[Vm-dev] float word order

Bryce Kampjes bryce at kampjes.demon.co.uk
Sun Apr 19 13:43:50 UTC 2009

On Sat, 2009-04-18 at 18:15 -0700, Eliot Miranda wrote:
> Hi All,
>     I see that Float 32-bit word order is big-endian (PowerPC) on all
> platforms.  This is a pain for performance and a pain for code
> generation in Cog.  For example using SSE2 instructions it is trivial
> to swizzle a PowerPC-layout Float into an xmm register using the
> PSHUFD SSE2 instruction but tediously verbose to swizzle on write,
> because one has to swizzle to an xmm register which is hence
> destructive, which means three instructions (shuffle, write,
> unshuffle) just to write a Float result.  Yes, ok 2 extra instructions
> is small potatoes, but they're still starch.  So I wonder what would
> the impact be of maintaining Floats in platform order?  There are a
> number of possible solutions.
> 1. Floats are always in platform order and swizzled on image load when
> moving from little-endian to big-endian or vice verce.  Image code
> must be rewritten to take the platform's endianness into account.
> (requires an image rewrite)
> 2.  As for 1 but the image is isolated from the change by providing
> two primitives, primitiveFloatAt and primitiveFloatAtPut which are
> implemented with selectors at: basicAt: at:put: and basicAt:put: on
> Float.  These primitives map index 1 onto the most significant word
> and index 2 onto the least significant word.  (requires no image
> rewrite, but does require a file-in of the four implementations)

I'd like to see Floats stored in native format too.  Don't forget about
the 32 bit floats in Float arrays.


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