[llvm] [LangRef] Mention allocation elision (PR #177592)

[Ralf Jung via llvm-commits]

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@@ -2077,6 +2077,15 @@ For example:
     The first three options are mutually exclusive, and the remaining options
     describe more details of how the function behaves. The remaining options
     are invalid for "free"-type functions.
+
+    Calls to functions annotated with ``allockind`` are subject to allocation
+    elision: Calls to allocator functions can be removed, and the allocation
+    served from a virtual allocator instead. Notably, this is allowed even if
+    the allocator calls have side-effects.
+
+    If multiple allocation functions operate on the same allocation (for
+    example, an "alloc" followed by "free"), allocation elision is only allowed
+    if all involved functions have the same ``"alloc-family"``.
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RalfJung wrote:

It has come up in a few papers, e.g. https://sf.snu.ac.kr/publications/ccr.pdf. But I don't know a canonical citation.

NB is basically the natural interpretation of what it means to do demonic non-deterministic choice over the empty set of possible options. (This is dual to how UB is angelic choice over the empty set.)

https://github.com/llvm/llvm-project/pull/177592