Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!usc!csun!kithrup!sef
From: sef@kithrup.COM (Sean Eric Fagan)
Newsgroups: comp.arch
Subject: Re: Let's pretend
Keywords: Intel, 586, windows
Message-ID: <1990Dec19.223934.1568@kithrup.COM>
Date: 19 Dec 90 22:39:34 GMT
References: <3058@crdos1.crd.ge.COM> <1990Dec19.052338.3911@kithrup.COM> <3068@crdos1.crd.ge.COM>
Organization: Kithrup Enterprises, Ltd.
Lines: 53

In article <3068@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen) writes:
>  I meant what I said - "quantify" rather than qualify. Yes optimization
>would be better and memory accesses would be down, but how much?

Well... I could suggest you go read any recent (last decade or so) papers on
compiler optimization techniques, which would be chock full of them.  Also
read papers on the RISC chips, and why the register and instruction sets
were chosen.

Here is a sample of code:

	r2 = r3 = inb (0x3b8);

	r2 |= 8;
	outb (0x3b8, r2);

(r0 through r7 are declared locally as 'unsigned long r0, r1, ...;', and
inb and outb are declared as 'static inline unsigned char ...', and written
using inline assembly)

Here is the code gcc generates for that:

	inb (%dx)
	movl $952,-220(%ebp)
	movw -220(%ebp),%dx
	inb (%dx)
	movb %al,-216(%ebp)
	movzbl -216(%ebp),%eax
	movl %eax,-216(%ebp)
	movzbl -216(%ebp),%eax
	movl %eax,-212(%ebp)
	movl %eax,-216(%ebp)
	movl -220(%ebp),%eax
	movl %eax,-220(%ebp)
	movl -216(%ebp),%eax
	orl $8,%eax
	movl %eax,-216(%ebp)
	movw -220(%ebp),%dx
	movb -216(%ebp),%al
	outb (%dx)

Excercise for reader:  assuming 16 reigsters, rewrite that code using only
r0 through r7 (which was all I had declared in my code).  Then, take out an
intel book on the '386, and figure out the timings of the old code and the
new code (assume that the new register set will be accessed in the same
amount of time as the old register set, since I'm talking about completely
trashing the instruction set and redesigning it).

-- 
Sean Eric Fagan  | "I made the universe, but please don't blame me for it;
sef@kithrup.COM  |  I had a bellyache at the time."
-----------------+           -- The Turtle (Stephen King, _It_)
Any opinions expressed are my own, and generally unpopular with others.