Re: MKSOFTFLOAT for evbppc

On 26 September 2012 20:37, Matt Thomas <[email protected]> wrote:
>
> On Sep 26, 2012, at 12:35 PM, David Brownlee wrote:
>
>> On 26 September 2012 19:48, Greg Troxel <[email protected]> wrote:
>>>
>>> So it's good to hear that trapfloat (?) works (did you install that
>>> build?), but 6% seems like enough that using softfloat is in order.
>>> But, I see the point that having one userland usable on all evbppc is
>>> nice, and perhaps not worth 6% in the standard distribution, since using
>>> softfloat on machines with hardfloat (not trapfloat) is surely vastly
>>> slower.
>>
>> How much of this might be gained back by just a softfloat compiled
>> libm and possibly libc (or is the ABI incompatible).
>
> ABI is incompatible since it changes what values are passed in normal
> .vs. floating point registers.

Thats disappointing, but understandable :)
Is there some variant that will get gcc to use the fp registers but
not the instructions (assuming it doesn't trap on fp register
usage..): -mno-fused-madd -mno-mulhw -msimple-fpu looks to turn off
some instructions but not all.

If enough of that 6% cost happens in libc or libm it may still be
worth the effort...