[compiler-rt] [flang] [llvm] [AArch64] fix trampoline implementation: use X15 (PR #126743)

[Eli Friedman via llvm-commits]

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@@ -1,35 +1,26 @@
-; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+;; Testing that nest uses x15 on all calling conventions (except Arm64EC)
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efriedma-quic wrote:

> llvm caller's runtime to correctly use VirtualAlloc2

The trampoline is allocated on the stack.  We can't mess with the properties of the stack, or else x86 code breaks.

> we might have a pretty hard time fitting the necessary exit thunk into 36 bytes

There's no reason the size of a trampoline in arm64ec needs to be the same size as a regular AArch64 trampoline.  But I'm not sure what "exit thunk" you're referring to.

In any case, let's leave this for when someone actually tries arm64ec fortran.

https://github.com/llvm/llvm-project/pull/126743