In order to do that, the standard would have to define that dereferencing a null pointer must produce a deterministic behavior. There are only two possible behaviors:
1. The program successfully reads/writes that memory location and retrieves/overwrites whatever is there without crashing. Then the program can continue on and execute the if even if the pointer was null.
2. The program crashes immediate whenever a null pointer is read/written.
#1 is problematic, because NULL is a single pointer value that can be applied to a variety of pointer types. What happens if you first write to (long )NULL and then read from (FILE )NULL?
#2 is very useful, and most platforms already crash any program that tries to read or write NULL. But if the standard requires this behavior, then this introduces an even stronger guarantee that a dereferenced pointer is not null, so there's no reason to remove that optimization.
C is not Haskell or Java. The C programmer may intend to interact with actual hardware and is not required to interact with some abstract machine. The standard can reflect this or it can attempt to convert C into a poorly designed high level language. Dereferencing the null pointer should be implementation dependent, but the compiler should be required to either detect and flag this as an error or compile it into the machine operations indicated. The actual execution in the second case may depend on the environment.
Sorry, but you are just wrong. The C standard does define an abstract machine.
> but the compiler should be required to either detect and flag [dereferencing the null pointer] as an error
How could the compiler detect at compile time the value of a run time variable? Sure, some instances might be detectable, but those are the extreme minority. Static analysis tools such as Clang are already capable of finding those compile time NULLs.
> or compile it into the machine operations indicated. The actual execution in the second case may depend on the environment.
Which is exactly what's done now. In most platforms accessing NULL causes a crash, so either the pointer is not null and the program doesn't crash, so the check in redundant; or the pointer is null and the does crash, so the check is never executed.
