[lld/mac] Implement support for -mark_dead_strippable_dylib

[thakis]

Summary

lld doesn't read MH_DEAD_STRIPPABLE_DYLIB to strip dead dylibs yet,
but now it can produce dylibs with it set.

While here, also switch an existing test that looks only at the main Mach-O
header from --all-headers to --private-header.

[int3]

lld/MachO/Options.td

338

nit: dead-strippable

lld/MachO/Config.h

struct Configuration {
   bool hasReexports = false;
   bool allLoad = false;
   bool forceLoadObjC = false;
   bool staticLink = false;
   bool implicitDylibs = false;
   bool isPic = false;
   bool headerPadMaxInstallNames = false;
   bool ltoNewPassManager = LLVM_ENABLE_NEW_PASS_MANAGER;
+  bool markDeadStrippableDylib = false;
   bool printEachFile = false;
   bool printWhyLoad = false;
   bool searchDylibsFirst = false;
   bool saveTemps = false;
   bool adhocCodesign = false;
   uint32_t headerPad;
   uint32_t dylibCompatibilityVersion = 0;
   uint32_t dylibCurrentVersion = 0;

lld/MachO/Driver.cpp

config->ltoNewPassManager =
       args.hasFlag(OPT_no_lto_legacy_pass_manager, OPT_lto_legacy_pass_manager,
                    LLVM_ENABLE_NEW_PASS_MANAGER);
   config->runtimePaths = args::getStrings(args, OPT_rpath);
   config->allLoad = args.hasArg(OPT_all_load);
   config->forceLoadObjC = args.hasArg(OPT_ObjC);
   config->demangle = args.hasArg(OPT_demangle);
   config->implicitDylibs = !args.hasArg(OPT_no_implicit_dylibs);
 
+  if (args.hasArg(OPT_mark_dead_strippable_dylib)) {
+    if (config->outputType != MH_DYLIB)
+      warn("-mark_dead_strippable_dylib: ignored, only has effect with -dylib");
+    else
+      config->markDeadStrippableDylib = true;
+  }
+
   if (const Arg *arg = args.getLastArg(OPT_static, OPT_dynamic))
     config->staticLink = (arg->getOption().getID() == OPT_static);
 
   if (const Arg *arg =
           args.getLastArg(OPT_flat_namespace, OPT_twolevel_namespace))
     config->namespaceKind = arg->getOption().getID() == OPT_twolevel_namespace
                                 ? NamespaceKind::twolevel
                                 : NamespaceKind::flat;

lld/MachO/Options.td

def dylib_install_name : Separate<["-"], "dylib_install_name">,
      Group<grp_dylib>;
 def dylinker_install_name : Separate<["-"], "dylinker_install_name">,
      MetaVarName<"<name>">,
      Alias<install_name>,
      HelpText<"Alias for -install_name">,
      Flags<[HelpHidden]>,
      Group<grp_dylib>;
 def mark_dead_strippable_dylib : Flag<["-"], "mark_dead_strippable_dylib">,
-     HelpText<"Clients can discard this dylib if it is unreferenced">,
-     Flags<[HelpHidden]>,
+     HelpText<"Mark output dylib as dead-strippable: When a client links against it but does not use any of its symbols, the dylib will not be added to the client's list of needed dylibs">,
      Group<grp_dylib>;
 def compatibility_version : Separate<["-"], "compatibility_version">,
      MetaVarName<"<version>">,
      HelpText<"Compatibility <version> of this library">,
      Group<grp_dylib>;
 def dylib_compatibility_version : Separate<["-"], "dylib_compatibility_version">,
      MetaVarName<"<version>">,
      Alias<compatibility_version>,

lld/MachO/SyntheticSections.cpp

void MachHeaderSection::writeTo(uint8_t *buf) const {
   hdr->flags = MachO::MH_DYLDLINK;
 
   if (config->namespaceKind == NamespaceKind::twolevel)
     hdr->flags |= MachO::MH_NOUNDEFS | MachO::MH_TWOLEVEL;
 
   if (config->outputType == MachO::MH_DYLIB && !config->hasReexports)
     hdr->flags |= MachO::MH_NO_REEXPORTED_DYLIBS;
 
