explain why invalid bool is truly arbitrary UB

diff --git a/personal/_posts/2020-07-15-unused-data.md b/personal/_posts/2020-07-15-unused-data.md
index 500ecef44b6d23b8b97c75d8f8bd02acf567880d..dcd6b78ad9b28513550d882db6a2adf292a5f618 100644 (file)
--- a/personal/_posts/2020-07-15-unused-data.md
+++ b/personal/_posts/2020-07-15-unused-data.md
@@ -30,6 +30,7 @@ fn example(b: bool) -> i32 {
 
 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.
 
 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.

+For example, the compiler might use a [jump table](https://en.wikipedia.org/wiki/Branch_table); an out-of-bounds index in that table could literally execute any code, so there is no way to bound the behavior in that case.

 (This is a compiler-understood *validity invariant* that is fixed in the language specification, which is very different from a user-defined *safety invariant*.
 See [this earlier post]({% post_url 2018-08-22-two-kinds-of-invariants %}) for more details on that distinction.)
 
 (This is a compiler-understood *validity invariant* that is fixed in the language specification, which is very different from a user-defined *safety invariant*.
 See [this earlier post]({% post_url 2018-08-22-two-kinds-of-invariants %}) for more details on that distinction.)