Ruling out operations such as comparison on uninitialized data is useful, because it means the compiler does not have to "remember" which exact bit pattern an uninitialized variable has!
A well-behaved (UB-free) program cannot observe that bit pattern anyway.
So each time an uninitialized variable gets used, we can just use *any* machine register---and for different uses, those can be different registers!
-So, one time we "look" at `x` it can be at least 150, and then when we look at it again it is less than 120, even though `x` did not change.
+So, one time we "look" at `x` it can be at least 150, and then when we look at it again it is at most 120, even though `x` did not change.
`x` was just uninitialized all the time.
That explains why our compiled example program behaves the way it does.
## "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).
+*Only* 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.[^sanitizer]
+
+[^sanitizer]: This does imply that tools like valgrind, that work on the final assembly, can never reliably detect *all* UB.
+
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.
The Rust abstract machine *does* make a distinction between "relaxed" and "release"/"acquire", and your program will go wrong if you ignore that fact.
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