Some of the embedded
PowerPC processors do not have a floating point unit(FPU),
and so must perform all floating point operations in software. Others do have a
hardware FPU unit and may perform operations in hardware.
If your application has very intensive floating point requirements, you may
need to switch to fixed point or choose a target processor which does have an
FPU.
Floating point can be performed either by instruction emulation in the kernel,
or by compiling
everything with
-msoft-float. In particular, it's
important that all the libraries, whether dynamically or statically linked are
compiled with the same options as the binaries that use them. Unless you're
using a toolkit where this has already been done for you, this will generally
mean that if you wish to use
-msoft-float to gain maximum performance, you
need to (re)compile
everything, including:
- All static and shared libraries
- The compiler's internal libraries, such as :libgcc.a:
- All executables
You will almost certainly not get this right first time. The primary symptom
is that
printf gives bogus numbers for simple floating point values.However,
if you succeed, you can save space by configuring the kernel without
floating point emulation.
See
http://lists.linuxppc.org/listarcs/linuxppc-embedded/199911/msg00056.html
To add kernel math emulation to the 2.2.13 kernel, see
http://lists.linuxppc.org/listarcs/linuxppc-embedded/199912/msg00017.html
The "paranoia" test should give no complaints if everything is working
correctly. See
http://www.enseeiht.fr/NetLib/paranoia/index.html
If your
CPU has an
FPU, you'll want to use it. Make sure that everything is
consistently compiled for hardware floating point, especially if you've
assembled your toolkit yourself, or are using an 8xx toolkit to compile for the
8260.
Programs compiled for hardware floating point can still run on a
CPU without
an
FPU, provided the kernel is built with the floating point instruction
emulator. This incurs a further performance penalty on
CPUs lacking an
FPU.
If you are using a single set of shared libraries, you cannot mix the two
techniques in the one system. If you link statically or try
really hard by
creating two sets of shared libraries, the two can co-exist since programs
compiled with
-msoft-float never generate floating point instructions
requiring kernel emulation.
In general, you're better off choosing one or the other, and choosing hardware
floating point with kernel instruction emulation is much easier to get working
correctly.
