问题
I need to have a function busy-wait.
for(long int j=0; j<50000000; ++j)
;
When I compile in release mode, this gets optimized out. Other than compiling in debug mode, is there some way to cause this to not get optimized out? I don't particularly care about the actual number of the loop, but it must be a noticeable busy-delay.
回答1:
I don't know why you need to keep the CPU busy, but let's assume that you really have a good reason, like making sure you keep the CPU busy so it doesn't think about that breakup it went through last week and get all depressed but I digress...
The problem you are seeing is that the compiler performs "dead code elimination": it sees that the loop does nothing (i.e. has no side-effects) and so cuts it out. So you could make it have a side-effect.
A simple solution would be this function:
void busywait(long iterations)
{
for(volatile long i = 0; i != iterations; i++)
;
}
By marking i as volatile you are ensuring that the loop has side-effects, since stores to volatile objects (i.e. the incrementing we perform) are treated as having side-effects.
来源：https://stackoverflow.com/questions/13921753/how-to-prevent-optimization-of-busy-wait