[Mach-O] Add -no_function_starts

macho/cmdline.cc

@@ -74,6 +74,7 @@ Options:
   -needed-framework <NAME>[,<SUFFIX>]
                               Search for a given framework
   -no_deduplicate             Ignored
+  -no_function_starts         Do not generate an LC_FUNCTION_STARTS load command
   -no_uuid                    Do not generate an LC_UUID load command
   -o <FILE>                   Set output filename
   -objc_abi_version <VERSION> Ignored
@@ -408,6 +409,8 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       remaining.push_back("-needed_framework");
       remaining.push_back(std::string(arg));
     } else if (read_flag("-no_deduplicate")) {
+    } else if (read_flag("-no_function_starts")) {
+      ctx.arg.function_starts = false;
     } else if (read_flag("-no_uuid")) {
       ctx.arg.uuid = UUID_NONE;
     } else if (read_arg("-o")) {

macho/main.cc

@@ -324,6 +324,10 @@ static void create_synthetic_chunks(Context<E> &ctx) {
   // Create a __DATA,__objc_imageinfo section.
   ctx.image_info = ObjcImageInfoSection<E>::create(ctx);
 
+  // Create a __LINKEDIT,__func_starts section.
+  if (ctx.arg.function_starts)
+    ctx.function_starts.reset(new FunctionStartsSection(ctx));
+
   // Handle -sectcreate
   for (SectCreateOption arg : ctx.arg.sectcreate) {
     MappedFile<Context<E>> *mf =

macho/mold.h

@@ -876,6 +876,7 @@ struct Context {
     bool dynamic = true;
     bool export_dynamic = false;
     bool fatal_warnings = false;
+    bool function_starts = true;
     bool ignore_optimization_hints = false;
     bool mark_dead_strippable_dylib = false;
     bool noinhibit_exec = false;
@@ -974,10 +975,10 @@ struct Context {
   BindSection<E> bind{*this};
   LazyBindSection<E> lazy_bind{*this};
   ExportSection<E> export_{*this};
-  FunctionStartsSection<E> function_starts{*this};
   SymtabSection<E> symtab{*this};
   StrtabSection<E> strtab{*this};
 
+  std::unique_ptr<FunctionStartsSection<E>> function_starts;
   std::unique_ptr<ObjcImageInfoSection<E>> image_info;
   std::unique_ptr<CodeSignatureSection<E>> code_sig;
 

macho/output-chunks.cc

@@ -191,8 +191,8 @@ static std::vector<u8> create_function_starts_cmd(Context<E> &ctx) {
 
   cmd.cmd = LC_FUNCTION_STARTS;
   cmd.cmdsize = buf.size();
-  cmd.dataoff = ctx.function_starts.hdr.offset;
-  cmd.datasize = ctx.function_starts.hdr.size;
+  cmd.dataoff = ctx.function_starts->hdr.offset;
+  cmd.datasize = ctx.function_starts->hdr.size;
   return buf;
 }
 
@@ -275,7 +275,8 @@ static std::vector<std::vector<u8>> create_load_commands(Context<E> &ctx) {
     vec.push_back(create_uuid_cmd(ctx));
   vec.push_back(create_build_version_cmd(ctx));
   vec.push_back(create_source_version_cmd(ctx));
-  vec.push_back(create_function_starts_cmd(ctx));
+  if (ctx.arg.function_starts)
+    vec.push_back(create_function_starts_cmd(ctx));
 
   for (DylibFile<E> *file : ctx.dylibs)
     if (file->dylib_idx != BIND_SPECIAL_DYLIB_MAIN_EXECUTABLE)
@@ -971,6 +972,9 @@ void ExportSection<E>::copy_buf(Context<E> &ctx) {
   enc.write_trie(buf, enc.root);
 }
 
+// LC_FUNCTION_STARTS contains function start addresses encoded in
+// ULEB128. I don't know what tools consume this table, but we create
+// it anyway by default for the sake of compatibility.
 template <typename E>
 void FunctionStartsSection<E>::compute_size(Context<E> &ctx) {
   std::vector<std::vector<u64>> vec(ctx.objs.size());

test/macho/no-function-starts.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+CC="${TEST_CC:-cc}"
+CXX="${TEST_CXX:-c++}"
+GCC="${TEST_GCC:-gcc}"
+GXX="${TEST_GXX:-g++}"
+OBJDUMP="${OBJDUMP:-objdump}"
+MACHINE="${MACHINE:-$(uname -m)}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+t=out/test/macho/$MACHINE/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+int main() {}
+EOF
+
+clang --ld-path=./ld64 -o $t/exe1 $t/a.o
+otool -l $t/exe1 | grep -q LC_FUNCTION_STARTS
+
+clang --ld-path=./ld64 -o $t/exe2 $t/a.o -Wl,-no_function_starts
+otool -l $t/exe2 > $t/log
+! grep -q LC_FUNCTION_STARTS $t/log || false
+
+echo OK