[Haskell-cafe] Re: Detecting system endianness

wren ng thornton wren at freegeek.org
Sat Dec 20 01:30:22 EST 2008

Maurí­cio wrote:
> But why would you want that? I understand the only
> situation when talking about number of bytes
> makes sense is when you are using Foreign and
> Ptr. Besides that, you can only guess the amount
> of memory you need to deal with your data (taking
> laziness, GC etc. into account).

Because I'm using both Ptr and Foreign? ;)

See my recent announcement for bytestring-trie. One of the optimizations 
I'm working on is to read off a full natural word at a time, instead of 
just one byte. To do this properly I need to detect the word size so 
that I don't accidentally read garbage off the end of the ByteString 
when there's less than a natural word left.

Detecting endianness is similar because it determines how to interpret 
that word as if it were an array of bytes, which is needed to get the 
correct behavior when interpreting the word as a bit-vector for trieing. 
That is, if you only read the "first" two bytes on a big-endian machine, 
then you're skipping the 4/6/? bytes which are actually at the beginning 
of the bytestring.

I'm not sure how important physical endianness of bytes within a word 
is. For IntMap a common case is to get large contiguous chunks of keys, 
so logical big-endian trieing improves performance over logical 
little-endian. I'm not sure how common large contiguous chunks of 
bytestring keys are, though. Reading a word then changing the physical 
endianness of the bytes seems expensive.

Live well,

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