Question regarding ARMV7M FPU context switch and ISRs

[Chris Johns]

Sorry as I sent this I could see the subject and 7M.

Chris

On 9/2/2024 1:33 pm, Chris Johns wrote:
> On 8/2/2024 9:55 pm, Sebastian Huber wrote:
>> On 08.02.24 11:43, Cedric Berger wrote:
>>> On 08.02.2024 11:09, Sebastian Huber wrote:
>>>> On 08.02.24 10:53, Cedric Berger wrote:
>>>>>
>>>>> This would also simplify the context switching code, by centralizing of the
>>>>> saving of the FPU context in RTEMS only, and enabling optimisation like only
>>>>> saving/restoring the FPU when switching between tasks defined with
>>>>> RTEMS_FLOATING_POINT.
>>>>>
>>>>> What do you think? I'm missing something? would it be a good idea?
>>>>
>>>> From experience, working with the RTEMS_FLOATING_POINT in applications is
>>>> quite annoying. Is there really a measurable and significant performance
>>>> improvement if you enable the deferred FPU switching? Can you guarantee that
>>>> the compiler will not generate FPU or vector instructions for integer
>>>> operations? In this version or a GCC release in the future?
>>>
>>> Obviously, since I'm not God, I won't be able to provide any guarantee
>>> regarding the future :)
>>>
>>> But I believe that if GCC started to use FPU for integer operations, many
>>> people would complain:
>>>
>>> FreeBSD requires fpu_kern_enter/fpu_kern_leave to use FPU in the kernel, and
>>> Linux requires kernel_fpu_begin/kernel_fpu_end to use FPU ops in the kernel.
>>>
>>> I'm pretty sure Linus will give GCC developpers a hard time if they start to
>>> use FPU for integer operations anytime soon...
>>
>> I am definitely sure that on PowerPC the AltiVec unit is used to optimize memory
>> copies and initializations. I agree that it is unlikely that GCC will use the
>> FPU for integer operations.
> 
> FPU hardware often have wider data paths so they can be used for high speed
> copies, ie 128bit at a time, so it is not about integers ...
> 
> https://github.com/RTEMS/sourceware-mirror-newlib-cygwin/blob/master/newlib/libc/machine/arm/memcpy-armv7a.S#L31
> 
> Please be careful with any changes in this area.
> 
> Chris
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