4 ms·
EDIT: This is about modifying code in memory; the article is actually more about modifying the executable file on disk — oops (: ---- One example: My underst
by interroboink 5y ago
EDIT: This is about modifying code in memory; the article is actually more about modifying the executable file on disk — oops (:
----
One example:
My understanding is that static initialization in C/C++ is sometimes done with self-modifying code.
Eg:
ExpensiveObject &getInstance() {
static ExpensiveObject obj;
return obj;
}
So, the first time through, the generated assembly would run some "if object is not initialized, run the constructor" conditional code. Then, it would rewrite itself to not do that check for subsequent calls.
Obviously, that's all in the "hidden behind the scenes" generated assembly code, and the rust compiler could pull similar tricks, but you might also want to allow client code to do such things, for extreme performance cases.
- mmozeiko 5y agoThat's not really what's happening with such C++ code. `static` variables are placed in normal writeable global memory. Only difference with such static is that compiler generates extra "bool" variable and checks it to initialize obj only on first run (threadsafe on C++11 and up). You can see example here: https://godbolt.org/z/7f1qbh5r5 https://godbolt.org/z/7f1qbh5r5 There are no code modifications - all compiler generated is extra byte check ("if" statement) and then acquire/release lock around constructor to be threadsafe.
- interroboink 5y agoSome more good discussion here: https://stackoverflow.com/questions/63568992/can-compilers-generate-self-modifying-code https://stackoverflow.com/questions/63568992/can-compilers-g... I guess it's not commonplace these days, but it is still allowed, it seems. I feel like my initial run-in with the concept was on some embedded systems stuff long ago, so maybe that was a more esoteric compiler.