sparc-rtems4.7-gcc versus fpu

Stephan Gruenfelder stephan.gruenfelder at
Wed Jun 13 08:38:10 UTC 2007


I agree with Joel that this is not an RTEMS topic. The answer most likely is the 
-mv8 option of gcc, which enables the FPU. However, if you compile mission 
critical code for the AT697E (AT697E-2E-E), then you might want to turn off the 
FPU anyway, because of a Multiplier/Divider failure in the LEON2 VHDL model (see 
errata sheet of the AT697).


Joel Sherrill wrote:
> Peter A. Krauss wrote:
>> Hello,
>> I have downloaded sparc-rtems4.7 and would like to use it for a leon2 
>> target with fpu. I am puzzled on how the compiler inserts fpu 
>> instructions. My summary is:
>> - The compiler does insert fpu instructions, if no fpu command line 
>> option is given or if "-mhard-quad-float" is used.
>> - The compiler does not insert fpu instructions, if "-mfpu" or 
>> "-mhard-float" is used.
>> Why?
> It would take a gcc expert to answer for sure but I recall a recent
> discussion on the gcc list which sounds like it is happening here.
> I suspect that since hard FPU is the default, -mfpu is toggling the
> default and turning it into soft float.
> I tried this with gcc 4.2.0 and got your behavior.  I tried it with
> the RTEMS 4.6 gcc 3.2.3 and it still generated hardware FPU instructions.
> Can you send this report to gcc AT and CC me?  I think
> someone over there will have a better answer.
> --joel


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