uVision5 still skips codes under -O0 optimization level

Hiilda said:

I am new to Keil and I am implementing some crypto algorithms on a platform using ARM cortex M23. The platform has 3KB RAM and 30KB ROM.

3 KB RAM is very less. You need to be careful when you define the stack and maybe heap.

Hiilda said:

When I debug, if the plain text size is small (2048bytes), everything is fine. But if I increase the size of plain text to 3072 or 4096, the encryption will be skipped and I cannot see the result.

Do you keep the strings that should be encrypted in RAM? Did you check your map file if your application does not allocate more than 3 KB? I could imagine that the original string, as well as the encrypted string, needs to be stored in RAM. If this is the case, 3 KB RAM would be way too less.

Hiilda said:

I added breakpoint right before and after the encryption function call but it doesn't stop. It goes back to the beginning of the whole program directly after I click "step over" before function call.

This looks like a reset or exception which is caused by your encryption algorithm. You should see the reason when you single step through your encryption algorithm and if you watch the Exceptions dialog. I'm sure that no code is skipped or not translated.

Hiilda said:

I am new to Keil and I am implementing some crypto algorithms on a platform using ARM cortex M23. The platform has 3KB RAM and 30KB ROM.

3 KB RAM is very less. You need to be careful when you define the stack and maybe heap.

Hiilda said:

When I debug, if the plain text size is small (2048bytes), everything is fine. But if I increase the size of plain text to 3072 or 4096, the encryption will be skipped and I cannot see the result.

Do you keep the strings that should be encrypted in RAM? Did you check your map file if your application does not allocate more than 3 KB? I could imagine that the original string, as well as the encrypted string, needs to be stored in RAM. If this is the case, 3 KB RAM would be way too less.

Hiilda said:

I added breakpoint right before and after the encryption function call but it doesn't stop. It goes back to the beginning of the whole program directly after I click "step over" before function call.

This looks like a reset or exception which is caused by your encryption algorithm. You should see the reason when you single step through your encryption algorithm and if you watch the Exceptions dialog. I'm sure that no code is skipped or not translated.

Hiilda said:

I am using a open source library, so I don't know much details about how the author allocates stack and heap.. but I will manage that.

The stack and heap is usually not defined in the library. You define both in the startup files or in your linker scatter file.

Hiilda said:

By the way, how can I know whether there is stack overflow?

There is no stack overflow exception unless you don't use the MPU for the stack. A stack overflow will overwrite other variables or the application will crash when returning from a subroutine. In the map file, you see where the stack is located. This application note 316 tells you how to analyze the stack consumption:

https://developer.arm.com/documentation/kan316/latest/

Even if you don't use a Real-Time OS, you should read the paragraph "Main stack watermarking". Then you could see the current stack consumption in the uVision debugger.

Hiilda said:

I didn't keep the string in RAM, I kept it in ROM. And the encrypted string is 128bits, so I don't think it will run out RAM space.

OK, this looks reasonable.

Hiilda said:

Where can I see the Exceptions dialog?

Menu 'Peripherals - Core Peripherals - Fault Reports'.

Hiilda said:

Total RW  Size (RW Data + ZI Data)              1032 (   1.01kB)

This looks extremely low but might be correct.