[lld/mac] Set ordinal on dynamic undefined symbols in symbol table

[thakis]

Summary

This lets nm -m print "(from libfoo)" in its output, which is more
accessible than dumping the bind table.

See https://reviews.llvm.org/D57190#2455761 for the somewhat
surprising AltEntry that appears in symtab.s.

[int3]

lgtm

lld/MachO/SyntheticSections.cpp

757–759

why not just pass nList->n_desc into SET_LIBRARY_ORDINAL? I suppose this is a bit more explicit about the fact that we're modifying n_desc, but I think it's already pretty obvious from the SET_ in the name...

[thakis]

Thanks!

lld/MachO/SyntheticSections.cpp

757–759

Because SET_LIBRARY_ORDINAL wants a uint16_t& but nList->n_desc is a llvm::support::ulittle16_t. I suppose we could templatize SET_LIBRARY_ORDINAL instead, but I could see us wanting to reaad n_desk once at the top of the loop, then modify it in the body, and then write it once at the end anyways, so that's a step in that direction.

lld/MachO/SyntheticSections.cpp

if (auto *defined = dyn_cast<Defined>(entry.sym)) {
       } else {
         nList->n_type =
             (defined->isExternal() ? MachO::N_EXT : 0) | MachO::N_SECT;
         nList->n_sect = defined->isec->parent->index;
         // For the N_SECT symbol type, n_value is the address of the symbol
         nList->n_value = defined->getVA();
       }
       nList->n_desc |= defined->isWeakDef() ? MachO::N_WEAK_DEF : 0;
+    } else if (auto *dysym = dyn_cast<DylibSymbol>(entry.sym)) {
+      uint16_t n_desc = nList->n_desc;
+      MachO::SET_LIBRARY_ORDINAL(n_desc, dysym->file->ordinal);
+      nList->n_desc = n_desc;
     }
     ++nList;
   }
 }
 
 IndirectSymtabSection::IndirectSymtabSection()
     : LinkEditSection(segment_names::linkEdit,
                       section_names::indirectSymbolTable) {}

lld/test/MachO/dylink.s

 # CHECK-LABEL: Bind table:
 # CHECK-DAG: __DATA_CONST __got  0x{{0*}}[[#%x, HELLO_RIP + HELLO_OFF]]               pointer 0   libhello   _hello_world
 # CHECK-DAG: __DATA_CONST __got  0x{{0*}}[[#%x, HELLO_ITS_ME_RIP + HELLO_ITS_ME_OFF]] pointer 0   libhello   _hello_its_me
 # CHECK-DAG: __DATA_CONST __got  0x{{0*}}[[#%x, GOODBYE_RIP + GOODBYE_OFF]]           pointer 0   libgoodbye _goodbye_world
 # CHECK-DAG: __DATA       __data 0x[[#%x, DATA_ADDR:]]                                pointer 0   libhello   _hello_world
 # CHECK-DAG: __DATA       __data 0x{{0*}}[[#%x, DATA_ADDR + 8]]                       pointer 8   libhello   _hello_its_me
 # CHECK-DAG: __DATA       __data 0x{{0*}}[[#%x, DATA_ADDR + 16]]                      pointer -15 libgoodbye _goodbye_world
 
+# RUN: llvm-nm -m %t/dylink | FileCheck --check-prefix=NM %s
+
+# NM-DAG: _goodbye_world (from libgoodbye)
+# NM-DAG: _hello_its_me (from libhello)
+# NM-DAG: _hello_world (from libhello)
+
 # RUN: llvm-objdump --macho --all-headers %t/dylink | FileCheck %s \
 # RUN:   --check-prefix=LOAD --implicit-check-not LC_LOAD_DYLIB
 ## Check that we don't create duplicate LC_LOAD_DYLIBs.
 # RUN: %lld -o %t/dylink -L%t -lhello -lhello -lgoodbye -lgoodbye %t/dylink.o
 # RUN: llvm-objdump --macho --all-headers %t/dylink | FileCheck %s \
 # RUN:   --check-prefix=LOAD --implicit-check-not LC_LOAD_DYLIB
 
 # LOAD:              cmd LC_LOAD_DYLIB

lld/test/MachO/symtab.s

 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
 # CHECK-NEXT:     Name: dyld_stub_binder (15)
 # CHECK-NEXT:     Type: Undef (0x0)
 # CHECK-NEXT:     Section:  (0x0)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
-# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     Flags [ (0x100)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x0
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
 # CHECK-NEXT:     Name: _dynamic (80)
 # CHECK-NEXT:     Type: Undef (0x0)
 # CHECK-NEXT:     Section:  (0x0)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
-# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     Flags [ (0x200)
+# CHECK-NEXT:       AltEntry (0x200)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x0
 # CHECK-NEXT:   }
 # CHECK-NEXT: ]
 # CHECK-NEXT: Dysymtab {
 # CHECK-NEXT:   ilocalsym: 0
 # CHECK-NEXT:   nlocalsym: 1
 # CHECK-NEXT:   iextdefsym: 1