When compiling with Nios II SBT, the optimize option (-O2) causes it to stop working.

There were no problems with build or compile.
However, when I run it, execution stops near the following code

```
sthio zero,0(r2)
```

If you do not take `zero` as an operand as shown above, you can execute the function without any problem.
Specifically, I tried the following two
* Using pragma for the function in question and not optimizing it so that it takes a non-zero register as its operand
* using `volatile` for a variable that is an operand, so that it takes a non-zero register as an operand

If I build and compile in these ways, it runs without any problem.

Therefore, we thought that the problem might be related to the execution of the operand `zero` for the opcode `sthio`.