Re: [gentoo-embedded] pxa270 cflags?

[Martin Guy <martinwguy@gmail.com>]

Strictly speaking, iWMMXt is a coprocessor of the ARM, not a CPU, so
technically it is like an FPU except that it does not do floating
point, so it is modelled as a cpu in GCC, with an instruction set that
is a superset of the armv5te set. It's a bit like the difference
between pentium and pentium MMX. It only exists in silicon as a
coprocessor for armv5te cores as far as I know, hence "iwmmxt is
armv5te with extra stuff".
  Yes, it has its own old-ABI, which requires 8-byte alignment of
double words. EABI (v4) has this anyway so it may work.
  Furthermore, it hijacks the same coprocessor interface as the old
FPA floating point accelerator, or in other words, the bit patterns of
its instructions overlap with the FPA instruction set, so you must use
soft float and cannot use kernel emulation of FPA instructions. That's
not really a problem since no silicon exists with both iwmmxt coproc
and a real FPU of any kind, so you would want to use softfloat anyway.
  The main instructions that iwmmxt adds to the armv5te set are real
64-bit integer operations and a slew of vector instructions for those
tight inner loops. Whether it works, of course... have you tried
building gcc for it and running the testsuite on iwmmxt hardware? Prod
me if you need runes to do this.

>  gcc fails saying that "-march=armv5te -mtune=iwmmxt" is not kosher

Indeed. iwmmxt is modeled as both a cpu and an ISA, but unless there
are iwmmxt insns included, the tuning would be a no-op. It would be
like cpu=armv4 arch=armv4t - the armv4 can't run all armv4t
instructions.

Did you run into problems with cpu=iwmmxt?

   M