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From: wallach@motcid.UUCP (Cliff H. Wallach)
Newsgroups: comp.arch
Subject: Re: Let's pretend
Keywords: Intel, 586, windows
Message-ID: <5874@avocado5.UUCP>
Date: 20 Dec 90 19:04:43 GMT
References: <3058@crdos1.crd.ge.COM> <1990Dec19.052338.3911@kithrup.COM> <3068@crdos1.crd.ge.COM> <1990Dec19.223934.1568@kithrup.COM>
Organization: Motorola Inc. - Cellular Infrastructure Div., Arlington Heights, IL 60004
Lines: 67

In article <1990Dec19.223934.1568@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes:
-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)

Is this code for real?

-
-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).
-


Exercise for compiler writers: Generate optimized code for a current
architecture.  Maybe something like:

	xor	eax,eax
	mov	edx,3b8h
	in	al,edx
	mov	r3[bp],eax
	or	al,8
	out	edx,al
	mov	r2[bp],eax


Cliff Wallach				...uunet!motcid!wallach