From: bobduff@world.std.com (Robert A Duff)
Subject: Re: aliased & packed?
Date: 1997/05/13
Date: 1997-05-13T00:00:00+00:00 [thread overview]
Message-ID: <EA4nII.803@world.std.com> (raw)
In-Reply-To: 1997May13.065907.1@eisner
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
next prev parent reply other threads:[~1997-05-13 0:00 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
1997-05-11 0:00 aliased & packed? tmoran
1997-05-11 0:00 ` Robert Dewar
1997-05-11 0:00 ` Tom Moran
1997-05-12 0:00 ` Robert Dewar
1997-05-12 0:00 ` Robert A Duff
[not found] ` <1997May13.065907.1@eisner>
1997-05-13 0:00 ` Robert Dewar
1997-05-13 0:00 ` Robert A Duff [this message]
1997-05-13 0:00 ` Larry Kilgallen
1997-05-14 0:00 ` Richard Kenner
-- strict thread matches above, loose matches on Subject: below --
1997-05-09 0:00 Tom Moran
1997-05-09 0:00 ` Stephen Leake
1997-05-09 0:00 ` Tom Moran
1997-05-10 0:00 ` Robert Dewar
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