C in such a
way as to control the exact sequence of instructions.  (This is the whole
reason for the ++ and -- operators, for example, since they allow fairly
direct control of the pdp11 autoincrement/decrement addressing modes.)
Code that has been done this way for the sake of absolute maximum speed
is inherently fairly machine-specific.  ON THE OTHER HAND, if the code
avoids making stupid assumptions, it WILL work on other machines, although
a lot of the efficiency tailoring may be wasted.  Such code is usually hard
to read, but its meaning is often machine-independent even though the
idioms used to express it are not.

In the absence of very good compilers, C written without such machine-
specific tweaking will be less efficient than C written with it.

HOWEVER.  If you are not concerned with squeezing every last microsecond
out of the code in arcane ways, and are willing to content yourself with
the standard of efficiency set by typical C compilers (not superb, but
good compared to a lot of other high-level languages and their compilers),
the situation changes.  Then there is generally little or no penalty for
writing portable code, assuming you know what you're doing.

Since only a minute fraction of all C code is sufficiently crucial to
justify machine-specific code greasing, the best general statement is that
well-written portable C is not significantly less efficient than unportable
C.  This is true even without considering how "efficient" the code is when
it won't run on your new faster machine.
-- 
"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology
the dust.  Which shall it be?"	{allegra,ihnp4,decvax,pyramid}!utzoo!henry