X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/0750a867ea3132ffb722bf921ff9cf0d185553ca..f84e95a2f2e8a40f8a8560305588167dff59059e:/ralf/_drafts/provenance-matters.md

diff --git a/ralf/_drafts/provenance-matters.md b/ralf/_drafts/provenance-matters.md
index 959aadd..7ad9f1a 100644
--- a/ralf/_drafts/provenance-matters.md
+++ b/ralf/_drafts/provenance-matters.md
@@ -236,9 +236,10 @@ But in a language such as Rust, C, or C++ that supports pointer-integer casts, t
 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.
+Pointer provenance is just a particularly good (and subtle) example.
+The warm-up above is another trivial case of this (albeit one where the existing specification is sufficient): loop-invariant code motion of arithmetic operations and UB on arithmetic overflow can both be *correct*, but not in the same IR.
 For another example, see [§2.3 of this paper](https://plv.mpi-sws.org/validc/paper.pdf) (Figure 3 contains the code) which 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.
-The warm-up example is another trivial case of this: loop-invariant code motion of arithmetic operations and UB on arithmetic overflow can both be *correct*, but not in the same IR.
+Finally, this [paper on `undef` and `poison`](https://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf) gives examples for optimizations that are broken by the presence of `undef` in LLVM, and describes some of the trade-offs that arise when defining the semantics of `poison`.
 
 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](https://llvm.org/docs/LangRef.html), 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.