---
title: "Pointers Are Complicated II, or: We need better language specs"
-categories: rust
+categories: rust research
forum: https://internals.rust-lang.org/t/pointers-are-complicated-ii-or-we-need-better-language-specs/13562
+license: CC BY-SA 4.0
+license-url: https://creativecommons.org/licenses/by-sa/4.0/
---
Some time ago, I wrote a blog post about how [there's more to a pointer than meets the eye]({% post_url 2018-07-24-pointers-and-bytes %}).
{% endhighlight %}
However, that transformation is actually incorrect.
If we imagine a caller using this function as `sum_up(INT_MAX, 1, 0)`, then this is a perfectly correct way to call `sum_up`: the loop is never entered, so the overflowing addition `INT_MAX+1` is never performed.
-However, after the desired optimization, the program now causes a signed integer overflow, which is UB (Undefined Behavior) and thus May Never Happen!
+However, after the desired optimization, the program now causes a signed integer overflow, which is UB (Undefined Behavior) and thus May Never Happen![^signed-int-overflow]
[^loop]: If you are a regular reader of my blog, you will recognize this as the same optimization that already played a crucial role in [a previous post of mine]({% post_url 2020-07-15-unused-data %}). Loop-invariant code motion is a great optimization to look at when considering corner cases of IR semantics.
+[^signed-int-overflow]: If you are coming here with a Rust mindset, imagine `i + j` was written as `i.unchecked_add(j)`.
+
One might be tempted to ignore this problem because the UB on integer overflow is a compiler-only concept; every target supported by the compiler will do the obvious thing and just produce an overflowing result.
However, there might be other compiler passes running after the optimization we are considering.
One such pass might inline `sum_up`, and another pass might notice the `INT_MAX+1` and replace it by `unreachable` since UB code is "by definition" unreachable, and another pass might then just remove all our code since it is unreachable.
This is exactly what we also expect from the specification of a programming language such as C, which is why I think we should consider compiler IRs as proper programming languages in their own right, and specify them with the same diligence as we would specify "normal" languages.[^ub-difference]
Sure, no human is going to write many programs in LLVM IR, so their syntax barely matters, but clang and rustc produce LLVM IR programs all the time, and as we have seen understanding the exact rules governing the behavior of programs is crucial to ensuring that the optimizations LLVM performs do not change program behavior.
-[^cheat]: If now you feel like we somehow cheated, since we can always translate the program from C to LLVM IR, optimize there, and translate back, consider this: translating from LLVM IR to C is really hard! In particular, singed integer addition in LLVM IR can *not* be translated into signed integer addition in C, since the former is well-defined with `poison` result in case of overflow, but the latter says overflow is UB. C has strictly more UB than LLVM IR (for integer arithmetic), which makes translation in one direction easy, while the other direction is hard.
+[^cheat]: If now you feel like we somehow cheated, since we can always translate the program from C to LLVM IR, optimize there, and translate back, consider this: translating from LLVM IR to C is really hard! In particular, signed integer addition in LLVM IR can *not* be translated into signed integer addition in C, since the former is well-defined with `poison` result in case of overflow, but the latter says overflow is UB. C has strictly more UB than LLVM IR (for integer arithmetic), which makes translation in one direction easy, while the other direction is hard.
[^ub-difference]: In particular, two different variants of the IR with different rules for UB are really *two different programming languages*. A program that is well-defined in one language may have UB in another, so great care needs to be taken when the program is moved from being governed by one set of rules to another.
To see why, consider the two expressions `(char*)(uintptr_t)(p+1)` and `(char*)(uintptr_t)q`:
if the optimization of removing pointer-integer-pointer roundtrips is correct, the first operation will output `p+1` and the second will output `q`, which we just established are two different pointers (they differ in their provenance).
The only way to explain this is to say that the input to the `(char*)` cast is different, since the program state is otherwise identical in both cases.
-But we know that the integer values computed by `(uintptr_t)(p+1)` and `(uintptr_t)q` (i.e., the bit pattern of length 32 that serve as input to the `(char*)` casts) are the same, and hence a difference can only arise if these integers consist of more than just this bit pattern---just like pointers, integers have provenance.
+But we know that the integer values computed by `(uintptr_t)(p+1)` and `(uintptr_t)q` (i.e., the bit pattern as stored in some CPU register) are the same, and hence a difference can only arise if these integers consist of more than just this bit pattern---just like pointers, integers have provenance.
Finally, let us consider the first optimization.
Here, a successful equality test `iq == ip` prompts the optimizer to replace one value by the other.
It also feeds back into the design of the UB rules by discovering patterns that people want or need to use but that are not currently accepted by Miri.
On the LLVM side, the main development in this area is [Alive](https://blog.regehr.org/archives/1722), a tool that can automatically validate[^validate] optimizations performed by LLVM.
-Alive has found [many bugs in LLVM optimizations](https://github.com/AliveToolkit/alive2/blob/master/BugList.md), and indeed much of the recent dialog with the LLVM community aimed at a more precise IR semantics is pushed by the people building Alive, lead by Nuno P. Lopes and John Regehr.
+Alive has found [many bugs in LLVM optimizations](https://github.com/AliveToolkit/alive2/blob/master/BugList.md), and indeed much of the recent dialog with the LLVM community aimed at a more precise IR semantics is pushed by the people building Alive, led by Nuno P. Lopes and John Regehr.
[^validate]: A note on terminology: "validating" an optimization means that given the program text before and after the optimization, the tool will try to prove that this particular transformation is correct. This is in contrast to "verifying" an optimization where a once-and-forall proof is carried out showing that the optimization will always perform a correct transformation. Verification gives a much stronger result, but is also extremely difficult to carry out, so validation is a great middle-ground that is still able to find plenty of bugs.
I hope this post can raise awareness for the subtle problems optimizing compilers are facing, and convince some people that figuring out the specification of compiler IRs is an important and interesting problem to work on. :)
That's all I have for today, thanks for sticking with me!
-As usual, this post can be [discussed in the Rust forums](https://internals.rust-lang.org/t/pointers-are-complicated-ii-or-we-need-better-language-specs/13562).
+As usual, this post can be [discussed in the Rust forums](https://internals.rust-lang.org/t/pointers-are-complicated-ii-or-we-need-better-language-specs/13562) and [on Reddit](https://www.reddit.com/r/rust/comments/kd157i/pointers_are_complicated_ii_or_we_need_better/).
I am curious what your thoughts are on how we can build compilers that do not suffer from the issues I have discussed here.
#### Footnotes
projects / web.git / blobdiff