gcc optimize busy waiting as dead loop

Question

I'm implementing a single-producer single-consumer queue, by which one thread waits for the global queue to be filled by another thread like this:

while (queue.head == queue.tail);

When I compiled the program will gcc -O0, it worked well. But when it was compiled with gcc -O1, deadloop happened. Then I looked into assembly code and found that the latter version checked (queue.head == queue.tail) only once, if it was not true, then jumped to a dead loop and never checked again.

I also tried to declare queue as volatile but it didn't work. How to make gcc aware that queue is shared among threads and stop optimizing like that? Many thanks.

P.S.

1 In a single-threaded program, it is OK to optimize like that. But in my program queue.tail can be modified by another thread.

2 My queue was declared like this:

typedef struct {
    struct my_data data[MAX_QUEUE_LEN];
    int head;
    int tail;
} my_queue_t;

volatile my_queue_t queue;

3 I've also tried to declare head and tail (but not the whole struct) as volatile, it didn't work. But after I declare queue, head, tail all as volatile, it works. So is volatile should be declared to all the related variables like this?

Answer 1

Have you tried declaring head/tail as volatile?

Answer 2

I compiled the following code:

struct my_data {
    int x;
};

typedef struct {
    struct my_data data[5];
    int head;
    int tail;
} my_queue_t;

volatile my_queue_t queue;

int main() {
    while (queue.head == queue.tail);
}

with :

g++ -S -c -O1  th.cpp

which (for the while loop) produced the folowing output:

       movl    $_queue+20, %edx
       movl    $_queue+24, %eax
L2:
       movl    (%edx), %ebx
       movl    (%eax), %ecx
       cmpl    %ecx, %ebx
       je      L2

where the head and tail are loaded & tested inside the loop. Could you post what assembler you are getting emitted?

Edit: Making head and tail volatile in the struct declaration, rather than declaring the struct instance volatile, resulted in identical code.