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From: sef@kithrup.COM (Sean Eric Fagan)
Newsgroups: comp.arch
Subject: Re: Unusual instructions and constructions
Message-ID: <1991Mar14.013109.16636@kithrup.COM>
Date: 14 Mar 91 01:31:09 GMT
References: <4748@eastapps.East.Sun.COM> <7571@mentor.cc.purdue.edu> <11964@pasteur.Berkeley.EDU>
Organization: Kithrup Enterprises, Ltd.
Lines: 38

In article <11964@pasteur.Berkeley.EDU> jbuck@galileo.berkeley.edu (Joe Buck) writes:
>Check out this short program:

[code deleted]
>It's not optimal: there are two divides.  Still, if you write kindly
>I'll bet you could talk RMS into seeing if he could recognize the
>pattern and produce one divide in gcc 2.0 (or for some higher number).


foop:
	pushl %ebp
	movl %esp,%ebp
	pushl %ebx
	movl 8(%ebp),%eax
	cltd
	idivl 12(%ebp)
	movl %eax,%ecx
	movl %edx,%ebx
	pushl %ebx
	pushl %ecx
	pushl $.LC0
	call printf
	leal -4(%ebp),%esp
	popl %ebx
	leave
	ret

It was already done, for gcc 1.3[89].  Good work, eh?  Yes, the code could
be optimal:  gcc could look at the entire function, and not bother moving
from eax to ecx, just pushing them directly.  But those are small amounts
(two or three cycles, I believe), and are *much* better than the 15+ cycles
the extra divide would have taken.

-- 
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.