[lld-macho] Add options -(no_)keep_dwarf_unwind

[gkm]

Summary

DWARF unwind info was superseded by compact unwind as-of macOS 10.9 and iOS 7.0
(circa 2013). Most often, DWARF unwind info is entirely redundant and can be
discarded. In rare instances, complex unwind info cannot be represented by a
compact-unwind entry, and we fall-back to the DWARF frame info.

The intelligent approach is to filter-out all redundant entries in the
__TEXT,__eh_frame section, leaving only those that are necessary fallbacks
when compact unwind is insufficient. However, that's not easy, so unless and
until (maybe later today!) we have a use case that requires the intelligent
approach, we offer the choice of all or nothing.

[thakis]

The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

[gkm]

In D104084#2813364, @thakis wrote:

The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

Yes, I will do it, but wanted this out quickly. (I changed the description to read "... unless and until (maybe later today!) we have a use case ..." :)

When I looked at the CU entries for Chromium Framework, I saw none that had type bits [24..27] == 4, which means UNWIND_X86_64_MODE_DWARF. What am I not seeing?

[int3]

Yes, I will do it, but wanted this out quickly.

No great rush to commit this though :)

[thakis]

In D104084#2813716, @gkm wrote:

In D104084#2813364, @thakis wrote:

The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

Yes, I will do it, but wanted this out quickly. (I changed the description to read "... unless and until (maybe later today!) we have a use case ..." :)

When I looked at the CU entries for Chromium Framework, I saw none that had type bits [24..27] == 4, which means UNWIND_X86_64_MODE_DWARF. What am I not seeing?

I didn't look in detail, I just noticed that ld64 keeps some of the section. So I thought the bits that remain are probably needed? Not sure though.

[int3]

clearing my queue

lld/MachO/Config.h

struct Configuration {
   llvm::StringRef installName;
   llvm::StringRef mapFile;
   llvm::StringRef outputFile;
   llvm::StringRef ltoObjPath;
   llvm::StringRef thinLTOJobs;
   bool deadStripDylibs = false;
   bool demangle = false;
   bool deadStrip = false;
+  bool keepDwarfUnwind = false;
   PlatformInfo platformInfo;
   NamespaceKind namespaceKind = NamespaceKind::twolevel;
   UndefinedSymbolTreatment undefinedSymbolTreatment =
       UndefinedSymbolTreatment::error;
   ICFLevel icfLevel = ICFLevel::none;
   llvm::MachO::HeaderFileType outputType;
   std::vector<llvm::StringRef> systemLibraryRoots;
   std::vector<llvm::StringRef> librarySearchPaths;

lld/MachO/Driver.cpp

#endif
 
   config->saveTemps = args.hasArg(OPT_save_temps);
 
   config->adhocCodesign = args.hasFlag(
       OPT_adhoc_codesign, OPT_no_adhoc_codesign,
       (config->arch() == AK_arm64 || config->arch() == AK_arm64e) &&
           config->platform() == PlatformKind::macOS);
 
+  config->keepDwarfUnwind =
+      args.hasFlag(OPT_keep_dwarf_unwind, OPT_no_keep_dwarf_unwind, false);
+
   if (args.hasArg(OPT_v)) {
     message(getLLDVersion());
     message(StringRef("Library search paths:") +
             (config->librarySearchPaths.empty()
                  ? ""
                  : "\n\t" + join(config->librarySearchPaths, "\n\t")));
     message(StringRef("Framework search paths:") +
             (config->frameworkSearchPaths.empty()

lld/MachO/InputFiles.cpp

 }
 
 template <class Section>
 void ObjFile::parseSections(ArrayRef<Section> sections) {
   subsections.reserve(sections.size());
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
 
   for (const Section &sec : sections) {
+    if (!config->keepDwarfUnwind &&
+        strcmp(sec.sectname, section_names::ehFrame) == 0)
+      continue;
     if (config->dedupLiterals && sectionType(sec.flags) == S_CSTRING_LITERALS) {
       if (sec.nreloc)
         fatal(toString(this) + " contains relocations in " + sec.segname + "," +
               sec.sectname +
               ", so LLD cannot deduplicate literals. Try re-running without "
               "--deduplicate-literals.");
 
       auto *isec = make<CStringInputSection>();

lld/MachO/Options.td

def ignore_optimization_hints : Flag<["-"], "ignore_optimization_hints">,
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,
     Group<grp_undocumented>;
 def init_offsets : Flag<["-"], "init_offsets">,
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,
     Group<grp_undocumented>;
 def keep_dwarf_unwind : Flag<["-"], "keep_dwarf_unwind">,
-    HelpText<"This option is undocumented in ld64">,
-    Flags<[HelpHidden]>,
+    HelpText<"Retain the __TEXT,__eh_frame section">,
     Group<grp_undocumented>;
 def no_keep_dwarf_unwind : Flag<["-"], "no_keep_dwarf_unwind">,
-    HelpText<"This option is undocumented in ld64">,
-    Flags<[HelpHidden]>,
+    HelpText<"Omit the __TEXT,__eh_frame section (default)">,
     Group<grp_undocumented>;
 def kext : Flag<["-"], "kext">,
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,
     Group<grp_undocumented>;
 def kext_objects_dir : Flag<["-"], "kext_objects_dir">,
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,

lld/test/MachO/eh_frame.s

+# REQUIRES: x86
+# RUN: rm -rf %t; mkdir -p %t
+
+## Verify that __TEXT,__eh_frame is either retained or omitted from the
+## linker output, according to command-line options -(no_)keep_dwarf_unwind
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/main.o
+# RUN: %lld -o %t/keep %t/main.o -lSystem -keep_dwarf_unwind
+# RUN: %lld -o %t/toss %t/main.o -lSystem -no_keep_dwarf_unwind
+# RUN: %lld -o %t/default %t/main.o -lSystem
+
+# RUN: llvm-objdump -h %t/keep | FileCheck %s --check-prefixes=KEEP
+# RUN: llvm-objdump -h %t/toss | FileCheck %s --check-prefixes=TOSS
+# RUN: llvm-objdump -h %t/default | FileCheck %s --check-prefixes=TOSS
+
+# KEEP-LABEL: {{^}}Sections:
+# KEEP: [[#]] __eh_frame {{.*}} DATA
+
+# TOSS-LABEL: {{^}}Sections:
+# TOSS-NOT: [[#]] __eh_frame {{.*}} DATA
+
+.text
+.global _main
+_main:
+  ret
+
+.section __TEXT,__eh_frame
+.fill 8, 8