[compiler-rt] [AArch64] Implement INIT/ADJUST_TRAMPOLINE (PR #70267)

[Carlos Eduardo Seo via llvm-commits]

================
@@ -5946,6 +5946,56 @@ static SDValue LowerFLDEXP(SDValue Op, SelectionDAG &DAG) {
   return Final;
 }
 
+SDValue AArch64TargetLowering::LowerADJUST_TRAMPOLINE(SDValue Op,
+                                                      SelectionDAG &DAG) const {
+  // Note: x18 cannot be used for the Nest parameter on Windows and macOS.
+  if (Subtarget->isTargetDarwin() || Subtarget->isTargetWindows())
+    report_fatal_error(
+        "ADJUST_TRAMPOLINE operation is only supported on Linux.");
+
+  return Op.getOperand(0);
+}
+
+SDValue AArch64TargetLowering::LowerINIT_TRAMPOLINE(SDValue Op,
+                                                    SelectionDAG &DAG) const {
+
+  // Note: x18 cannot be used for the Nest parameter on Windows and macOS.
+  if (Subtarget->isTargetDarwin() || Subtarget->isTargetWindows())
+    report_fatal_error("INIT_TRAMPOLINE operation is only supported on Linux.");
----------------
ceseo wrote:

Per the Arm PCS:

"The role of register r18 is platform specific. If a platform ABI has need of a dedicated general purpose register to carry inter-procedural state (for example, the thread context) then it should use this register for that purpose. If the platform ABI has no such requirements, then it should use r18 as an additional temporary register. The platform ABI specification must document the usage for this register."

Both macOS and Windows use this as the Platform Register and LLVM documents this in `AArch64CallingConvention.td`.

https://github.com/llvm/llvm-project/pull/70267