UB post: clarify

[web.git] / ralf / _posts / 2017-07-14-undefined-behavior.md

diff --git a/ralf/_posts/2017-07-14-undefined-behavior.md b/ralf/_posts/2017-07-14-undefined-behavior.md
index 46356689dfa90848d28f5d5b387758fc334bc2b3..851f737909324a3cb7d6d5d710ece80e84f44e88 100644 (file)
--- a/ralf/_posts/2017-07-14-undefined-behavior.md
+++ b/ralf/_posts/2017-07-14-undefined-behavior.md
@@ -52,6 +52,11 @@ However, if they *do* alias, that would violate the effective type restriction,
 As we have seen, in both of the possible cases, the reordering is correct; the compiler is thus free to perform the transformation.
 
 Undefined behavior moves the burden of proving the correctness of this optimization from the compiler to the programmer.
+In the example above, what the "effective type" rule really means is that every single memory read of a `float` comes with a *proof obligation*:
+The programmer has to show that that the last write to this memory actually happened through a `float` pointer (baring some exceptions around union and character pointers).
+Similarly, the (in)famous rule that [signed integer overflow is undefined behavior](https://stackoverflow.com/questions/16188263/is-signed-integer-overflow-still-undefined-behavior-in-c) means that every single arithmetic operation on signed integers comes with the proof obligation that this operation will never, ever, overflow.
+The compiler performs its optimization under the assumption that the programmer actually went through the effort and convinced itself that this is the case.
+
 Considering that the compiler can only be so smart, this is a great way to justify optimizations that would otherwise be difficult or impossible to perform.
 Unfortunately, it is often not easy to say whether a program has undefined behavior or not -- after all, such an analysis being difficult is the entire reason compilers have to rely on UB to perform their optimizations.
 Furthermore, while C compilers are happy to exploit the fact that a particular program *has* UB, they do not provide a way to test that executing a program *does not* trigger UB.
@@ -89,4 +94,6 @@ It is with such considerations in my mind that I have previously written about [
 Coming up next on this channel:  During my [internship]({{ site.baseurl }}{% post_url 2017-05-23-internship-starting %}), I am working on such a specification.
 My ideas are concrete enough now that I can write down a draft, which I will share with the world to see what the world thinks about it.
 
-**Uodate:** [Writing down has happened]({{ site.baseurl }}{% post_url 2017-07-17-types-as-contracts %}).
+**Update:** [Writing down has happened]({{ site.baseurl }}{% post_url 2017-07-17-types-as-contracts %}).
+
+**Update:** Clarified "Shifting Responsibility".