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From: golde@uw-beaver (Helmut Golde)
Newsgroups: net.lang.c
Subject: Re: Right shift vs. divide
Message-ID: <1733@uw-beaver>
Date: Fri, 3-Jan-86 02:14:54 EST
Article-I.D.: uw-beave.1733
Posted: Fri Jan  3 02:14:54 1986
Date-Received: Sat, 4-Jan-86 04:47:06 EST
References: <974@brl-tgr.ARPA> <3000070@uokvax.UUCP>
Reply-To: golde@uw-beaver.UUCP (Helmut golde)
Organization: U of Washington Computer Science
Lines: 31
Summary: 

Microsoft C for 8086/8088 seems to successfully optimizes integer divides
into right shift, when optimizing for reduced execution speed.  Here's is
the actual code produced (with comment for those unfamiliar with the 8086).

Y = X / 2     ------>

mov ax,X	;mov variable X into register ax
cwd		;convert AX to double word, high part in DX
sub ax, dx	;ax -= dx
sar ax,1	;shift arithmetic right ax 1 bit
mov Y,ax	;move ax to variable Y

So far so good.  It seems to work.  But get how it compiles larger divisions!

Y = X / 8  (or any other power of 2)     ------------>

mov ax, X	;register ax = variable X
cwd		;word to double word, high part in dx
xor ax,dx	;ax = ax xor dx
sub ax,dx	;ax -= dx
mov cx,3	;cx = 3 (or whatever the shift count should be)
sar ax,cx	;shift ax arithmetic right cx bits
xor ax,dx	;ax = ax xor dx
sub ax,dx	;ax -= dx
mov Y,ax	;variable Y = ax

It even seems to work, and I guess it is faster than an integer divide.
Amazing, though.


--Peter Golde