[lldb] Improve EXC_RESOURCE exception reason

[JDevlieghere]

Summary

Jason noted that the stop message we print for a memory high water mark notification (EXC_RESOURCE) could be clearer. Currently, the stop message looks like this:

* thread #3, queue = 'com.apple.CFNetwork.LoaderQ', stop reason = EXC_RESOURCE RESOURCE_TYPE_MEMORY (limit=14 MB, unused=0x0)
    frame #0: 0x00000001f6a8c09c dyld`dyld4::APIs::dyld_shared_cache_some_image_overridden()
dyld`dyld4::APIs::dyld_shared_cache_some_image_overridden:
->  0x1f6a8c09c <+0>:  pacibsp
    0x1f6a8c0a0 <+4>:  sub    sp, sp, #0x30
    0x1f6a8c0a4 <+8>:  stp    x20, x19, [sp, #0x10]
    0x1f6a8c0a8 <+12>: stp    x29, x30, [sp, #0x20]

It's hard to read the message because the exception and the types (EXC_RESOURCE RESOURCE_TYPE_MEMORY) blend together. Additionally, the "observed=0x0" should not be printed for memory limit exceptions.

I wanted to continue to include the resource type from <kern/exc_resource.h> while also explaining what it actually is. I used the wording from the comments in the header. With this path, the stop reason now looks like this:

* thread #5, stop reason = EXC_RESOURCE (RESOURCE_TYPE_MEMORY: high watermark memory limit exceeded) (limit=14 MB)
    frame #0: 0x000000022500bb90 libsystem_pthread.dylib`start_wqthread
libsystem_pthread.dylib`start_wqthread:
->  0x22500bb90 <+0>: stp    xzr, xzr, [sp, #-0x10]!
    0x22500bb94 <+4>: bl     0x22500bcd8               ; _pthread_wqthread
    0x22500bb98 <+8>: brk    #0x1

rdar://40466897

[jingham]

LGTM

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

#if defined(__APPLE__)
 
       code_label = "limit";
       code_desc = code_desc_buf;
       subcode_label = "observed";
       subcode_desc = subcode_desc_buf;
 
       switch (resource_type) {
       case RESOURCE_TYPE_CPU:
-        exc_desc = "EXC_RESOURCE RESOURCE_TYPE_CPU";
+        exc_desc =
+            "EXC_RESOURCE (RESOURCE_TYPE_CPU: CPU usage monitor tripped)";
         snprintf(code_desc_buf, sizeof(code_desc_buf), "%d%%",
                  (int)EXC_RESOURCE_CPUMONITOR_DECODE_PERCENTAGE(m_exc_code));
         snprintf(subcode_desc_buf, sizeof(subcode_desc_buf), "%d%%",
                  (int)EXC_RESOURCE_CPUMONITOR_DECODE_PERCENTAGE_OBSERVED(
                      m_exc_subcode));
         break;
       case RESOURCE_TYPE_WAKEUPS:
-        exc_desc = "EXC_RESOURCE RESOURCE_TYPE_WAKEUPS";
+        exc_desc = "EXC_RESOURCE (RESOURCE_TYPE_WAKEUPS: idle wakeups monitor "
+                   "tripped)";
         snprintf(
             code_desc_buf, sizeof(code_desc_buf), "%d w/s",
             (int)EXC_RESOURCE_CPUMONITOR_DECODE_WAKEUPS_PERMITTED(m_exc_code));
         snprintf(subcode_desc_buf, sizeof(subcode_desc_buf), "%d w/s",
                  (int)EXC_RESOURCE_CPUMONITOR_DECODE_WAKEUPS_OBSERVED(
                      m_exc_subcode));
         break;
       case RESOURCE_TYPE_MEMORY:
-        exc_desc = "EXC_RESOURCE RESOURCE_TYPE_MEMORY";
+        exc_desc = "EXC_RESOURCE (RESOURCE_TYPE_MEMORY: high watermark memory "
+                   "limit exceeded)";
         snprintf(code_desc_buf, sizeof(code_desc_buf), "%d MB",
                  (int)EXC_RESOURCE_HWM_DECODE_LIMIT(m_exc_code));
         subcode_desc = nullptr;
-        subcode_label = "unused";
+        subcode_label = nullptr;
         break;
 #if defined(RESOURCE_TYPE_IO)
       // RESOURCE_TYPE_IO is introduced in macOS SDK 10.12.
       case RESOURCE_TYPE_IO:
         exc_desc = "EXC_RESOURCE RESOURCE_TYPE_IO";
         snprintf(code_desc_buf, sizeof(code_desc_buf), "%d MB",
                  (int)EXC_RESOURCE_IO_DECODE_LIMIT(m_exc_code));
         snprintf(subcode_desc_buf, sizeof(subcode_desc_buf), "%d MB",
#endif
   if (m_exc_data_count >= 1) {
     if (code_desc)
       strm.Printf(" (%s=%s", code_label, code_desc);
     else
       strm.Printf(" (%s=%" PRIu64, code_label, m_exc_code);
   }
 
   if (m_exc_data_count >= 2) {
-    if (subcode_desc)
+    if (subcode_label && subcode_desc)
       strm.Printf(", %s=%s", subcode_label, subcode_desc);
-    else
+    else if (subcode_label)
       strm.Printf(", %s=0x%" PRIx64, subcode_label, m_exc_subcode);
   }
 
   if (m_exc_data_count > 0)
     strm.PutChar(')');
 
   m_description = std::string(strm.GetString());
   return m_description.c_str();