X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/7f79b2443f56a844eb41561dbed9fa3991a5b5b7..f5649511ac4cb0b606060b4fa28cc0f174dd8d89:/ralf/_drafts/unused-data.md

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----
-title: "Why even unused data needs to be valid"
-categories: rust
----
-
-`unsafe` code in Rust has a few ways that it can trigger [Undefined Behavior][ub], i.e., there are a few assumptions that the compiler makes about all code, and that for `unsafe` code the programmer is responsible for upholding.
-Those assumptions are [listed in the Rust reference](https://doc.rust-lang.org/reference/behavior-considered-undefined.html).
-The one that seems to be most surprising to many people is the clause which says that unsafe code may not produce "[...] an invalid value, even in private fields and locals".
-The reference goes on to explain that "*producing* a value happens any time a value is assigned to or read from a place, passed to a function/primitive operation or returned from a function/primitive operation".
-In other words, even just *constructing*, for example, an invalid `bool`, is Undefined Behavior---no matter whether that `bool` is ever actually "used" by the program.
-The purpose of this post is to explain why.
-
-[ub]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#undefined-behavior
-
-<!-- MORE -->
-
-First of all, let me clarify what is meant by "used" here, as that term is used to mean very different things.
-The following code "uses" `b`:
-
-{% highlight rust %}
-fn example(b: bool) -> i32 {
-  if b { 42 } else { 23 }
-}
-{% endhighlight %}
-
-I hope it is not very surprising that calling `example` on, e.g., `3` transmuted to `bool` is Undefined Behavior (UB).
-When compiling `if`, the compiler assumes that `0` and `1` are the only possible values; there is no saying what could go wrong when that assumption is violated.
-
-What is less obvious is why calling `example` on `3` is UB even when there is no such `if` being executed.
-To understand why that is important, let us consider the following example:
-
-{% highlight rust %}
-fn example(b: bool, num: u32) -> i32 {
-  let mut acc = 0;
-  for _i in 0..num {
-    acc += if b { 42 } else { 23 };
-  }
-  acc
-}
-{% endhighlight %}
-
-Now assume we were working in a slightly different version of Rust, where transmuting `3` to a `bool` is fine as long as you do not "use" the `bool`.
-That would mean that calling `example(transmute(3u8), 0)` is actually allowed, because in that case the loop never gets executed, so we never "use" `b`.
-
-However, this is a problem for a very important transformation called [loop-invariant code motion](https://en.wikipedia.org/wiki/Loop-invariant_code_motion).
-That transformation can be used to turn our `example` function into the following:
-
-{% highlight rust %}
-fn example(b: bool, num: u32) -> i32 {
-  let mut acc = 0;
-  let incr = if b { 42 } else { 23 }
-  for _i in 0..num {
-    acc += incr;
-  }
-  acc
-}
-{% endhighlight %}
-
-The increment `if b { 42 } else { 23 }` is "invariant" during the execution of the loop, and thus computing the increment can be moved out.
-Why is this a good transformation?
-Instead of determining the increment each time around the loop, we do that just once, thus saving a lot of conditional jumps that the CPU is unhappy about.