How to prevent GCC from optimizing out a busy wait loop?

Declare i and j variables as volatile. This will prevent compiler to optimize code involving these variables.

unsigned volatile char i, j;

I developed this answer after following a link from dmckee's answer, but it takes a different approach than his/her answer.

Function Attributes documentation from GCC mentions:

noinline This function attribute prevents a function from being considered for inlining. If the function does not have side-effects, there are optimizations other than inlining that causes function calls to be optimized away, although the function call is live. To keep such calls from being optimized away, put asm ("");

This gave me an interesting idea... Instead of adding a nop instruction at the inner loop, I tried adding an empty assembly code in there, like this:

unsigned char i, j;
j = 0;
while(--j) {
    i = 0;
    while(--i)
        asm("");
}

And it worked! That loop has not been optimized-out, and no extra nop instructions were inserted.

What's more, if you use volatile, gcc will store those variables in RAM and add a bunch of ldd and std to copy them to temporary registers. This approach, on the other hand, doesn't use volatile and generates no such overhead.

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Update: If you are compiling code using -ansi or -std, you must replace the asm keyword with __asm__, as described in GCC documentation.

In addition, you can also use __asm__ __volatile__("") if your assembly statement must execute where we put it, (i.e. must not be moved out of a loop as an optimization).