The ABI defines how user code-calls (library-)functions. There are three
(1) user-code calls by using the soft-abi, library's interface is
soft-abi and the library uses FPU emulation
(2) user-code calls by using the soft-abi, library's interface is
soft-abi and the library uses hardware-FPU (for example v*-commands on
(3) user-code calls by using hard-abi, library's interface is hard-abi
and the library uses hardware-FPU
Libraries are for example the libraries which come with the toolchain
(like libgcc). If libraries for all options are available and all code
of the user-application can be recompiled the 3rd method may have the
advantage that there is no need to transfer registers-content. I did not
look too close into the generated assembly-code to give information
about the "overhead", maybe somebody else can.
As far as I can see (I just made a few tests with STM32F4 Cortex M4 and
GNU tools) the main advantage of the soft-abi is the possiblity to "mix"
libraries which are only available as object code (archives/"a-files").
As long as the library has been compiled with soft-abi and it uses
FPU-emulation internally it can be linked with every other object-code
that uses the soft-abi and everything should work. For hard-ABI all
object-code in the application has to be compiled with this setting and
the hardware-FPU has be be enabled. If a library uses the soft-ABI but
the hardware-FPU interally the interface is compatible with other code
but if the FPU is not enabled there will be core-exceptions.
You may read the getting-started.pdf for Codebench lite from
Mentor/Codesourcery for some more information and the GNU manuals for
Hope this helps at least it little bit.