From:	nikki AT gameboutique DOT co (nikki)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: SHIFTS vs MULS
Date:	2 Feb 1997 11:51:58 GMT
Organization:	GameBoutique Ltd.
Lines:	22
Message-ID:	<5d1v4u$lkd@flex.uunet.pipex.com>
References:	<19970201 DOT 205029 DOT 4943 DOT 0 DOT chambersb AT juno DOT com>
	<32F48236 DOT 7F2C AT cs DOT com>
NNTP-Posting-Host:	www.gameboutique.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

>> This sounds a little fishy to me, especially considering the code sizes
>> of the two programs are (when stripped) roughly equivalent.  Is there any
>> reason that Gcc might skip the mul's in the shift&add one as an
>> optimization?  (ie looking ahead and seeing the value isn't used so not
>> doing the calculation, or something?)

if you compile with the -S switch you could look at the code and make sure
it's producing what you want it to :)

just for the record, you can get a shift down to one cycle on a pentium
by using the shrl $value,%%register version. however it only pairs in the u
pipe, so a few of these will cause problems. mul and imul are really awful
taking a minimum of 9 cycles and not pairable at all. so your shifts should
win by a factor of about 9 here.

regards,
nik


-- 
Graham Tootell           
nikki AT gameboutique DOT com  

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