[Mach-O] Support -flat_namespace flag

macho/cmdline.cc

@@ -354,6 +354,8 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.dependency_info = arg;
     } else if (read_flag("-dylib")) {
       ctx.output_type = MH_DYLIB;
+    } else if (read_flag("-flat_namespace")) {
+      ctx.arg.flat_namespace = true;
     } else if (read_hex("-headerpad")) {
       ctx.arg.headerpad = hex_arg;
     } else if (read_flag("-headerpad_max_install_names")) {

macho/mold.h

@@ -869,6 +869,7 @@ struct Context {
     bool dynamic = true;
     bool export_dynamic = false;
     bool fatal_warnings = false;
+    bool flat_namespace = false;
     bool function_starts = true;
     bool ignore_optimization_hints = true;
     bool mark_dead_strippable_dylib = false;

macho/output-chunks.cc

@@ -346,7 +346,7 @@ void OutputMachHeader<E>::copy_buf(Context<E> &ctx) {
   mhdr.filetype = ctx.output_type;
   mhdr.ncmds = cmds.size();
   mhdr.sizeofcmds = flatten(cmds).size();
-  mhdr.flags = MH_TWOLEVEL | MH_NOUNDEFS | MH_DYLDLINK | MH_PIE;
+  mhdr.flags = MH_NOUNDEFS | MH_DYLDLINK | MH_PIE;
 
   if (has_tlv(ctx))
     mhdr.flags |= MH_HAS_TLV_DESCRIPTORS;
@@ -357,6 +357,9 @@ void OutputMachHeader<E>::copy_buf(Context<E> &ctx) {
   if (ctx.arg.mark_dead_strippable_dylib)
     mhdr.flags |= MH_DEAD_STRIPPABLE_DYLIB;
 
+  if (!ctx.arg.flat_namespace)
+    mhdr.flags |= MH_TWOLEVEL;
+
   write_vector(buf + sizeof(mhdr), flatten(cmds));
 }
 

test/macho/flat_namespace.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+export LC_ALL=C
+set -e
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+t=out/test/macho/$(uname -m)/$testname
+mkdir -p $t
+
+cat <<EOF | cc -o $t/a.o -c -xc -
+int main() {}
+EOF
+
+cc --ld-path=./ld64 -o $t/exe $t/a.o -Wl,-flat_namespace
+otool -hv $t/exe | grep -qv TWOLEVEL
+
+cc --ld-path=./ld64 -o $t/exe $t/a.o
+otool -hv $t/exe | grep -v TWOLEVEL
+
+echo OK