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Re: MKSOFTFLOAT for evbppc
On Sep 27, 2012, at 8:23 AM, Eduardo Horvath wrote:
> On Wed, 26 Sep 2012, David Brownlee wrote:
>> 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.
> Well, if you have no FPU you have no FPU registers. If you have no FPU
> registers, any instructions to access them will fault. If you're going to
> fault anyway, you may as well do the whole FP operation in the kernel
> thaty way you only take one fault for the FPOP instead of one fault to
> read the register contents and another fault to write the results back.
An alternative is to always use the soft-float ABI and just have a
implementation of it that uses hard-float instructions. You may have a few
extra instructions to move arguments/results between register sets (as well as
the call itself) but it shouldn't affect performance that much.
I actually implemented one of those for the 85xx using the SPE FP instructions
and it gets loaded using LD_PRELOAD. Maybe I should do a build using that and
see how much time it saves over the normal softfloat implementation.
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