The As If Rule

There were several reasons for the K&R C rearrangement rules:

• The rearrangements provide many more opportunities for optimizations, such as compile-time constant folding.

• The rearrangements do not change the result of integral-typed expressions on most machines.

• Some of the operations are both mathematically and computationally commutative and associative on all machines.

The ANSI/ISO C Committee eventually became convinced that the rearrangement rules were intended to be an instance of the as if rule when applied to the described target architectures. ANSI/ISO C's as if rule is a general license that permits an implementation to deviate arbitrarily from the abstract machine description as long as the deviations do not change the behavior of a valid C program.

Thus, all the binary bitwise operators (other than shifting) are allowed to be rearranged on any machine because there is no way to notice such regroupings. On typical two's-complement machines in which overflow wraps around, integer expressions involving multiplication or addition can be rearranged for the same reason.

Therefore, this change in C does not have a significant impact on most C programmers.