Re: virtual machine efficiency

hannah@schlund.de (Hannah Schroeter)
30 Jan 2005 13:43:46 -0500

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Re: virtual machine efficiency hannah@schlund.de (2005-01-30)
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From: hannah@schlund.de (Hannah Schroeter)
Newsgroups: comp.compilers
Date: 30 Jan 2005 13:43:46 -0500
Organization: Schlund + Partner AG
References: 04-12-151 04-12-163
Keywords: VM, performance

Hello!


VBDis <vbdis@aol.com> wrote:
>[...]


>Even if I know that on some machines misaligned reads are expensive,
>does anybody know of a concrete machine where reading a misaligned
>"word" will definitely take longer than reading the according number
>of individual bytes???


There are architectures that just don't do misaligned memory accesses
at all, producing a fault instead. The OS might handle the fault and
software-emulate the misaligned access, but handling faults is usually
*much* more expensive than
    word = (a[0] << 24) | (a[1] << 16) | (a[2] << 8) | a[3]


Kind regards,


Hannah.



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