8345296: AArch64: VM crashes with SIGILL when prctl is disallowed

Reviewed-by: eastigeevich, phh, aph

Author: shipilev

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp

@@ -444,7 +444,19 @@ void VM_Version::initialize() {
   }
 
   if (UseSVE > 0) {
-    _initial_sve_vector_length = get_current_sve_vector_length();
+    int vl = get_current_sve_vector_length();
+    if (vl < 0) {
+      warning("Unable to get SVE vector length on this system. "
+              "Disabling SVE. Specify -XX:UseSVE=0 to shun this warning.");
+      FLAG_SET_DEFAULT(UseSVE, 0);
+    } else if ((vl == 0) || ((vl % FloatRegister::sve_vl_min) != 0) || !is_power_of_2(vl)) {
+      warning("Detected SVE vector length (%d) should be a power of two and a multiple of %d. "
+              "Disabling SVE. Specify -XX:UseSVE=0 to shun this warning.",
+              vl, FloatRegister::sve_vl_min);
+      FLAG_SET_DEFAULT(UseSVE, 0);
+    } else {
+      _initial_sve_vector_length = vl;
+    }
   }
 
   // This machine allows unaligned memory accesses