Reland "[mac/lld] Implement -why_load".

The problem was that `sym` became replaced in the call
to make<ObjFile> and referring to it afer that read memory that now
stored a different kind of symbol (a Defined instead of a LazySymbol).
Since this happens only once per archive, just copy the symbol to the
stack before make<ObjFile> and read the copy instead.

Originally reviewed at https://reviews.llvm.org/D92496

lld/MachO/Config.h

@@ -38,6 +38,7 @@ struct Configuration {
   bool staticLink = false;
   bool isPic = false;
   bool headerPadMaxInstallNames = false;
+  bool printWhyLoad = false;
   bool searchDylibsFirst = false;
   bool saveTemps = false;
   uint32_t headerPad;

lld/MachO/Driver.cpp

@@ -275,6 +275,8 @@ static InputFile *addFile(StringRef path, bool forceLoadArchive) {
         for (const ArchiveMember &member : getArchiveMembers(*buffer)) {
           inputFiles.push_back(
               make<ObjFile>(member.mbref, member.modTime, path));
+          printWhyLoad((forceLoadArchive ? "-force_load" : "-all_load"),
+                       inputFiles.back());
         }
       }
     } else if (config->forceLoadObjC) {
@@ -291,6 +293,7 @@ static InputFile *addFile(StringRef path, bool forceLoadArchive) {
           if (hasObjCSection(member.mbref)) {
             inputFiles.push_back(
                 make<ObjFile>(member.mbref, member.modTime, path));
+            printWhyLoad("-ObjC", inputFiles.back());
           }
         }
       }
@@ -637,6 +640,7 @@ bool macho::link(llvm::ArrayRef<const char *> argsArr, bool canExitEarly,
   config->headerPad = args::getHex(args, OPT_headerpad, /*Default=*/32);
   config->headerPadMaxInstallNames =
       args.hasArg(OPT_headerpad_max_install_names);
+  config->printWhyLoad = args.hasArg(OPT_why_load);
   config->outputType = getOutputType(args);
   config->runtimePaths = args::getStrings(args, OPT_rpath);
   config->allLoad = args.hasArg(OPT_all_load);

lld/MachO/Driver.h

@@ -19,6 +19,7 @@ namespace lld {
 namespace macho {
 
 class DylibFile;
+class InputFile;
 
 class MachOOptTable : public llvm::opt::OptTable {
 public:
@@ -45,6 +46,8 @@ llvm::Optional<DylibFile *> makeDylibFromTAPI(llvm::MemoryBufferRef mbref,
 
 uint32_t getModTime(llvm::StringRef path);
 
+void printWhyLoad(StringRef reason, const InputFile *);
+
 } // namespace macho
 } // namespace lld
 

lld/MachO/DriverUtils.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "Driver.h"
+#include "Config.h"
 #include "InputFiles.h"
 
 #include "lld/Common/Args.h"
@@ -185,3 +186,10 @@ uint32_t macho::getModTime(StringRef path) {
   warn("failed to get modification time of " + path);
   return 0;
 }
+
+void macho::printWhyLoad(StringRef reason, const InputFile *f) {
+  if (!config->printWhyLoad)
+    return;
+  lld::outs() << reason << " forced load of " << toString(f)
+              << '\n';
+}

lld/MachO/InputFiles.cpp

@@ -619,8 +619,17 @@ void ArchiveFile::fetch(const object::Archive::Symbol &sym) {
                                      "for the member defining symbol " +
                                      toMachOString(sym)));
 
+  // `sym` is owned by a LazySym, which will be replace<>() by make<ObjFile>
+  // and become invalid after that call. Copy it to the stack so we can refer
+  // to it later.
+  const object::Archive::Symbol sym_copy = sym;
+
   auto file = make<ObjFile>(mb, modTime, getName());
 
