[llvm] langref updates for aarch64 trampoline (PR #139740)

[Eli Friedman via llvm-commits]

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@@ -410,8 +410,9 @@ added in the future:
       calling convention: on most platforms, they are not preserved and need to
       be saved by the caller, but on Windows, xmm6-xmm15 are preserved.
 
-    - On AArch64 the callee preserve all general purpose registers, except X0-X8
-      and X16-X18.
+    - On AArch64 the callee preserve all general purpose registers, except
+      X0-X8 and X16-X18. Using this calling convention with nest is forbidden
+      and may crash llvm.
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efriedma-quic wrote:

We don't usually add a "not all backends support this construct" disclaimers to LangRef; it would pop up in a lot of places, and it wouldn't really be helpful in most of those places.

There is a way to print a proper diagnostic from the backend without crashing, we just don't use it in all the places we should.

https://github.com/llvm/llvm-project/pull/139740