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

[Jameson Nash via llvm-commits]

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@@ -523,6 +537,7 @@ def CC_AArch64_Preserve_None : CallingConv<[
   // We can pass arguments in all general registers, except:
   // - X8, used for sret
   // - X16/X17, used by the linker as IP0/IP1
+  // - X15, the nest register and used by Windows for stack allocation
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vtjnash wrote:

That sounds reasonable. If I read the code correctly, the comment about "X15 for stack allocation" is not quite accurate as well (the code doesn't necessarily have to clobber that, since it is implemented to use a temp register first--though such a register does need to be made available for it). Other comments here seem contradictory also, since it assigns X9 last "because it is needed as a scratch register"–but it seems like either it is needed as a scratch register (in which case it should not have been allowed to assign it), or it doesn't need it as a scratch register (in which case the comment is wrong since it doesn't get used as a scratch register)

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