[lldb] Do not produce field information for registers known not to exist

lldb/source/Plugins/Process/Utility/RegisterFlagsLinux_arm64.cpp

@@ -20,14 +20,18 @@
 #define HWCAP2_BTI (1ULL << 17)
 #define HWCAP2_MTE (1ULL << 18)
 #define HWCAP2_AFP (1ULL << 20)
+#define HWCAP2_SME (1ULL << 23)
 #define HWCAP2_EBF16 (1ULL << 32)
 
 using namespace lldb_private;
 
 LinuxArm64RegisterFlags::Fields
 LinuxArm64RegisterFlags::DetectSVCRFields(uint64_t hwcap, uint64_t hwcap2) {
   (void)hwcap;
-  (void)hwcap2;
+
+  if (!(hwcap2 & HWCAP2_SME))
+    return {};
+
   // Represents the pseudo register that lldb-server builds, which itself
   // matches the architectural register SCVR. The fields match SVCR in the Arm
   // manual.
@@ -40,7 +44,10 @@ LinuxArm64RegisterFlags::DetectSVCRFields(uint64_t hwcap, uint64_t hwcap2) {
 LinuxArm64RegisterFlags::Fields
 LinuxArm64RegisterFlags::DetectMTECtrlFields(uint64_t hwcap, uint64_t hwcap2) {
   (void)hwcap;
-  (void)hwcap2;
+
+  if (!(hwcap2 & HWCAP2_MTE))
+    return {};
+
   // Represents the contents of NT_ARM_TAGGED_ADDR_CTRL and the value passed
   // to prctl(PR_TAGGED_ADDR_CTRL...). Fields are derived from the defines
   // used to build the value.

lldb/source/Plugins/Process/Utility/RegisterFlagsLinux_arm64.h

@@ -38,8 +38,8 @@ class LinuxArm64RegisterFlags {
   /// For the registers listed in this class, detect which fields are
   /// present. Must be called before UpdateRegisterInfos.
   /// If called more than once, fields will be redetected each time from
-  /// scratch. If you do not have access to hwcap, just pass 0 for each one, you
-  /// will only get unconditional fields.
+  /// scratch. If the target would not have this register at all, the list of
+  /// fields will be left empty.
   void DetectFields(uint64_t hwcap, uint64_t hwcap2);
 
   /// Add the field information of any registers named in this class,
@@ -63,7 +63,7 @@ class LinuxArm64RegisterFlags {
 
   struct RegisterEntry {
     RegisterEntry(llvm::StringRef name, unsigned size, DetectorFn detector)
-        : m_name(name), m_flags(std::string(name) + "_flags", size, {{"", 0}}),
+        : m_name(name), m_flags(std::string(name) + "_flags", size, {}),
           m_detector(detector) {}
 
     llvm::StringRef m_name;

lldb/source/Target/RegisterFlags.cpp

@@ -54,10 +54,6 @@ unsigned RegisterFlags::Field::PaddingDistance(const Field &other) const {
 }
 
 void RegisterFlags::SetFields(const std::vector<Field> &fields) {
-  // We expect that the XML processor will discard anything describing flags but
-  // with no fields.
-  assert(fields.size() && "Some fields must be provided.");
-
   // We expect that these are unsorted but do not overlap.
   // They could fill the register but may have gaps.
   std::vector<Field> provided_fields = fields;