Use LC_DYLD_EXPORTS_TRIE to locate the dyld trie structure if present

The pointer to the dyld trie data structure which lldb needs to parse to get
"trampoline kinds" on Darwin used to be a field in the LC_DYLD_INFO load command. A
new load command was added recently dedicated to this purpose: LC_DYLD_EXPORTS_TRIE.
The format of the trie did not change, however. So all we have to do is use the new
command if present. The commands are supposed to be mutually exclusive, so I added
an lldb_assert to warn if they are not.

Differential Revision: https://reviews.llvm.org/D107673

Author: jimingham

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

@@ -2231,6 +2231,7 @@ size_t ObjectFileMachO::ParseSymtab() {
 
   llvm::MachO::symtab_command symtab_load_command = {0, 0, 0, 0, 0, 0};
   llvm::MachO::linkedit_data_command function_starts_load_command = {0, 0, 0, 0};
+  llvm::MachO::linkedit_data_command exports_trie_load_command = {0, 0, 0, 0};
   llvm::MachO::dyld_info_command dyld_info = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
   // The data element of type bool indicates that this entry is thumb
   // code.
@@ -2308,11 +2309,19 @@ size_t ObjectFileMachO::ParseSymtab() {
       }
     } break;
 
+    case LC_DYLD_EXPORTS_TRIE:
+      exports_trie_load_command.cmd = lc.cmd;
+      exports_trie_load_command.cmdsize = lc.cmdsize;
+      if (m_data.GetU32(&offset, &exports_trie_load_command.dataoff, 2) ==
+          nullptr) // fill in offset and size fields
+        memset(&exports_trie_load_command, 0,
+               sizeof(exports_trie_load_command));
+      break;
     case LC_FUNCTION_STARTS:
       function_starts_load_command.cmd = lc.cmd;
       function_starts_load_command.cmdsize = lc.cmdsize;
       if (m_data.GetU32(&offset, &function_starts_load_command.dataoff, 2) ==
-          nullptr) // fill in symoff, nsyms, stroff, strsize fields
+          nullptr) // fill in data offset and size fields
         memset(&function_starts_load_command, 0,
                sizeof(function_starts_load_command));
       break;
@@ -2456,16 +2465,24 @@ size_t ObjectFileMachO::ParseSymtab() {
       dyld_info.export_off += linkedit_slide;
       m_dysymtab.indirectsymoff += linkedit_slide;
       function_starts_load_command.dataoff += linkedit_slide;
+      exports_trie_load_command.dataoff += linkedit_slide;
     }
 
     nlist_data.SetData(m_data, symtab_load_command.symoff,
                        nlist_data_byte_size);
     strtab_data.SetData(m_data, symtab_load_command.stroff,
                         strtab_data_byte_size);
 
+    // We shouldn't have exports data from both the LC_DYLD_INFO command
+    // AND the LC_DYLD_EXPORTS_TRIE command in the same binary:
+    lldbassert(!((dyld_info.export_size > 0) 
+                 && (exports_trie_load_command.datasize > 0)));
     if (dyld_info.export_size > 0) {
       dyld_trie_data.SetData(m_data, dyld_info.export_off,
                              dyld_info.export_size);
+    } else if (exports_trie_load_command.datasize > 0) {
+      dyld_trie_data.SetData(m_data, exports_trie_load_command.dataoff,
+                             exports_trie_load_command.datasize);
     }
 
     if (m_dysymtab.nindirectsyms != 0) {