Modify CTL to save FPU selectively

An application I'm working on uses the FPU.

When the FPU is used with the CTL - all FPU registers are saved on the task's stack. FPU related resisters take up about 100 bytes of ram.

I'm going to modify the CTL by adding some flags to the task's TCB - one of which states whether floating point operations are used in that task. If no FP operations are used - then the time and resources associated with saving the FPU on the stack is wasted.

Can you see any pitfalls associated with selectively saving the FPU registers?

I would program the arm to use FP instructions but not perform FPU register stacking. When a task is made active or placed on hold, the modified CTL would push/pull the FPU registers only as required.

Each interrupt would have to know if a save was required also.

My application uses 5 tasks, of which only 2 use FP instructions --- so I would save approximately 300 bytes of ram.

Do you see why this is not possible?

Thanks

Michael Johnson

September 06, 2015 06:56

I started on this route - someone from ARM pointed out that there's no guarantee that a compiler won't use the FP registers for integer operations. So in general it's not an option - but it may be for your case.

Joe

September 06, 2015 12:23

The comment from the ARM guy sounds like a very general statement. If the FPU is present, then the compiler has more registers to work with.

Are you aware of gcc having this behavior?

September 06, 2015 12:29

I reviewed a UCOS-III implementation that did exactly what I'm asking -- After I understood what the implementation was doing -- it looked like an opportunity to save ram.

September 07, 2015 07:12

The cm4/cm7 ctl implementation will only save the FPU registers if they have been used...

I've not seen it - but/maybe if the register pressure is too high then gcc will use the FPU registers rather than spill to memory.

Modify CTL to save FPU selectively

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