[fpc-devel] FPC for Linux-ARM

[Bernd Mueller]

Henry Vermaak wrote:
> On 20/01/11 08:20, Michael Schnell wrote:
>> On 01/19/2011 05:13 PM, Henry Vermaak wrote:
>>>
>>> it's really expensive to trap the illegal instructions and emulate them.
>> AFAIK, the trapping is done by the ARM CPU, anyway, providing the
>> emulation functions just costs some memory, but of course it's even
>> slower and more intrusive than directly calling an FPU emulation
>> function in user space.
> 
> No, the cpu generates an exception for every illegal fpu operation, 
> which gets trapped by the kernel.  The kernel then has to catch this 
> instruction and perform it by emulating the _whole_ fpu.  This, together 
> with the exception handling causes the bad performance.  Look at the 
> code in arch/arm/nwfpe if you're interested.

three years ago, I compared an OABI kernel with double precision 
Netwinder FPE emulation against softfloat and the softfloat variant was 
about 2 until 10 times faster, depending on the floating point instructions.

> 
> Henry
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