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

[Nikita Popov via llvm-commits]

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@@ -2077,6 +2077,19 @@ 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. In other words, for each allocation
+    there there is a non-deterministic choice between calling the allocator as
+    usual, or using a virtual, side-effect-free allocator instead.
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nikic wrote:

Yes, in a specific sense. It is okay to elide *all* of the allocator side effects. But if the allocation is not elided, we also can't ignore the side effects.

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