anchao opened a new pull request #5645:
URL: https://github.com/apache/incubator-nuttx/pull/5645
## Summary
arch/arm: optimize context switch speed
The current context save implementation saves registers of each task
to xcp context, which is unnecessary because most of the arm registers are
already saved in the task stack, this commit replace the xcp context with
stack context to improve context switching performance and reduce the tcb
space occupation of tcb instance.
Signed-off-by: chao.an <anc...@xiaomi.com>
## Impact
arm context switch
## Testing
Qemu:
sabre-6quad/smp, sabre-6quad/netknsh
Cortex-m33: cycle count test
`#define DWT_CYCCNT (DWT_BASE + 0x0004) /* Cycle Count
Register */`
**thread 1: (priority 100)**
```
start = getreg32(DWT_CYCCNT);
sem_post(&g_exclsem);
```
**thread 2: (priority 200)**
```
sem_wait(&g_exclsem);
end = getreg32(DWT_CYCCNT);
```
Before:
`semaphore wait test: 797`
After:
`semaphore wait test: 604` + ~24%
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