Re: Robust detection of endianness at compile time.

[Nick Maclaren <nmm1 at cus dot cam dot ac dot uk>]

"Lee Rhodes" <lee@rhoba.com> wrote:
>
> I presume you mean an early FP coprocessor, where the CPU issues the
> instruction and the binary data sent to the FP unit may have different byte
> orders depending on the origin of the data?

Precisely.  And whether it uses it or not will depend on whether it
is there at start-up.  Such designs are staging a (perhaps temporary)
come-back - look up Clearspeed and SGI/AMD/Intel's plans for hybrid
SMP designs.

> Sorry, when I said "compiler" I meant compiler/assembler. Nonetheless, what
> do you mean "only for C integers"?  

The compiler does not actually need to know the layout of
floating-point, even in C99 - after all, C90 had to support the
designs mentioned above.  And a Fortran compiler doesn't need to
do so even for integers.  There is nothing in the languages that
expose those to the program at compile time.

> That is certainly true for the byte shuffle operations for conversion. But a
> run-time detection of endianness might require a memory operation.  I would
> have to think about it.

Oh, it does.  Probably lots of them.  But you need do it only once.
Thereafter, the shuffling can be incredibly cache-friendly.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1@cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679