GDB on subtle lines when next or step

Question

I use GDB to debug an embedded linux target board. On target board gdbserver is running the stripped binary application. On PC side I run application which has debug symbols attached.

From PC via ethernet connection I do "target remote ip:port" and there is a healthy connection to gdbserver. I add break points in some functions and continue. Debugger successfully break as expected when I trigger external event and program breaks in my simple function. Weird behaviour starts at this point. When I do step or next in GDB console instead of next line it goes to previous line than goes to two lines forward and goes back two lines backward. Each time I do next or step this continues. There is no loops or jumps, code should just go line by line but it doesn't. Why GDB behaves like this? What should I do to solve this problem.

Answer 1

This is normal behaviour for code to which a high level of optimisation has been applied. Unless you are actually intending to debug the optimiser, executing optimised code in a debugger is almost useless.

From the manual:

When you debug a program compiled with ‘-g -O’, remember that the optimizer has rearranged your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file!