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Date:      Thu, 22 May 1997 18:12:07 -0600
From:      Brett Glass <brett@lariat.org>
To:        Jim Shankland <jas@flyingfox.com>, gpalmer@FreeBSD.ORG
Cc:        HARDWARE@FreeBSD.ORG
Subject:   Re: isa bus and boca multiport boards
Message-ID:  <3.0.1.32.19970522181207.0072d920@lariat.org>
In-Reply-To: <199705221647.JAA03333@biggusdiskus.flyingfox.com>

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At 09:47 AM 5/22/97 -0700, Jim Shankland wrote:
 
>This "tuned asm" thing is perilously close to an urban myth, anyway.
>To the extent there's any "lift" to be gotten from asm at all, it's
>from tiny little pieces in performance-critical inner loops, 

Exactly. The guts of the sio driver ARE some performance-critical inner loops.

>especially
>if there's a hardware-specific instruction that the compiler won't
>generate.

I'd have to look at the generated code to see how well I/O is done. But
I'd HOPE that the macros do the right thing.

>It's one thing to say, "I've just spent 3 weeks of backbreaking labor
>tuning the sio driver, and made it X% faster; then I experimented and
>found that, by replacing the following N lines of C code with assembler,
>I made it Y% faster still"; and another to wave one's hands about
>a fast sio driver written in assembler.  Given the costs in portability,
>maintainability, and effort required for assembler, arguments for its
>use must be supported by strong, empirical evidence of benefit.

My experience on other OS platforms dictates that there IS a large measure
of efficiency to be gained -- on the order of 25% time savings. The
improvements are partly from ASM and partly from good algorithms.

--Brett




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