RFC: PowerPC floating point features

[Joseph S. Myers]

On Fri, 4 Apr 2008, Michael Eager wrote:

> Xilinx has a PowerPC 405 processor with an attached
> single precision floating point processor.  I have a
> patch which supports this FP unit, but want to clean
> it up a bit before submitting it.

What do you propose as the function call and return ABI when this is in 
use?  At present we (32-bit GNU/Linux and EABI) have a hard-float ABI 
(TARGET_HARD_FLOAT && TARGET_FPRS), which relies on 64-bit FPRs to be able 
to pass double (so isn't usable if this processor only has 32-bit FPRs), 
and a soft-float ABI, also used on E500, passing floating point types in 
GPRs (32 bits of each GPR, not using the E500 high parts).  Are you going 
to use the soft-float ABI and take the performance cost of doing so, or do 
you propose a hybrid ABI passing float in FPRs and double in GPRs?  If you 
propose the latter then you should work with the Power.org ABI working 
group on defining the ABI properly.

(Whatever you do, you may wish to consider how to define flags relating to 
the ABI separately from those relating to code generation.)

-- 
Joseph S. Myers
joseph@codesourcery.com