Merge pull request #570 from RalfJung/llvm

add list of assumptions we make about LLVM

Author: CAD97

resources/llvm-assumptions.md

@@ -0,0 +1,13 @@
+Some of the things we want people to do with Rust can currently not be expressed in LLVM in a way that is fully backed by the LLVM LangRef.
+Let's collect a list of those cases here.
+
+## List of LLVM assumptions not backed by the spec
+
+- To implement `ptr.addr()`, we assume that a pointer-to-int transmute yields the address.
+  The LangRef is quiet about this (as it is about almost everything related to provenance).
+  Alive says that this yields poison.
+- To implement the desired semantics for `MaybeUninit<$int>` we need a type of arbitrary size that can hold arbitrary data -- including provenance.
+  LLVM currently has no such type, the only type that is fully guaranteed to support provenance is `ptr` and that has a fixed size.
+- This one is not about current Rust but about possible future extensions:
+  when LLVM returns `poison` for some operation, we can *not* say that this corresponds to `uninit` in Rust. We *must* declare this immediate UB.
+  The reason for this is that LLVM does not really support `poison` being stored in memory; Rust's `uninit` can therefore only correspond to LLVM's `undef`.