Cortex-M0+ core Hang up in FFFF FFFEh address when executing DSB instruction.

 Hi, I am porting FreeRTOS to my M0+ device now.

I faced an issue CPU hang-up when DSB instruction executed.

Then Program Counter showed "FFFF FFFEh" and Exc value of xpsr is 0x03. So, I knew Hard_Fault error interrupt is happened.

But it has interrupt routine other address of FFFF FFFEh. So, I did not understand why CPU showed FFFF FFFEh address as PC value.

This is also weird, it was happening when I set -O0 of optimize level using GCC. It has never happen when using other optimize level...

I also check execute DSP instruction before starting RTOS of FreeRTOS. It can work properly....

Is there any hint to solve this issue?