[lld/mac] Implement -final_output

[thakis]

Summary

This is one of two flags clang passes to the linker when giving calling
clang with multiple -arch flags.

I think it'd make sense to also use finalOutput instead of outputFile
in CodeSignatureSection() and when replacing @executable_path, but
ld64 doesn't do that, so I'll at least put those in separate commits.

[int3]

lld/test/MachO/final-output.s

9

how about llvm-objdump --macho --dylib-id?

[thakis]

Thanks!

lld/test/MachO/final-output.s

9

Turns out llvm-otool -D does that too, so switched to that :)

lld/MachO/Config.h

struct Configuration {
   bool timeTraceEnabled = false;
   bool dataConst = false;
   bool dedupLiterals = true;
   uint32_t headerPad;
   uint32_t dylibCompatibilityVersion = 0;
   uint32_t dylibCurrentVersion = 0;
   uint32_t timeTraceGranularity = 500;
   std::string progName;
+
+  // For `clang -arch arm64 -arch x86_64`, clang will:
+  // 1. invoke the linker twice, to write one temporary output per arch
+  // 2. invoke `lipo` to merge the two outputs into a single file
+  // `outputFile` is the name of the temporary file the linker writes to.
+  // `finalOutput `is the name of the file lipo writes to after the link.
+  llvm::StringRef outputFile;
+  llvm::StringRef finalOutput;
+
   llvm::StringRef installName;
   llvm::StringRef mapFile;
-  llvm::StringRef outputFile;
   llvm::StringRef ltoObjPath;
   llvm::StringRef thinLTOJobs;
   llvm::StringRef umbrella;
   uint32_t ltoo = 2;
   bool deadStripDylibs = false;
   bool demangle = false;
   bool deadStrip = false;
   PlatformInfo platformInfo;

lld/MachO/Driver.cpp

config->explicitUndefineds.push_back(symtab->addUndefined(
         arg->getValue(), /*file=*/nullptr, /*isWeakRef=*/false));
   }
 
   for (const Arg *arg : args.filtered(OPT_U))
     symtab->addDynamicLookup(arg->getValue());
 
   config->mapFile = args.getLastArgValue(OPT_map);
   config->outputFile = args.getLastArgValue(OPT_o, "a.out");
+  if (const Arg *arg = args.getLastArg(OPT_final_output))
+    config->finalOutput = arg->getValue();
+  else
+    config->finalOutput = config->outputFile;
   config->astPaths = args.getAllArgValues(OPT_add_ast_path);
   config->headerPad = args::getHex(args, OPT_headerpad, /*Default=*/32);
   config->headerPadMaxInstallNames =
       args.hasArg(OPT_headerpad_max_install_names);
   config->printDylibSearch =
       args.hasArg(OPT_print_dylib_search) || getenv("RC_TRACE_DYLIB_SEARCHING");
   config->printEachFile = args.hasArg(OPT_t);
   config->printWhyLoad = args.hasArg(OPT_why_load);

 #endif
 
   if (const Arg *arg = args.getLastArg(OPT_install_name)) {
     if (config->outputType != MH_DYLIB)
       warn(arg->getAsString(args) + ": ignored, only has effect with -dylib");
     else
       config->installName = arg->getValue();
   } else if (config->outputType == MH_DYLIB) {
-    config->installName = config->outputFile;
+    config->installName = config->finalOutput;
   }
 
   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;
   }

lld/MachO/InputFiles.cpp

static DylibFile *findDylib(StringRef path, DylibFile *umbrella,
 
   // TODO: Handle -dylib_file
 
   SmallString<128> newPath;
   if (config->outputType == MH_EXECUTE &&
       path.consume_front("@executable_path/")) {
     // ld64 allows overriding this with the undocumented flag -executable_path.
     // lld doesn't currently implement that flag.
+    // FIXME: Consider using finalOutput instead of outputFile.
     path::append(newPath, path::parent_path(config->outputFile), path);
     path = newPath;
   } else if (path.consume_front("@loader_path/")) {
     fs::real_path(umbrella->getName(), newPath);
     path::remove_filename(newPath);
     path::append(newPath, path);
     path = newPath;
   } else if (path.startswith("@rpath/")) {

lld/MachO/Options.td

def e : Separate<["-"], "e">,
     HelpText<"Make <symbol> the entry point of an executable (default is \"start\" from crt1.o)">,
     Group<grp_rare>;
 def w : Flag<["-"], "w">,
     HelpText<"Suppress all warnings">,
     Flags<[HelpHidden]>,
     Group<grp_rare>;
 def final_output : Separate<["-"], "final_output">,
     MetaVarName<"<name>">,
-    HelpText<"Specify the dylib install name if -install_name is not used--used by compiler driver for multiple -arch arguments">,
-    Flags<[HelpHidden]>,
+    HelpText<"Specify dylib install name if -install_name is not used; used by compiler driver for multiple -arch arguments">,
     Group<grp_rare>;
 def arch_multiple : Flag<["-"], "arch_multiple">,
     HelpText<"Augment error and warning messages with the architecture name">,
     Flags<[HelpHidden]>,
     Group<grp_rare>;
 def dot : Separate<["-"], "dot">,
     MetaVarName<"<path>">,
     HelpText<"Write a graph of symbol dependencies to <path> as a .dot file viewable with GraphViz">,

lld/MachO/SyntheticSections.cpp

 }
 
 static_assert((CodeSignatureSection::blobHeadersSize % 8) == 0, "");
 static_assert((CodeSignatureSection::fixedHeadersSize % 8) == 0, "");
 
 CodeSignatureSection::CodeSignatureSection()
     : LinkEditSection(segment_names::linkEdit, section_names::codeSignature) {
   align = 16; // required by libstuff
+  // FIXME: Consider using finalOutput instead of outputFile.
   fileName = config->outputFile;
   size_t slashIndex = fileName.rfind("/");
   if (slashIndex != std::string::npos)
     fileName = fileName.drop_front(slashIndex + 1);
   allHeadersSize = alignTo<16>(fixedHeadersSize + fileName.size() + 1);
   fileNamePad = allHeadersSize - fixedHeadersSize - fileName.size();
 }
 

lld/test/MachO/final-output.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t.o %s
+
+## -final_output sets the default for -install_name, but an explicit
+## -install_name wins
+# RUN: %lld -dylib -o %t.dylib -final_output /lib/foo.dylib %t.o
+# RUN: llvm-otool -D %t.dylib | FileCheck -DID=/lib/foo.dylib %s
+
+# RUN: %lld -dylib -o %t.dylib -install_name /foo/bar.dylib \
+# RUN:     -final_output /lib/foo.dylib %t.o
+# RUN: llvm-otool -D %t.dylib | FileCheck -DID=/foo/bar.dylib %s
+
+# CHECK: [[ID]]
+
+.globl __Z3foo
+__Z3foo:
+  ret