10.6 Optimization and Ordering

Sequence points limit the compiler’s freedom to reorder operations arbitrarily, but optimizations can still reorder them if the compiler concludes that this won’t alter the results. Thus, in this code,

x++;
y = z;
x++;

there is a sequence point after each statement, so the code is supposed to increment x once before the assignment to y and once after. However, incrementing x has no effect on y or z, and setting y can’t affect x, so the code could be optimized into this:

y = z;
x += 2;

Normally that has no effect except to make the program faster. But there are special situations where it can cause trouble due to things that the compiler cannot know about, such as shared memory. To limit optimization in those places, use the volatile type qualifier (see volatile).