Path: utzoo!utgpu!attcan!uunet!portal!cup.portal.com!bcase
From: bcase@cup.portal.com (Brian bcase Case)
Newsgroups: comp.arch
Subject: Re: Binaries and Max Speed
Message-ID: <13225@cup.portal.com>
Date: 5 Jan 89 19:45:21 GMT
References: <22745@apple> <2700003@prisma> <5847@polya.Stanford.EDU> <19240@shemp.CS.UCLA.EDU> <13114@cup.portal.com> <10596@gumby.mips.COM>
Organization: The Portal System (TM)
Lines: 38

>>I claim this isn't true.  Getting rid of NOPs changes code addresses.
>>You'll have to find all places where code addresses are referenced,
>>including tables for case statements and other places where code
>>addresses are computed at run time (procedure parameters?), and change
>>them too.  This is a hard problem.
>
>Hard problem? Not really, just you must be careful to modify all the
>addresses correctly.  Also, if your coding style (or the compilers) is
>to intermix code and data in a hap-hazzard way or if you like to 
>write into the code area (:-{) these add significant difficulty.
>
>The Mips program pixie is used to add insturmentation to a program
>for performance analysis.  It must change the addresses to account
>for the added code.  Deleting code is much the same.

Yes, I believe you probably can do it for MIPS code.  The first thing
that makes it possible is fixed-size and fixed-alignment instructions.
These are attributes that make distinguishing code from data either
trivial or unimportant (you can translate data as if it were code
without harm, it would amount to a pessimistic assumption).  Plus, it
seems that PIXIE uses an address correspondence array (something that
I thought *I* had thought of, sigh), which also helps (it keeps track
of the mapping of old code addresses to new code addresses so that
indirect branches will work OK).

Thus, yes, I believe that you think this is not a hard problem for
*your* environment.  In general, however, which is how I took the
original poster's words and how I intended my response, this is certainly
a very hard problem.  Talk to Hunter Systems.  Guys like Phoenix
Technologies, who use dynamic analysis, have a much easier time, of
course.

Also, note that the address correspondence array is an extra level of
indirection that will have some effect on the performance of the
converted program.

I would like to know more about PIXIE, especially the techniques used.
Are there published reports to which I can refer?  Thanks....