Path: utzoo!attcan!uunet!husc6!ukma!mailrus!uflorida!haven!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Efficient coding considered harmful?
Message-ID: <14444@mimsy.UUCP>
Date: 9 Nov 88 19:15:49 GMT
References: <3105@hubcap.UUCP> <34112@XAIT.XEROX.COM> <1700@dataio.Data-IO.COM> <23459@amdcad.AMD.COM>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 42

In article <23459@amdcad.AMD.COM> tim@crackle.amd.com (Tim Olson) writes:
>A good compiler can also recognize a signed integer divided by a power
>of 2, and use the following trick:
[copy and add sign bit]

It is worth noting that Sun's PCC-based compiler does something similar:

	f() {
		register i, j;
		...
		i = j / 2;
		...
		i = (unsigned)j / 2;
		...
	}

compiles (using the SunOS 3.5 compiler running under SunOS 3.2) to

	movl	d6,d0
	tstl	d0
	jge	L2000000
	addql	#1,d0
L2000000:
	asrl	#1,d0
	movl	d0,d7

(clearly not the best code in the world) and

	movl	d6,d0
	lsrl	#0x1,d0
	movl	d0,d7

(why some constants in hex and others decimal we may never know :-) ).
The optimiser transforms the latter to the more appropriate

	movl	d6,d7
	lsrl	#1,d7

(yes, the 0x disappears).
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris