From 6ea5be5ce86fa20f99063cbc35a594956b763542 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sat, 13 Aug 2022 12:14:19 -0400 Subject: [PATCH 1/1] nod to Rust readers --- ralf/_posts/2020-12-14-provenance.md | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/ralf/_posts/2020-12-14-provenance.md b/ralf/_posts/2020-12-14-provenance.md index ee3f5b6..b591c00 100644 --- a/ralf/_posts/2020-12-14-provenance.md +++ b/ralf/_posts/2020-12-14-provenance.md @@ -55,10 +55,12 @@ int sum_up(int i, int j, unsigned int n) { // optimized version {% 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 from Rust, 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. -- 2.30.2