[lld/mac] Don't add names of unreferenced symbols to string table

[thakis]

Summary

Before this, a hello world program would contain many many unnecessary
entries in its string table.

No behavior change, just makes the string table in the output smaller
and more like ld64's.

[compnerd]

The change on L722 took me a few moments to catch - that was subtle.

lld/MachO/SyntheticSections.cpp

699

Why not emplace_back?

[thakis]

Thanks!

lld/MachO/SyntheticSections.cpp

699

That's annoying to use with structs pre c++20 (https://stackoverflow.com/questions/13812703/c11-emplace-back-on-vectorstruct) and in practice it won't matter.

(But that's the retconned answer. The actual reason is that the LHS used push_back and I didn't think about it ;) )

lld/MachO/SyntheticSections.cpp

for (Defined *defined : symbolsNeedingStabs) {
     }
   }
 
   if (!stabs.empty())
     emitEndSourceStab();
 }
 
 void SymtabSection::finalizeContents() {
+  auto addSymbol = [&](std::vector<SymtabEntry> &symbols, Symbol *sym) {
+    uint32_t strx = stringTableSection.addString(sym->getName());
+    symbols.push_back({sym, strx});
+  };
+
   // Local symbols aren't in the SymbolTable, so we walk the list of object
   // files to gather them.
   for (InputFile *file : inputFiles) {
     if (auto *objFile = dyn_cast<ObjFile>(file)) {
       for (Symbol *sym : objFile->symbols) {
         // TODO: when we implement -dead_strip, we should filter out symbols
         // that belong to dead sections.
         if (auto *defined = dyn_cast<Defined>(sym)) {
-          if (!defined->isExternal()) {
-            uint32_t strx = stringTableSection.addString(sym->getName());
-            localSymbols.push_back({sym, strx});
-          }
+          if (!defined->isExternal())
+            addSymbol(localSymbols, sym);
         }
       }
     }
   }
 
   // __dyld_private is a local symbol too. It's linker-created and doesn't
   // exist in any object file.
-  if (Defined* dyldPrivate = in.stubHelper->dyldPrivate) {
-    uint32_t strx = stringTableSection.addString(dyldPrivate->getName());
-    localSymbols.push_back({dyldPrivate, strx});
-  }
+  if (Defined* dyldPrivate = in.stubHelper->dyldPrivate)
+    addSymbol(localSymbols, dyldPrivate);
 
   for (Symbol *sym : symtab->getSymbols()) {
-    uint32_t strx = stringTableSection.addString(sym->getName());
     if (auto *defined = dyn_cast<Defined>(sym)) {
       assert(defined->isExternal());
-      externalSymbols.push_back({sym, strx});
+      addSymbol(externalSymbols, sym);
     } else if (auto *dysym = dyn_cast<DylibSymbol>(sym)) {
       if (dysym->isReferenced())
-        undefinedSymbols.push_back({sym, strx});
+        addSymbol(undefinedSymbols, sym);
     }
   }
 
   emitStabs();
   uint32_t symtabIndex = stabs.size();
   for (const SymtabEntry &entry :
        concat<SymtabEntry>(localSymbols, externalSymbols, undefinedSymbols)) {
     entry.sym->symtabIndex = symtabIndex++;

lld/test/MachO/symtab.s

 # CHECK-NEXT: Dysymtab {
 # CHECK-NEXT:   ilocalsym: 0
 # CHECK-NEXT:   nlocalsym: 2
 # CHECK-NEXT:   iextdefsym: 2
 # CHECK-NEXT:   nextdefsym: 3
 # CHECK-NEXT:   iundefsym: 5
 # CHECK-NEXT:   nundefsym: 2
 
-## Verify that the first entry in the StringTable is a space.
+## Verify that the first entry in the StringTable is a space, and that
+## unreferenced symbols aren't emitted.
 # RUN: obj2yaml %t/test | FileCheck %s --check-prefix=YAML
 # YAML:      StringTable:
 # YAML-NEXT: ' '
+# YAML-NOT: _unreferenced
 
 #--- libfoo.s
 .globl _dynamic
 _dynamic:
 
 #--- test.s
-.globl _main, _external, _external_weak
+.globl _main, _external, _external_weak, _unreferenced
 
 .data
 _external:
   .space 0
 _local:
   .space 0
 
 .text
 .weak_definition _external_weak
 _external_weak:
   .space 0
 
 _main:
   callq _dynamic
   mov $0, %rax
   ret