[lld-macho] Fix bug in makeSyntheticInputSection when -dead_strip flag is specified (#86878)

Previously, `makeSyntheticInputSection` would create a new
`ConcatInputSection` without setting `live` explicitly for it. Without
`-dead_strip` this would be OK since `live` would default to `true`.
However, with `-dead_strip`, `live` would default to false, and it would
remain set to `false`.
This hasn't resulted in any issues so far since no code paths that
exposed this issue were present.
However a recent change - ObjC relative method lists
(https://github.com/llvm/llvm-project/pull/86231) exposes this issue by
creating relocations to the `SyntheticInputSection`.
When these relocations are attempted to be written, this ends up with a
crash(assert), since the `SyntheticInputSection` they refer to is marked
as dead (`live` = `false`).

With this change, we set the correct behavior - `live` will always be
`true`. We add a test case that before this change would trigger an
assert in the linker.

lld/MachO/ConcatOutputSection.cpp

@@ -323,11 +323,7 @@ void TextOutputSection::finalize() {
       thunkInfo.isec =
           makeSyntheticInputSection(isec->getSegName(), isec->getName());
       thunkInfo.isec->parent = this;
-
-      // This code runs after dead code removal. Need to set the `live` bit
-      // on the thunk isec so that asserts that check that only live sections
-      // get written are happy.
-      thunkInfo.isec->live = true;
+      assert(thunkInfo.isec->live);
 
       StringRef thunkName = saver().save(funcSym->getName() + ".thunk." +
                                          std::to_string(thunkInfo.sequence++));

lld/MachO/InputSection.cpp

@@ -281,6 +281,9 @@ ConcatInputSection *macho::makeSyntheticInputSection(StringRef segName,
   Section &section =
       *make<Section>(/*file=*/nullptr, segName, sectName, flags, /*addr=*/0);
   auto isec = make<ConcatInputSection>(section, data, align);
+  // Since this is an explicitly created 'fake' input section,
+  // it should not be dead stripped.
+  isec->live = true;
   section.subsections.push_back({0, isec});
   return isec;
 }

lld/MachO/InputSection.h

@@ -149,6 +149,7 @@ public:
 };
 
 // Initialize a fake InputSection that does not belong to any InputFile.
+// The created ConcatInputSection will always have 'live=true'
 ConcatInputSection *makeSyntheticInputSection(StringRef segName,
                                               StringRef sectName,
                                               uint32_t flags = 0,

lld/MachO/SymbolTable.cpp

@@ -377,7 +377,7 @@ static void handleSectionBoundarySymbol(const Undefined &sym, StringRef segSect,
     // live. Marking the isec live ensures an OutputSection is created that the
     // start/end symbol can refer to.
     assert(sym.isLive());
-    isec->live = true;
+    assert(isec->live);
 
     // This runs after gatherInputSections(), so need to explicitly set parent
     // and add to inputSections.

lld/MachO/SyntheticSections.cpp

@@ -850,7 +850,7 @@ ConcatInputSection *ObjCSelRefsHelper::makeSelRef(StringRef methname) {
                                 S_LITERAL_POINTERS | S_ATTR_NO_DEAD_STRIP,
                                 ArrayRef<uint8_t>{selrefData, wordSize},
                                 /*align=*/wordSize);
-  objcSelref->live = true;
+  assert(objcSelref->live);
   objcSelref->relocs.push_back({/*type=*/target->unsignedRelocType,
                                 /*pcrel=*/false, /*length=*/3,
                                 /*offset=*/0,

lld/MachO/Writer.cpp

@@ -1375,9 +1375,7 @@ void macho::createSyntheticSections() {
       segment_names::data, section_names::data, S_REGULAR,
       ArrayRef<uint8_t>{arr, target->wordSize},
       /*align=*/target->wordSize);
-  // References from dyld are not visible to us, so ensure this section is
-  // always treated as live.
-  in.imageLoaderCache->live = true;
+  assert(in.imageLoaderCache->live);
 }
 
 OutputSection *macho::firstTLVDataSection = nullptr;

lld/test/MachO/objc-relative-method-lists-simple.s

@@ -8,6 +8,10 @@
 # RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -objc_relative_method_lists
 # RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib  | FileCheck %s --check-prefix=CHK_REL
 
+## Test arm64 + relative method lists + dead-strip
+# RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -objc_relative_method_lists -dead_strip
+# RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib  | FileCheck %s --check-prefix=CHK_REL
+
 ## Test arm64 + traditional method lists (no relative offsets)
 # RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -no_objc_relative_method_lists
 # RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib  | FileCheck %s --check-prefix=CHK_NO_REL