[llvm-objcopy][MachO] Keep symbols having the flag REFERENCED_DYNAMICALLY set

[alexander-shaposhnikov]

Summary

Do not strip symbols having the flag REFERENCED_DYNAMICALLY set.

This is compatible with the behavior of cctools' strip and consistent with the documentation ("Mach-O Runtime Architecture"):

"REFERENCED_DYNAMICALLY (0x10)—Must be set for any symbol that might be referenced
by another image. The strip tool uses this bit to avoid removing symbols that must exist:
If the symbol has this bit set, strip does not strip it."

Test plan: make check-all

[smeenai]

LGTM

[jhenderson]

LGTM, with nit.

llvm/test/tools/llvm-objcopy/MachO/Inputs/strip-all-with-dwarf.yaml

295

It's probably worth a comment showing what this n_desc value represents.

llvm/test/tools/llvm-objcopy/MachO/Inputs/strip-all-with-dwarf.yaml

- cmd:             LC_BUILD_VERSION
     cmdsize:         24
     platform:        1
     minos:           658944
     sdk:             0
     ntools:          0
   - cmd:             LC_SYMTAB
     cmdsize:         24
     symoff:          2136
-    nsyms:           3
-    stroff:          2184
-    strsize:         20
+    nsyms:           4
+    stroff:          2204
+    strsize:         40
   - cmd:             LC_DYSYMTAB
     cmdsize:         80
     ilocalsym:       0
     nlocalsym:       1
     iextdefsym:      1
     nextdefsym:      1
     iundefsym:       2
     nundefsym:       1
     tocoff:          0
     ntoc:            0
     modtaboff:       0
     nmodtab:         0
     extrefsymoff:    0
     nextrefsyms:     0
     indirectsymoff:  0
     nindirectsyms:   0
     extreloff:       0
     nextrel:         0
     locreloff:       0
     nlocrel:         0
 LinkEditData:
   NameList:
-    - n_strx:          1
+    - n_strx:          21
       n_type:          0x0E
       n_sect:          2
       n_desc:          0
       n_value:         36
-    - n_strx:          11
+    - n_strx:          31
       n_type:          0x0F
       n_sect:          1
       n_desc:          0
       n_value:         0
-    - n_strx:          6
+    ## __mh_execute_header
+    - n_strx:          1
+      n_type:          0xF
+      n_sect:          1
+      ## ReferencedDynamically (0x10)
+      n_desc:          16
+      n_value:         4294967296
+    - n_strx:          26
       n_type:          0x01
       n_sect:          0
       n_desc:          512
       n_value:         4
   StringTable:
     - ''
+    - __mh_execute_header
     - _bar
     - _foo
     - _main
     - ''
     - ''
     - ''
 DWARF:
   debug_str:

llvm/test/tools/llvm-objcopy/MachO/strip-all.test

 # DWARF-NOT:     Name: __debug_macinfo
 # DWARF-NOT:     Name: __apple_names
 # DWARF-NOT:     Name: __apple_objc
 # DWARF-NOT:     Name: __apple_namespac
 # DWARF-NOT:     Name: __apple_types
 # DWARF-NOT:     Name: __debug_line
 # DWARF:       ]
 
-## Make sure that all relocations and symbols are removed.
+## Make sure that all relocations and symbols
+## (except those which have the flag ReferencedDynamically set) are removed.
 # COMMON:      Relocations [
 # COMMON-NEXT: ]
-# COMMON:      Symbols [
+# COMMON-NEXT: Symbols [
+# COMMON-NEXT:    Symbol {
+# COMMON-NEXT:      Name: __mh_execute_header
+# COMMON-NEXT:      Extern
+# COMMON-NEXT:      Type: Section
+# COMMON-NEXT:      Section: __text
+# COMMON-NEXT:      RefType: UndefinedNonLazy
+# COMMON-NEXT:      Flags [ (0x10)
+# COMMON-NEXT:        ReferencedDynamically (0x10)
+# COMMON-NEXT:      ]
+# COMMON-NEXT:      Value:
+# COMMON-NEXT:    }
 # COMMON-NEXT: ]

llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp

if (I != Config.SymbolsToRename.end())
       Sym.Name = std::string(I->getValue());
   }
 
   auto RemovePred = [Config, &Obj](const std::unique_ptr<SymbolEntry> &N) {
     if (N->Referenced)
       return false;
     if (Config.KeepUndefined && N->isUndefinedSymbol())
       return false;
+    if (N->n_desc & MachO::REFERENCED_DYNAMICALLY)
+      return false;
     if (Config.StripAll)
       return true;
     if (Config.DiscardMode == DiscardType::All && !(N->n_type & MachO::N_EXT))
       return true;
     // This behavior is consistent with cctools' strip.
     if (Config.StripSwiftSymbols && (Obj.Header.Flags & MachO::MH_DYLDLINK) &&
         Obj.SwiftVersion && *Obj.SwiftVersion && N->isSwiftSymbol())
       return true;