+  if (config->markDeadStrippableDylib)
+    hdr->flags |= MachO::MH_DEAD_STRIPPABLE_DYLIB;
+
   if (config->outputType == MachO::MH_EXECUTE && config->isPic)
     hdr->flags |= MachO::MH_PIE;
 
   if (in.exports->hasWeakSymbol || in.weakBinding->hasNonWeakDefinition())
     hdr->flags |= MachO::MH_WEAK_DEFINES;
 
   if (in.exports->hasWeakSymbol || in.weakBinding->hasEntry())
     hdr->flags |= MachO::MH_BINDS_TO_WEAK;

lld/test/MachO/header.s

 # REQUIRES: x86, aarch64
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/x86_64-test.o
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t/arm64-test.o
 # RUN: %lld -arch x86_64 -platform_version macos 10.5.0 11.0 -o %t/x86-64-executable %t/x86_64-test.o
 # RUN: %lld -arch arm64 -o %t/arm64-executable %t/arm64-test.o
 # RUN: %lld -arch x86_64 -dylib -o %t/x86-64-dylib %t/x86_64-test.o
 # RUN: %lld -arch arm64  -dylib -o %t/arm64-dylib %t/arm64-test.o
 
-# RUN: llvm-objdump --macho --all-headers %t/x86-64-executable | FileCheck %s -DCAPS=LIB64
-# RUN: llvm-objdump --macho --all-headers %t/arm64-executable | FileCheck %s -DCAPS=0x00
-# RUN: llvm-objdump --macho --all-headers %t/x86-64-dylib | FileCheck %s -DCAPS=0x00
-# RUN: llvm-objdump --macho --all-headers %t/arm64-dylib | FileCheck %s -DCAPS=0x00
+# RUN: llvm-objdump --macho --private-header %t/x86-64-executable | FileCheck %s -DCAPS=LIB64
+# RUN: llvm-objdump --macho --private-header %t/arm64-executable | FileCheck %s -DCAPS=0x00
+# RUN: llvm-objdump --macho --private-header %t/x86-64-dylib | FileCheck %s -DCAPS=0x00
+# RUN: llvm-objdump --macho --private-header %t/arm64-dylib | FileCheck %s -DCAPS=0x00
 
 # CHECK:      magic        cputype cpusubtype  caps     filetype {{.*}} flags
 # CHECK-NEXT: MH_MAGIC_64  {{.*}}         ALL  [[CAPS]] {{.*}}          NOUNDEFS {{.*}} TWOLEVEL
 
 .globl _main
 _main:
   ret

lld/test/MachO/mark-dead-strippable-dylib.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t.o %s
+
+# RUN: %no_fatal_warnings_lld -o %t.exec %t.o -mark_dead_strippable_dylib 2>&1 \
+# RUN:     | FileCheck --check-prefix=WARN %s
+# RUN: llvm-objdump --macho --private-header %t.exec \
+# RUN:     | FileCheck --check-prefix=NO-DS %s
+
+# RUN: %no_fatal_warnings_lld -bundle -o %t.bundle %t.o \
+# RUN:     -mark_dead_strippable_dylib 2>&1 \
+# RUN:     | FileCheck --check-prefix=WARN %s
+# RUN: llvm-objdump --macho --private-header %t.bundle \
+# RUN:     | FileCheck --check-prefix=NO-DS %s
+
+# RUN: %lld -dylib -o %t.dylib %t.o -mark_dead_strippable_dylib 2>&1
+# RUN: llvm-objdump --macho --private-header %t.dylib \
+# RUN:     | FileCheck --check-prefix=DS %s
+
+# WARN: warning: -mark_dead_strippable_dylib: ignored, only has effect with -dylib
+
+# NO-DS-NOT: DEAD_STRIPPABLE_DYLIB
+# DS:        DEAD_STRIPPABLE_DYLIB
+
+.globl _main
+_main:
+  ret