[lld-macho] Emit empty string as first entry of string table

[int3]

Summary

ld64 emits string tables which start with a space and a zero byte. We
match its behavior here since some tools depend on it.

Similar rationale as D89561: [MC] Adjust StringTableBuilder for linked Mach-O binaries.

[int3]

Note that unlike D89561, this diff doesn't need to make any alignment-related changes as LLD should already be handling that correctly. See the linkedit-contiguity.s test for more details.

[smeenai]

Do we have any logic to emit an n_strx for an empty string? If so, that would need to be adjusted to emit 1 instead of 0. If not, LGTM.

[int3]

Do we have any logic to emit an n_strx for an empty string?

Nope. In general, we don't make any attempt to dedup strings (I think ld64 does the same thing)

[smeenai]

LGTM

lld/MachO/SyntheticSections.h

 public:
   StringTableSection();
   // Returns the start offset of the added string.
   uint32_t addString(StringRef);
   uint64_t getRawSize() const override { return size; }
   void writeTo(uint8_t *buf) const override;
 
 private:
-  // An n_strx value of 0 always indicates the empty string, so we must locate
-  // our non-empty string values at positive offsets in the string table.
-  // Therefore we insert a dummy value at position zero.
-  std::vector<StringRef> strings{"\0"};
-  size_t size = 1;
+  // ld64 emits string tables which start with a space and a zero byte. We
+  // match its behavior here since some tools depend on it.
+  std::vector<StringRef> strings{" "};
+  size_t size = 2;
 };
 
 struct SymtabEntry {
   Symbol *sym;
   size_t strx;
 };
 
 struct StabsEntry {

lld/test/MachO/symtab.s

 # REQUIRES: x86
 # RUN: rm -rf %t && split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/libfoo.s -o %t/libfoo.o
 # RUN: %lld -dylib %t/libfoo.o -o %t/libfoo.dylib
 # RUN: %lld -lSystem %t/test.o %t/libfoo.dylib -o %t/test
-# RUN: llvm-readobj --syms --macho-dysymtab %t/test | FileCheck %s
 
+# RUN: llvm-readobj --syms --macho-dysymtab %t/test | FileCheck %s
 # CHECK:      Symbols [
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: _local (1)
+# CHECK-NEXT:     Name: _local (2)
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __data (0x4)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:     Flags [ (0x0)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: _main (8)
+# CHECK-NEXT:     Name: _main (9)
 # CHECK-NEXT:     Extern
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __text (0x1)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:     Flags [ (0x0)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: _external (54)
+# CHECK-NEXT:     Name: _external (55)
 # CHECK-NEXT:     Extern
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __data (0x4)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:     Flags [ (0x0)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: _external_weak (64)
+# CHECK-NEXT:     Name: _external_weak (65)
 # CHECK-NEXT:     Extern
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __text (0x1)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:     Flags [ (0x80)
 # CHECK-NEXT:       WeakDef (0x80)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: __dyld_private (102)
+# CHECK-NEXT:     Name: __dyld_private (103)
 # CHECK-NEXT:     Extern
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __data (0x4)
 # CHECK-NEXT:     RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:     Flags [ (0x0)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value: 0x1{{[0-9a-f]*}}
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: dyld_stub_binder (14)
+# 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:     ]
 # CHECK-NEXT:     Value: 0x0
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
-# CHECK-NEXT:     Name: _dynamic (79)
+# 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:     ]
 # CHECK-NEXT:     Value: 0x0
 # CHECK-NEXT:   }
 # CHECK-NEXT: ]
 # CHECK-NEXT: Dysymtab {
 # CHECK-NEXT:   ilocalsym: 0
 # CHECK-NEXT:   nlocalsym: 1
 # CHECK-NEXT:   iextdefsym: 1
 # CHECK-NEXT:   nextdefsym: 4
 # CHECK-NEXT:   iundefsym: 5
 # CHECK-NEXT:   nundefsym: 2
 
+## Verify that the first entry in the StringTable is a space.
+# RUN: obj2yaml %t/test | FileCheck %s --check-prefix=YAML
+# YAML:      StringTable:
+# YAML-NEXT: ' '
+
 #--- libfoo.s
 .globl _dynamic
 _dynamic:
 
 #--- test.s
 .globl _main, _external, _external_weak
 
 .data