[libunwind] [libunwind][AArch64] Disable ZA before resuming from unwinding (on Linux) (PR #165451)

[Benjamin Maxwell via cfe-commits]

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@@ -1908,25 +1917,43 @@ class _LIBUNWIND_HIDDEN Registers_arm64 {
 private:
   uint64_t lazyGetVG() const;
 
+  void zaDisable() const {
+    if (!_misc_registers.__has_sme)
+      return;
+    if (!__libunwind_Registers_arm64_za_disable())
+      _LIBUNWIND_ABORT("SME ZA disable failed");
+  }
+
+  static bool checkHasSME() {
+#if defined(HAVE_SYS_AUXV_H)
+    constexpr int hwcap2_sme = (1 << 23);
+    unsigned long hwcap2 = getauxval(AT_HWCAP2);
+    return (hwcap2 & hwcap2_sme) != 0;
+#endif
+    // TODO: Support other platforms.
+    return false;
+  }
+
   struct GPRs {
-    uint64_t __x[29]; // x0-x28
-    uint64_t __fp;    // Frame pointer x29
-    uint64_t __lr;    // Link register x30
-    uint64_t __sp;    // Stack pointer x31
-    uint64_t __pc;    // Program counter
-    uint64_t __ra_sign_state; // RA sign state register
+    uint64_t __x[29] = {};        // x0-x28
----------------
MacDue wrote:

It's to remove the constructor that memsets the whole `Registers_arm64` structure to zero, which would overwrite the `checkHasSME()` initialization. 

https://github.com/llvm/llvm-project/pull/165451