more precise about what the HW does

[web.git] / ralf / _posts / 2019-07-14-uninit.md

diff --git a/ralf/_posts/2019-07-14-uninit.md b/ralf/_posts/2019-07-14-uninit.md
index b1c479ab9a4a5c66049c12ec9d07bad2180bb033..72b26c76ab5207fec12d8d5c3bdfbdff5b4ff9e1 100644 (file)
--- a/ralf/_posts/2019-07-14-uninit.md
+++ b/ralf/_posts/2019-07-14-uninit.md
@@ -54,7 +54,7 @@ However, if you [run the example](https://play.rust-lang.org/?version=stable&mod
 ## What *is* uninitialized memory?
 
 How is this possible?
-The answer is that every byte in memory cannot just have a value in `0..256` (this is Rust syntax for a left-inclusive right-exclusive range), it can also be "uninitialized".
+The answer is that every byte in memory cannot just have a value in `0..256` (this is Rust/Ruby syntax for a left-inclusive right-exclusive range), it can also be "uninitialized".
 Memory *remembers* if you initialized it.
 The `x` that is passed to `always_return_true` is *not* the 8-bit representation of some number, it is an uninitialized byte.
 Performing operations such as comparison on uninitialized bytes is undefined behavior.
@@ -101,12 +101,14 @@ At least for LLVM, that [might change though](http://www.cs.utah.edu/~regehr/pap
 
 ## "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.