Keil debug view variable prompt not in scope

Screenshot of the problem ：
There was a warning when compiling ：…\src\user_main.c(155): warning: #550-D: variable “crc_buffer” was set but never used

I started by changing the variable type , from uint32_t Changed to uint8_t,uint16_t,uint64_t No way to find out , I wonder if it has been optimized , Then the optimization level is changed, as shown in the figure below , from Level 3 Changed to Level 0 I can't find it .

But I still want to be optimized , But I didn't think of a way .
I solved the warning , This variable is used , Even if this variable is used in the wrong way , But I can see the value , Here's the picture .

I consulted my master who is now in the company ( Miss Zhao ), Then he prompted me to use volatile, Then I found that I could see the value , Then he let me change to static, I found that it can also , Then I was prompted to think about the area where my parameter variables were placed ( Stack ), Let me think for myself .

My thoughts ( If you make a mistake, please correct it )：
Compiler optimization , Command variables in the optimization stack , If you don't optimize variables in the stack , So it's like I declared many variables but didn't use them , It can easily lead to stack overflow problems . Add volatile The keyword is to prevent the compiler from optimizing this variable in the stack , Use static This variable is not stored on the stack , It is stored in the static variable area ( Pile up ), Then it will not be optimized .

After the above thought , I just thought if that was the case , If I change a variable into a global variable, there is no problem , The fact is that there is no problem .

I changed the optimization level again , from Level 0 Changed to not use (default), And the variables are still on the stack ( local variable ) in , Discover or not in scope

I will read many articles to solve this problem , Is to write and change the optimization level , But it was not written on the dot , Why change the optimization level ？ Are there other ways to prevent optimization ？

After this, I also know a little about compiler optimization .