[lkorinth]

If we have the code "if (x != null) { x.foo = 42; }", then it can rewritten as x.foo = 42 (with some extra magic in the runtime). If x is not null it will be faster if the cost of a branch is higher than zero. If x is null, the (slow) trick with SEGV can be used.

The trick of catching the SEGV can also be used by an AOT compiler (https://llvm.org/docs/FaultMaps.html), but the AOT compiler would need profiling data to do this optimization that is more expensive to do if the variable x happens to be null often. Even if you have profile data for your application, and that profile data will correspond to your application profile of this particular run (on average), you can not handle the case when x != null for five hours and then is set to null for five hours. If you use an AOT compiler you would have exponential blow-up of code generations for combinations of variables that can be null (if you would try to compile all combinations), and you would basically reinvent a JIT compiler --- badly.

Theoretically, a JIT can do anything an AOT can do, but the reverse is not true.

The article is talking about optimizations the JIT is capable of doing for your code. It is well written, and although it is talking about code throwing a NullPointerException, I think you can see that the same optimization can be done for my example at the top (that is applicable to c++ as well). So my comparison is not apples to oranges.

But your observation that most compiled languages do not have NullPointerExceptions is interesting. It could very much be because that it is too expensive with an AOT, have you thought about that?

[kaba0]

Also, in an AOT-way you couldn’t really play the same SEGFAULT trick multiple times without some expensive tracking of where did it come from, where to return to, etc, at which point you are doing borderline JIT compiler with precached method bodies.