floats / Porting GCC on a new arch

[Kai Ruottu]

Florent DEFAY wrote:

> My target machine doesn't support floats operations
 > so I defined these macros :
> #define TARGET_FLOAT_FORMAT IEEE_FLOAT_FORMAT
> 
> #undef  TARGET_DECIMAL_FLOAT_SUPPORTED_P
> #define TARGET_DECIMAL_FLOAT_SUPPORTED_P        target_decimal_float_supported_p
> // returns always false
> #undef  TARGET_FIXED_POINT_SUPPORTED_P
> #define TARGET_FIXED_POINT_SUPPORTED_P      target_fixed_point_supported_p
> // returns always false
> 
> and that's all about floats.
> 
> What should I additionally implement ?

The normal way with no-FPU CPUs seems to be to use the
'config/fp-bit.c' which implements the basic soft-float
routines in C.  These routines then will be a part of the
'libgcc.a', the "GCC helper library". Another way is to
implement these in the opsys kernel like in x86 with the
old i386, i486SX, "embedded x86" etc. "no-*87 FPU inside"
CPUs in the Linux kernel. The idea being that there could
be that FPU in the system but when there isn't, every float
opcode causes a "trap" or "exception" which the opsys kernel
will handle... Third way is to implement the soft-float ops
in the C library, I remember in Linux/PPC this being the
case, the 'glibc' for it includes the soft-float operations.

But with embedded "no FPU anywhere" CPUs the 'libgcc.a'
normally has those basic soft-float routines for add/sub/mul/div
for floats and doubles...  Just "spy" the already ported
CPUs, their "target Makefile-fragments", the 't-<cpu>' or
something files in 'gcc/config/<cpu>'...