## "What the hardware does" considered harmful
-Maybe the most important lesson to take away from this post is that "what the hardware does" is most of the time *irrelevant* when discussing what a Rust/C/C++ program does.
+Maybe the most important lesson to take away from this post is that "what the hardware does" is most of the time *irrelevant* when discussing what a Rust/C/C++ program does, unless you *already established that there is no undefined behavior*.
Sure, hardware (well, [most hardware](https://devblogs.microsoft.com/oldnewthing/20040119-00/?p=41003)) does not have a notion of "uninitialized memory".
But *the Rust program you wrote does not run on your hardware*.
It runs on the Rust abstract machine, and that machine (which only exists in our minds) *does* have a notion of "uninitialized memory".
The real, physical hardware that we end up running the compiled program on is a very efficient *but imprecise* implementation of this abstract machine, and all the rules that Rust has for undefined behavior work together to make sure that this imprecision is not visible for *well-behaved* (UB-free) programs.
But for programs that do have UB, this "illusion" breaks down, and [anything is possible](https://raphlinus.github.io/programming/rust/2018/08/17/undefined-behavior.html).
+UB-free programs can be made sense of by looking at their assembly, but *whether* a program has UB is impossible to tell on that level.
+For that, you need to think in terms of the abstract machine.
This does not just apply to uninitialized memory: for example, in x86 assembly, there is no difference between "relaxed" and "release"/"acquire"-style atomic memory accesses.
But when writing Rust programs, even when writing Rust programs that you only intend to compile to x86, "what the hardware does" just does not matter.
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