malloc and gcc optimization 2

Question

while(count < 30000000){
    malloc(24);
    count++;
}

the above code runs in about 170 ms on my computer compiled with gcc -O0. However, compiling with -Ox where x > 0, the optimizer cleverly figures out that the memory being requested will never be used and so it is excluded from the optimized executable. How does it do this?

Answer 1

Well the compiler sees malloc return value is never used so it optimizes it out. If you want to prevent malloc call to be optimzed out even in -O3 you can use the volatile qualifier:

while(count < 30000000){
    void * volatile p = malloc(24);
    count++;
}