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From: bcase@amdcad.UUCP
Newsgroups: comp.arch,comp.lang.c
Subject: Re: String Processing Instruction
Message-ID: <15314@amdcad.UUCP>
Date: Fri, 27-Mar-87 18:11:50 EST
Article-I.D.: amdcad.15314
Posted: Fri Mar 27 18:11:50 1987
Date-Received: Sat, 28-Mar-87 16:03:18 EST
References: <15292@amdcad.UUCP> <978@ames.UUCP> <5969@mimsy.UUCP>
Reply-To: bcase@amdcad.UUCP (Brian Case)
Distribution: na
Organization: Advanced Micro Devices, Sunnyvale, California
Lines: 23
Keywords: instruction set architectures, Am29000
Xref: utgpu comp.arch:681 comp.lang.c:1355

In article <5969@mimsy.UUCP> mark@mimsy.UUCP (Mark Weiser) writes:
>In article <978@ames.UUCP> lamaster@pioneer.UUCP (Hugh LaMaster) writes:
>>...How likely is it that a compiler [can] detect the case when it can use 
>> an instruction like this and generate code automatically to use it?
>
>This is of course a good point, and it seems that this is going to be
>one of those hard cases for compiler writers.  But it opens the issue
>of subroutine libraries as providing an environment which also needs
>support in the instruction set.  The Unix string routines are a good
>example, because they are even now often recoded in assembly for efficiency.
>Various pieces of Unix kernel frequently suffer the same fate
>(e.g. tty buffers).  But once one is into instructions which are good
>for certain operating systems or environments, rather than certain languages,
>one has narrowed one's market.  For instance, the Am29000 special string
>compare is useless if strings are encoded as a length followed by the chars.  

Yes, you are right:  when the length is known, things are *much* easier.
But, the instruction is still a useful primitive:  it can be used to find
out, in one cycle, whether or not a 32-bit word contains a byte of
interest.  The compiler may not generate it, but it can be used in hand-
coded inner loops for this purpose.

    bcase