[PATCH] D104084: [lld-macho] Add options -(no_)keep_dwarf_unwind

[Greg McGary via Phabricator via llvm-commits]

gkm created this revision.
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DWARF unwind info was superseded by compact unwind as-of macOS 10.9 and iOS 7.0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 we have a use case that requires the intelligent approach, we offer the
choice of all or nothing.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D104084

Files:
  lld/MachO/Config.h
  lld/MachO/Driver.cpp
  lld/MachO/InputFiles.cpp
  lld/MachO/Options.td
  lld/test/MachO/eh_frame.s


Index: lld/test/MachO/eh_frame.s
===================================================================
--- /dev/null
+++ lld/test/MachO/eh_frame.s
@@ -0,0 +1,28 @@
+# 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
Index: lld/MachO/Options.td
===================================================================
--- lld/MachO/Options.td
+++ lld/MachO/Options.td
@@ -1245,12 +1245,10 @@
     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">,
Index: lld/MachO/InputFiles.cpp
===================================================================
--- lld/MachO/InputFiles.cpp
+++ lld/MachO/InputFiles.cpp
@@ -265,6 +265,9 @@
   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 + "," +
Index: lld/MachO/Driver.cpp
===================================================================
--- lld/MachO/Driver.cpp
+++ lld/MachO/Driver.cpp
@@ -1185,6 +1185,9 @@
       (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:") +
Index: lld/MachO/Config.h
===================================================================
--- lld/MachO/Config.h
+++ lld/MachO/Config.h
@@ -128,6 +128,7 @@
   bool deadStripDylibs = false;
   bool demangle = false;
   bool deadStrip = false;
+  bool keepDwarfUnwind = false;
   PlatformInfo platformInfo;
   NamespaceKind namespaceKind = NamespaceKind::twolevel;
   UndefinedSymbolTreatment undefinedSymbolTreatment =


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