[clang] [llvm] [AArch64][llvm] Add support for Permission Overlays Extension 2 (FEAT_S1POE2) (PR #164912)

[Jonathan Thackray via llvm-commits]

================
@@ -625,6 +625,19 @@ def FeatureF16F32DOT : ExtensionWithMArch<"f16f32dot", "F16F32DOT", "FEAT_F16F32
 def FeatureF16F32MM : ExtensionWithMArch<"f16f32mm", "F16F32MM", "FEAT_F16F32MM",
   "Enable Armv9.7-A Advanced SIMD half-precision matrix multiply-accumulate to single-precision", [FeatureNEON, FeatureFullFP16]>;
 
+//===----------------------------------------------------------------------===//
+//  Future Architecture Technologies
+//===----------------------------------------------------------------------===//
+
+def FeatureBTIE: ExtensionWithMArch<"btie", "BTIE", "FEAT_BTIE",
+  "Enable Enhanced Branch Target Identification extension">;
+
+def FeatureS1POE2: ExtensionWithMArch<"poe2", "POE2", "FEAT_S1POE2",
+  "Enable Stage 1 Permission Overlays Extension 2 instructions", [FeatureBTIE]>;
----------------
jthackray wrote:

The POE2 spec says:
```
"If FEAT_S1POE2 is implemented then FEAT_BTIE is also implemented."
```
but I don't think our sync with GNU recorded this; I think I only spotted this later. I will check again next week. I think if POE2 is enabled, we should also enable `+btie`, although I don't think GNU want a flag for `+btie`. We'll sort these nits out next week.

https://github.com/llvm/llvm-project/pull/164912