[dsymutil] Don't emit N_AST symbol entries in the Mach-O companion file

[JDevlieghere]

Summary

Treat N_AST symbol table entries like N_OSO entries and don't emit them in the linked binary.

[JDevlieghere]

llvm/tools/dsymutil/MachOUtils.cpp

167

Previously the code below would filter out N_OSO entries, relying on the fact that N_OSO entries come after N_SO entries. That means that technically the N_OSO could be omitted here, but I think this is much clearer.

[friss]

llvm/tools/dsymutil/MachOUtils.cpp

167

It's not only the N_OSO entries. The debugging entries are somewhat hierarchical and the top-level element is N_SO. An N_SO with a filename opens a debugging scope and another one without a name closes it. We don't transfer anything that is in the debug scope (N_OSO, N_FUN, N_ENSYM, N_BNSYM in the example I'm looking at).

[JDevlieghere]

llvm/tools/dsymutil/MachOUtils.cpp

167

So would you prefer to add a comment below describing this and removing it from the if clause?

[friss]

llvm/tools/dsymutil/MachOUtils.cpp

167

Yep, I think so

[JDevlieghere]

Address @friss' comments

[friss]

LGTM

llvm/test/tools/dsymutil/X86/swift-ast-x86_64.test

 DSYMUTIL: DEBUG MAP OBJECT:{{.*}}swift-ast.swiftmodule
 
 READOBJ: Name:{{.*}}__swift_ast
 READOBJ: |SWIFTMODULE DATA|
 READOBJ-NEXT: |.|
 
 RUN: dsymutil -oso-prepend-path %p/.. %p/../Inputs/swift-ast.macho.x86_64 -no-output -verbose 2>&1 | FileCheck %s --check-prefix=TIMESTAMP
 TIMESTAMP: warning: Timestamp mismatch
+
+RUN: dsymutil -s %T/swift-ast.dSYM/Contents/Resources/DWARF/swift-ast.macho.x86_64 | FileCheck %s --check-prefix=NAST
+NAST-NOT: N_AST

llvm/tools/dsymutil/MachOUtils.cpp

 static bool transferSymbol(NListTy NList, bool IsLittleEndian,
                            StringRef Strings, SmallVectorImpl<char> &NewSymtab,
                            NonRelocatableStringpool &NewStrings,
                            bool &InDebugNote) {
   // Do not transfer undefined symbols, we want real addresses.
   if ((NList.n_type & MachO::N_TYPE) == MachO::N_UNDF)
     return false;
 
+  // Do not transfer N_AST symbols as their content is copied into a section of
+  // the Mach-O companion file.
+  if (NList.n_type == MachO::N_AST)
+    return false;
+
   StringRef Name = StringRef(Strings.begin() + NList.n_strx);
+
+  // An N_SO with a filename opens a debugging scope and another one without a
+  // name closes it. Don't transfer anything in the debugging scope.
   if (InDebugNote) {
     InDebugNote =
         (NList.n_type != MachO::N_SO) || (!Name.empty() && Name[0] != '\0');
     return false;
   } else if (NList.n_type == MachO::N_SO) {
     InDebugNote = true;
     return false;
   }