Re: Software FP Support

["Joseph S. Myers" <joseph@codesourcery.com>]

On Tue, 30 Mar 2010, Paulo J. Matos wrote:

> I have 16 BITS_PER_UNIT for this target, with 1 UNITS_PER_WORD. If I
> define FLOAT_TYPE_SIZE to be 32, then a float is an HFmode and it's
> not that gcc uses this.
> Defining BITS_PER_UNIT to 16 shouldn't generate problems inside the
> compiler given that GCC has generic treatment for these situations

The GCC support for BITS_PER_UNIT values other than 8 is extremely 
bitrotten and parts of it have never worked; there are no such targets in 
the GCC source tree, no patches are tested for such targets and it is easy 
to have hidden dependencies on 8-bit bytes.  It is very inadvisable to 
make your first port one to such an unusual target; substantially more GCC 
experience and expertise is needed for doing a port to a system different 
from all supported systems and that challenges hidden assumptions in the 
GCC code.  There will be a large amount of work finding and fixing issues 
related to non-8-bit bytes that will need doing before you have a working 
integer port, and all that should be done before you look at 
floating-point issues.

If seriously interested in fixing support for non-8-bit bytes, first 
understand exactly what the C semantics are for such systems (including 
how strings are meant to work).  Then read my various past messages to the 
GCC lists discussing aspects of support for such systems in GCC (which you 
should be able to find for yourself in the list archives).  Improving wide 
string support for normal 8-bit-byte systems (so that optimization etc. 
infrastructure can work with both wide and narrow strings) may be a useful 
starting point for making some things work when the characters in narrow 
strings are more than one host byte.

-- 
Joseph S. Myers
joseph@codesourcery.com