+#### What went wrong?
+
+Clearly, one of the three optimizations is incorrect in the sense that it introduced a change in program behavior.
+But which one is it?
+
+In an ideal world, we would have a sufficiently precise semantics for LLVM IR that we would just have to read the docs (or, even better, run some Miri-like tool) to figure out the answer.
+However, describing language semantics at this level of precision is *hard*, and full of trade-offs.
+That's why the LLVM LangRef will not give us a clear answer here, and indeed obtaining a clear answer requires some decisions that have not been explicitly made yet.
+
+To proceed, we will use the three optimizations that we considered above as cues: assuming that the optimization is correct for LLVM IR, what does that tell us about the semantics?
+
+We start with the last optimization, where the `print` argument is changed from `q[0]` to `0`.
+This optimization is based on alias analysis:
+`q[0]` gets initialized to `0` at the beginning of the program, and the only write between that initialization and the `print` is to the pointer `p+1`.
+Since `q` and `p` point to different local variables, a pointer derived from `p` cannot alias `q[0]`, and hence we know that this write cannot affect the value stored at `q[0]`.
+
+Looking more closely, however, reveals that things are not quite so simple!
+`p+1` is a one-past-the-end pointer, so it actually *can* have the same address as `q[0]`
+(and, in fact, inside the conditional we know this to be the case).
+However, LLVM IR (just like C) does not permit memory accesses through one-past-the-end pointers.
+It makes a difference whether we use `p+1` or `q` inside the `if`, even though we know (in that branch) that both pointers point to the same memory location.
+This demonstrates that in LLVM IR, there is more to a pointer than just the address it points to---it also matters how this address was computed.
+This something extra is what we typically call *provenance*.
+It is impossible to argue for the *correct*ness of the third optimization without acknowledging that provenance is a real part of the semantics of an LLVM IR program.
+In a flat memory model where pointers are just integers (such as most assembly languages), this optimization is simply wrong.
+
+Now that we know that provenance exists in pointers, we have to also consider what happens to provenance when a pointer gets cast to an integer and back.
+The second optimization gives us a clue into this aspect of LLVM IR semantics: casting a pointer to an integer and back is optimized away, which means that *integers have provenance*.
+To see why, consider the two expressions `(char*)(int)(p+1)` and `(char*)(int)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 value (i.e., the bit pattern of length 32) that serves as input to the `(char*)` cast is the same, and hence a difference can only arise if 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.
+This optimization demonstrates that *integers do not have provenance*:
+the optimization is only correct if a successful run-time equality test implies that the two values are equivalent in the "Abstract Machine" that is used to describe the language semantics.
+But this means that the Abstract Machine version of this value cannot have any "funny" extra parts that are not represented at run-time.
+Of course, provenance is exactly such a "funny" extra part.
+A different way to phrase the same argument is to say that this optimization is correct only if `iq == ip` implies that both values have the same provenance.
+This would be a possible definition of `==` in LLVM IR, but only in principle---in practice this means the LLVM backends have to compile `==` in a way that pointer provenance is taken into account, which of course is impossible.
+
+*Take-away:*
+By considering each of these three optimizations in terms of what they tell us about the semantics of LLVM IR, we learned that pointers have provenance, that integers remember the provenance of the pointer they come from in case of a pointer-to-integer cast, and that integers do not have provenance.
+This is a contradiction, and this contradiction explains why we saw incorrect compilation results when applying all three optimizations to the same program.
+
+#### How can we fix this?
+
+To fix the problem, we will have to declare one of the three optimizations incorrect and stop performing it.
+Speaking in terms of the LLVM IR semantics, this corresponds to deciding whether pointers and/or integers have provenance:
+* We could say both pointers and integers have provenance, which invalidates the first optimization.
+* We could say pointers have provenance but integers do not, which invalidates the second optimization.
+* We could say nothing has provenance, which invalidates the third optimization.
+
+In my opinion, the first and last options are not tenable.
+Removing provenance altogether kills all but the most simple alias analyses.[^alias]
+On the other hand, declaring that integers have provenance does not just disable the first optimization in the chain shown above, it also disables common arithmetic optimizations such as `x - x` being equivalent to `0`.
+Even achieving commutativity and associativity of `+` becomes non-trivial once integers have provenance.
+
+[^alias]: Sadly, I do not know of a systematic study of the performance impact of this decision. It is my understanding that many compiler developers "obviously" consider this way too costly of a fix to the problem, but it would still be great to underpin this with some proper data.
