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Warning: This post may contain opinions. You have been warned.
-## When are optimizations legal?
+## When are Optimizations Legal?
Currently, we have a pretty good understanding of what the intended behavior of *safe* Rust is.
That is, there is general agreement (modulo some [bugs](https://github.com/rust-lang/rust/issues/27868)) about the order in which operations are to be performed, and about what each individual operation does.
Now, it turns out that it is often really hard to obtain precise aliasing information.
This could be the end of the game: No alias information, no way to verify our assumptions, no optimizations.
-## Shifting responsibility
+## Shifting Responsibility
-However, it turns out that compilers writers consider these optimizations important enough that they came up with an alternative solution:
+However, it turns out that compiler writers consider these optimizations important enough that they came up with an alternative solution:
Instead of having the compiler verify such assumptions, they declared the programmer responsible.
For example, the C standard says that memory accesses have to happen with the right "effective type": If data was stored with a `float` pointer, it must not be read with an `int` pointer.
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.
-It also turns out that programmers' intuition often [does not match what the compiler does](https://www.cl.cam.ac.uk/~pes20/cerberus/notes50-survey-discussion.html), which leads to miscompilations (in the eye of the programmer) and sometimes to security [vulerabilities](https://lwn.net/Articles/342330/).
+It also turns out that programmers' intuition often [does not match what the compiler does](https://www.cl.cam.ac.uk/~pes20/cerberus/notes50-survey-discussion.html), which leads to miscompilations (in the eye of the programmer) and sometimes to security [vulnerabilities](https://lwn.net/Articles/342330/).
As a consequence, UB has a pretty bad reputation.
There are various sanitizers that watch a program while it is being executed and try to detect UB, but they are not able to catch all possible sources of UB.
It is with such considerations in my mind that I have previously written about [miri as an executable specification]({{ site.baseurl }}{% post_url 2017-06-06-MIR-semantics %}).
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.
-I have a draft ready now, and I want to share it with the world to see what the world thinks about it.
+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 %}).
projects / web.git / blobdiff