What exactly is the “as-if” rule?

Question

As the title says,

What exactly is the \"as-if\" rule?

An typical answer one would get is:

<

Answer 1

In C, C++, Ada, Java, SML... in any programming language well specified by describing the (usually many possible, non-deterministic) behavior(s) of a program (exposed to series of interactions on I/O ports), there is no distinct as-if rule.

An example of distinct rule is the one that says that a division by zero raises an exception (Ada, Caml) or a null dereference raises an exception (Java). You could change the rule to specify something else and you would end up with a different language (that some people would rather call a "dialect"(*). A distinct rule is there to specify some distinct uses of a programming language like a distinct grammatical rule cover some syntax constructs.

(*) A dialect according to some linguists is a language with an "army". in that context, that could mean a programming language without a committee and a specific industry of compiler editors.

The as-if rule is not a distinct rule; it doesn't cover any program in particular and is not even a rule that could be discussed, removed, or altered in any way: the so called "rule" simply reiterates that program semantics is defined, and can only be portably (universally) defined, in term of the visible interactions of an execution of the program with the "external" world.

The external world can be I/O interfaces (stdio), a GUI, even an interactive interpreter that output the resulting value of a pure applicative language. In C and C++ is includes the (vaguely specified) accesses to volatile objects, which is another way of saying that some objects at given point must be represented in memory strictly according to the ABI (Application Binary Interface) without ever mentioning the ABI explicitly.

The definition of what is a trace of execution, also called the visible or observable behavior defines what is meant by "as-if rule". The as-if rule tries to explain it, but by doing so, it confuses people more than it clarifies things as it gives the expression of being an additional semantic rule giving more leeway to the implementation.

Summary:

• The so called "as-if rule" does not relax any constraints on implementations.
• You cannot remove the as-if rule in any programming language specified in term of visible behavior (execution traces composed for interaction with the external world) to get a distinct dialect.
• You cannot add the as-if rule to any programming language not specified in term of visible behavior.