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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: aliased & packed?
Date: 1997/05/13
Message-ID: <EA4nII.803@world.std.com>#1/1
X-Deja-AN: 241324747
References: <5l5llv$4e@lotho.delphi.com> <dewar.863449539@merv>
 <EA33z0.2ts@world.std.com> <1997May13.065907.1@eisner>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1997-05-13T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <1997May13.065907.1@eisner>,
Larry Kilgallen <kilgallen@eisner.decus.org> wrote:
>But that is for somebody who wants _only_ a single byte.  In a great
>many programs one wants several bytes in succession, and once a longword
>or quadword has been loaded it can be shifted and masked several times to
>produce the desired bytes.  Since one memory access can produce up to
>8 bytes, there is a clear performance benefit on a newer machine for
>many programs to _avoid_ byte memory accesses.

Is this really true?  I mean, if you access one byte at the start of a
word (using byte instructions on the 21164), then won't the next seven
be sitting there in the cache waiting for you?

And it seems like more than a couple of instructions to shift and mask
an 8-byte word (I mean, using 21064 instructions only), in the case
where the byte you want is dynamic (i.e. you're indexing randomly into
an array of bytes -- on the other hand, most array accesses are probably
sequential).

>Which I believe supports my claim that this a compatibility rather than
>a pure performance move.

Quite likely true.  ObAda: I doubt it was compatibility with Ada's rules
about tasking and independent addressability that drove the decision.
;-)

- Bob