Re: [gentoo-dev] Profile-enforced big-endian USE flag

[Joshua Kinard <kumba@gentoo.org>]

On 06/28/2017 18:29, James Le Cuirot wrote:
> On Wed, 28 Jun 2017 17:52:26 -0400
> Mike Gilbert <floppym@gentoo.org> wrote:
> 
>> On Tue, Jun 27, 2017 at 6:44 PM, James Le Cuirot <chewi@gentoo.org> wrote:
>>> I am therefore proposing a new global big-endian flag. This could be
>>> masked by default and unmasked + forced in the relevant profiles under
>>> arch. I will apply this according to the mapping defined in tc-endian of
>>> toolchain-funcs.eclass.  
> 
> I've just been putting the patch together. I made it slightly simpler
> by masking *and* forcing it by default so that it only needs to be
> unmasked were necessary.
> 
>> A possible alternative would be to create a new USE_EXPAND variable
>> for this. That would allow for easier expansion in case we ever
>> support something other than big/little endian machines.
> 
> That way madness lies? Wikipedia talks about middle-endian as being the
> catch all for other random orderings that have appeared over the years
> but I don't think any of them were used on a system-wide basis. I can't
> imagine Linux ever supporting such a thing. Unless you're talking about
> dealing with soft vs hard float here too?

As far as I know, Linux only supports big-endian and little-endian systems,
along with supporting a fairly-wide variety of platforms and architectures,
beaten really only by NetBSD.  If NetBSD lacks anything for middle-endian, then
it's probably safe to ignore it.

That said, Wikipedia doesn't offer much on the nature of middle-endian (which
is called "PDP-endian"), except to state that "The ARM architecture can also
produce this format when writing a 32-bit word to an address 2 bytes from a
32-bit word alignment."
https://en.wikipedia.org/wiki/Endianness#Middle

And this resource:
http://unixpapa.com/incnote/byteorder.html

Recommends to ignore middle-endian, as it believes no processor was ever
implemented that stored integers in middle-endian format.

-- 
Joshua Kinard
Gentoo/MIPS
kumba@gentoo.org
6144R/F5C6C943 2015-04-27
177C 1972 1FB8 F254 BAD0 3E72 5C63 F4E3 F5C6 C943

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