As-if rule

Allows any and all code transformations that do not change the observable behavior of the program.

The C compiler is permitted to perform any changes to the program as long as the following remains true:

1) At every sequence point, the values of all volatile	(until C11)
1) Accesses (reads and writes) to volatile objects occur strictly according to the semantics of the expressions in which they occur. In particular, they are not reordered	(since C11)

2) At program termination, data written to files is exactly as if the program was executed as written.

3) Prompting text which is sent to interactive devices will be shown before the program waits for input.

4) If the pragma #pragma STDC FENV_ACCESS is supported and is set to ON, the changes to the floating-point environment

the result of any floating-point expression other than cast and assignment may have range and precision of a floating-point type different from the type of the expression (see FLT_EVAL_METHOD
notwithstanding the above, intermediate results of any floating-point expression may be calculated as if to infinite range and precision (unless #pragma STDC FP_CONTRACT is OFF

(since C99)