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From: Paul Rubin <no.email@nospam.invalid>
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Subject: Re: AI12-0218:  What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Mon, 14 May 2018 17:43:53 -0700
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> What's "BSWAP"? It's not an instruction I'm familar with on x86. 

http://www.felixcloutier.com/x86/BSWAP.html

> (Admittedly, I don't usually use newer instructions because of the
> need to have some sort of alternative to run on older processors.)

It was introduced with the 486 so I think it runs on almost any x86 that
anyone is currently using.

> In any case, one has to build such support into the back-end.

In GCC you can write intrinsics by using pragmas that include the
desired assembly code.