[LLGS] Limit jThreadsInfo to only the most important registers for now

[labath]

Summary

It seems that reading of register data is the biggest bottleneck in LLGS at the moment. Sending
four registers instead of the full GPR set increases the jThreadsInfo processing time about
6-fold. Until we figure out where is this time going, this commit limits the amount of data we
send to provide a more fluid debugging experience.

[ovyalov]

LGTM

[tberghammer]

Looks good

lldb/trunk/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp

 {
     Log *log (GetLogIfAnyCategoriesSet (LIBLLDB_LOG_THREAD));
 
     NativeRegisterContextSP reg_ctx_sp = thread.GetRegisterContext ();
     if (! reg_ctx_sp)
         return nullptr;
 
     JSONObject::SP register_object_sp = std::make_shared<JSONObject>();
+
+#ifdef LLDB_JTHREADSINFO_FULL_REGISTER_SET
     // Expedite all registers in the first register set (i.e. should be GPRs) that are not contained in other registers.
     const RegisterSet *reg_set_p = reg_ctx_sp->GetRegisterSet(0);
     if (! reg_set_p)
         return nullptr;
-
     for (const uint32_t *reg_num_p = reg_set_p->registers; *reg_num_p != LLDB_INVALID_REGNUM; ++reg_num_p)
     {
-        const RegisterInfo *const reg_info_p = reg_ctx_sp->GetRegisterInfoAtIndex(*reg_num_p);
+        uint32_t reg_num = *reg_num_p;
+#else
+    // Expedite only a couple of registers until we figure out why sending registers is
+    // expensive.
+    static const uint32_t k_expedited_registers[] = {
+        LLDB_REGNUM_GENERIC_PC, LLDB_REGNUM_GENERIC_SP, LLDB_REGNUM_GENERIC_FP, LLDB_REGNUM_GENERIC_RA
+    };
+    for (uint32_t generic_reg: k_expedited_registers)
+    {
+        uint32_t reg_num = reg_ctx_sp->ConvertRegisterKindToRegisterNumber(eRegisterKindGeneric, generic_reg);
+        if (reg_num == LLDB_INVALID_REGNUM)
+            continue; // Target does not support the given register.
+#endif
+
+        const RegisterInfo *const reg_info_p = reg_ctx_sp->GetRegisterInfoAtIndex(reg_num);
         if (reg_info_p == nullptr)
         {
             if (log)
                 log->Printf("%s failed to get register info for register index %" PRIu32,
-                        __FUNCTION__, *reg_num_p);
+                        __FUNCTION__, reg_num);
             continue;
         }
 
         if (reg_info_p->value_regs != nullptr)
             continue; // Only expedite registers that are not contained in other registers.
 
         RegisterValue reg_value;
         Error error = reg_ctx_sp->ReadRegister(reg_info_p, reg_value);
         if (error.Fail())
         {
             if (log)
                 log->Printf("%s failed to read register '%s' index %" PRIu32 ": %s", __FUNCTION__,
-                        reg_info_p->name ? reg_info_p->name : "<unnamed-register>", *reg_num_p,
+                        reg_info_p->name ? reg_info_p->name : "<unnamed-register>", reg_num,
                         error.AsCString ());
             continue;
         }
 
         StreamString stream;
         WriteRegisterValueInHexFixedWidth(stream, reg_ctx_sp, *reg_info_p, &reg_value);
 
-        register_object_sp->SetObject(std::to_string(*reg_num_p),
+        register_object_sp->SetObject(std::to_string(reg_num),
                 std::make_shared<JSONString>(stream.GetString()));
     }
 
     return register_object_sp;
 }
 
 static const char *
 GetStopReasonString(StopReason stop_reason)