Why is GDB "jumping back" when debugging with c source codes

Question

I'm debugging the goldfish android kernel (version 3.4), with kernel sources.

Now I found that gdb sometimes jump back and forth between lines, e.g consider c source code like the following:

char *XXX;
int a;
...

if (...)
{

}

When I reached the if clause, I type in n and it will jump back to the int a part. Why is that?

If I execute that command again, it would enter the brackets in the if.

If possible, I want to avoid that part, and enter the if directly (of course, if condition matches)

I've not debugged in that environment, but I would guess it's to do with optimisations / code-rearragnement that the compiler has done. For example, if the `a` was only used inside the `if`, then it may not get created/initialised unless the `if` is entered. — TripeHound, Jun 24 '14 at 14:32

score 10 · Accepted Answer · edited May 23 '17 at 11:53

When I reached the if clause, I type in n and it will jump back to the int a part. Why is that?

Because your code is compiled with optimization on, and the compiler can (and often does) re-arrange instructions of your program in such a way that instructions "belonging" to different source lines are interleaved (code motion optimizations attempt (among other things) to move load instructions to long before their results are needed; this helps to hide memory latency).

If you are using gcc-4.8 or later, build your sources with -Og. Else, see this answer.

Why is GDB "jumping back" when debugging with c source codes

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