[titzer]

C "supports" whatever the hardware and compiler feel like supporting. It absolutely does not mandate anything. In particular, there are a number of particularly simple compiler optimizations that are not forbidden, though they are not technically correct according to IEEE 754, such as algebraic reassociation and simple commutativity. Moreover, C allows subexpressions to be computed in higher precision (e.g. 80 bit "long double"), which is observable. That last one is primarily due to the x87 FPU coprocessor design that has given us a good 35 years of headaches. Good riddance to that!