+
+So, I think that the issue should be resolved by saying that pointers have provenance but integers do not, which means that it is the second optimization that is wrong.
+This also corresponds to [what has been recently proposed to the C standard committee](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2577.pdf).
+That's why [LLVM bug #34548](https://bugs.llvm.org/show_bug.cgi?id=34548) says that optimizing away pointer-integer-pointer roundtrips is incorrect, and LLVM should stop doing this in the general case.
+There might still be special cases where this can be done, but figuring out the limits of this really needs a more precise description of LLVM IR semantics such as what we proposed [in this paper](https://people.mpi-sws.org/~jung/twinsem/twinsem.pdf).
+
+But ultimately, it will be up to the LLVM community to make this decision.
+All I can say for sure is that they have to make a choice, because the status quo of doing all three of these optimizations leads to incorrect compilation results.
+
+## Conclusion
+
+What did we learn?
+First of all, pointers are complicated.
+Precisely describing their semantics in a way that is consistent with common alias analyses requires adding a notion of "provenance".
+In a language such as Java or ML where pointers are opaque types whose representation cannot be observed, this is actually fairly easy to do.
+But in a language such as Rust, C or C++ that supports pointer-integer casts, the introduction of provenance poses some really tricky questions, and at least one of the commonly performed optimizations in this space has to give.
+
+We also learned that LLVM has a bug, but that was *not* the point of this blog post.
+The GCC developers [made exactly the same mistake](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82282), and I got word that MSVC and ICC have the same issue (though I do not know how to verify this).
+And I cannot blame them; the way compiler development typically works, I think bugs like this are inevitable: when exactly UB arises in an IR is often only loosely specified, so evaluating whether some optimization is *correct* in the sense defined above is basically impossible.
+Pointer provenance is just a particularly good example of where the current approach failed, but it is not the only case.
+For example, [ยง2.3 of this paper](https://plv.mpi-sws.org/validc/paper.pdf) (see Figure 3 for the code) shows how a sequence of two optimizations can lead to a miscompilation, where the first optimization is correct under the LLVM concurrency model, and the second optimization is correct under the C++11 concurrency model---but there is no concurrency model under which *both* optimizations are correct, so each compiler (or rather, each compiler IR) needs to pick one or the other.
+
+Which brings me to my main conclusion for this post: to avoid the problem of incompatible optimizations, I think we need to take compiler IRs more serious as programming languages in their own right, and give them a precise specification---including all the UB.
+Now, you may object by saying that LLVM has an extensive LangRef.
+And still, by reading the LLVM specification one could convince oneself that each of the three optimizations above is correct, which as we have seen is contradictory.
+What is missing? Do we need a formal mathematical definition to avoid such ambiguities?
+I do not think so; I think there is something simpler that will already help a lot.
+The source of the contradiction is that some parts of the specification *implicitly* assume that pointers have provenance, which is easy to forget when considering other operations.
+That's why I think it is very important to make such assumptions more explicit: the specification should *explicitly* describe the information that "makes up" a value, which will include things like provenance and whether the value is (wholly or partially) initialized.[^C]
+This information needs to be extensive enough such that a hypothetical interpreter can use it to detect all the UB.
+Doing so makes it obvious that a pointer has provenance, since otherwise it is impossible to correctly check for out-of-bounds pointer arithmetic while also permitting one-past-the-end pointers.
+
+[^C]: The astute reader will notice that this information is also absent in the C and C++ specifications. That is indeed one my main criticisms of these specifications. It leads to many open questions not only when discussing provenance but also when discussing indeterminate and unspecified values.
+
+This is my bar for what I consider a sufficiently precise language specification: it needs to contain all the information needed such that writing a UB-checking interpreter is just a matter of "putting in the work", but does not require introducing anything new that is not described in the specification.
+
+Ideally, the interpreter is not just hypothetical.
+This is really my main motivation for working on the [Miri interpreter](https://github.com/rust-lang/miri/).
+Of course, practically speaking, its main purpose is to help unsafe code authors avoid UB, but for me personally, I find it equally important that it helps us think about the semantics of Rust and MIR in a different way.
+
+I hope this was educational, and thanks for reading. :)
+As usual, this post can be discussed in the Rust forums.
+I am curious what your thoughts are on how we can build compilers that do not suffer from the issues I have discussed here.
projects / web.git / blobdiff