[lldb] Do not produce field information for registers known not to exist (#95125)

Currently the logic is generate field information for all registers in
LinuxArm64RegisterFlags and then as we walk the existing register info,
only those that are in that existing info will get the new fields
patched in.

This works fine but on a review for FreeBSD support it was pointed out
that this is not obvious from the source code.

So instead I've allowed the construction of empty lists of fields, and
field detection methods can return an empty field list if they think
that the register will never exist.

Then the pre-existing code will see the empty field list, and never look
for that register in the register info.

I think removing the assert is ok because the GDB classes filter out
empty field lists at runtime, and anyone updating the built in field
information would presumably notice if none of the fields they intended
to add were displayed.

mte_ctrl and svcr are the only registers that need this so far.

There is no extra testing here as the behaviour is the same, it doesn't
add field information to regiters that don't exist. The mechanism is
just clearer now.

Author: DavidSpickett

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

@@ -108,10 +108,6 @@ uint64_t RegisterFlags::Field::GetMask() 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;