+  // ld64 doesn't demangle sym here even with -demangle. Match that, so
+  // intentionally no call to toMachOString() here.
+  printWhyLoad(sym_copy.getName(), file);
+
   symbols.insert(symbols.end(), file->symbols.begin(), file->symbols.end());
   subsections.insert(subsections.end(), file->subsections.begin(),
                      file->subsections.end());

lld/MachO/Options.td

@@ -426,8 +426,7 @@ def commons : Separate<["-"], "commons">,
 def grp_introspect : OptionGroup<"introspect">, HelpText<"INTROSPECTING THE LINKER">;
 
 def why_load : Flag<["-"], "why_load">,
-     HelpText<"Log the symbol that compels loading of each object file from a static library">,
-     Flags<[HelpHidden]>,
+     HelpText<"Log why each object file is loaded from a static library">,
      Group<grp_introspect>;
 def whyload : Flag<["-"], "whyload">,
      Alias<why_load>,

lld/test/MachO/why-load.s

@@ -0,0 +1,83 @@
+# REQUIRES: x86
+# RUN: rm -rf %t
+# RUN: split-file %s %t
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/objc.o %t/objc.s
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/foo.o %t/foo.s
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/bar.o %t/bar.s
+# RUN: llvm-ar csr  %t/lib.a %t/objc.o %t/foo.o %t/bar.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/main.o %t/main.s
+
+# The first line checks that we never demangle symbols in -why_load output.
+# RUN: %lld %t/main.o %t/lib.a -o /dev/null -why_load -demangle | \
+# RUN:     FileCheck %s --check-prefix=WHY
+# RUN: %lld %t/main.o -force_load %t/lib.a -o /dev/null -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYFORCE
+# RUN: %lld %t/main.o %t/lib.a -o /dev/null -all_load -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYALL
+# RUN: %lld %t/main.o -force_load %t/lib.a -o /dev/null -all_load -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYALLFORCE
+
+# RUN: %lld %t/main.o %t/lib.a -o /dev/null -ObjC -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYOBJC
+# RUN: %lld %t/main.o -force_load %t/lib.a -o /dev/null -ObjC -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYOBJCFORCE
+# RUN: %lld %t/main.o %t/lib.a -o /dev/null -ObjC -all_load -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYOBJCALL
+# RUN: %lld %t/main.o -force_load %t/lib.a -o /dev/null -ObjC -all_load -why_load | \
+# RUN:     FileCheck %s --check-prefix=WHYOBJCALLFORCE
+
+# WHY-DAG: _bar forced load of lib.a(bar.o)
+# WHY-DAG: __Z3foo forced load of lib.a(foo.o)
+# WHY-NOT: {{.*}} forced load of lib.a(objc.o)
+
+# WHYFORCE-DAG: -force_load forced load of lib.a(bar.o)
+# WHYFORCE-DAG: -force_load forced load of lib.a(foo.o)
+# WHYFORCE-DAG: -force_load forced load of lib.a(objc.o)
+
+# WHYALL-DAG: -all_load forced load of lib.a(bar.o)
+# WHYALL-DAG: -all_load forced load of lib.a(foo.o)
+# WHYALL-DAG: -all_load forced load of lib.a(objc.o)
+
+# WHYALLFORCE-DAG: -force_load forced load of lib.a(bar.o)
+# WHYALLFORCE-DAG: -force_load forced load of lib.a(foo.o)
+# WHYALLFORCE-DAG: -force_load forced load of lib.a(objc.o)
+
+# WHYOBJC-DAG: _bar forced load of lib.a(bar.o)
+# WHYOBJC-DAG: __Z3foo forced load of lib.a(foo.o)
+# WHYOBJC-DAG: -ObjC forced load of lib.a(objc.o)
+
+# WHYOBJCFORCE-DAG: -force_load forced load of lib.a(bar.o)
+# WHYOBJCFORCE-DAG: -force_load forced load of lib.a(foo.o)
+# WHYOBJCFORCE-DAG: -force_load forced load of lib.a(objc.o)
+
+# WHYOBJCALL-DAG: -all_load forced load of lib.a(bar.o)
+# WHYOBJCALL-DAG: -all_load forced load of lib.a(foo.o)
+# WHYOBJCALL-DAG: -all_load forced load of lib.a(objc.o)
+
+# WHYOBJCALLFORCE-DAG: -force_load forced load of lib.a(bar.o)
+# WHYOBJCALLFORCE-DAG: -force_load forced load of lib.a(foo.o)
+# WHYOBJCALLFORCE-DAG: -force_load forced load of lib.a(objc.o)
+
+#--- objc.s
+.section	__DATA,__objc_catlist
+.quad 0x1234
+
+#--- foo.s
+.globl __Z3foo
+__Z3foo:
+  ret
+
+#--- bar.s
+.globl _bar
+_bar:
+  callq __Z3foo
+  ret
+
+#--- main.s
+.globl _main
+_main:
+  callq _bar
+  callq __Z3foo
+  ret