[lld-macho] Omit __llvm_addrsig metadata from the output (#98913)

This section contains metadata that's only relevant for Identical Code
Folding at link time, we should not include it in the output.

We still treat it like a regular section during input file parsing (e.g.
create a `ConcatInputSection` for it), as we want its relocations to be
parsed. But it should not be passed to `addInputSection`, as that's what
assigns it to an `OutputSection` and adds it to the `inputSections`
vector which specifies the inputs to dead-stripping and relocation
scanning.

This fixes a "__DATA,__llvm_addrsig, offset 0: fixups overlap" error
when using `--icf=safe` alongside `-fixup_chains`. This occurs because
all `__llvm_addrsig` sections are 8 bytes large, and the relocations
which signify functions whose addresses are taken are all at offset 0.

This makes the fix in 5fa24ac ("Category Merger: add support for
addrsig references") obsolete, as we no longer try to resolve symbols
referenced in `__llvm_addrsig` when writing the output file. When we do
iterate its relocations in `markAddrSigSymbols`, we do not try to
resolve their addresses.

lld/MachO/Driver.cpp

@@ -1247,6 +1247,9 @@ static void gatherInputSections() {
       // contrast, EH frames are handled like regular ConcatInputSections.)
       if (section->name == section_names::compactUnwind)
         continue;
+      // Addrsig sections contain metadata only needed at link time.
+      if (section->name == section_names::addrSig)
+        continue;
       for (const Subsection &subsection : section->subsections)
         addInputSection(subsection.isec);
     }

lld/MachO/ObjC.cpp

@@ -449,7 +449,6 @@ class ObjcCategoryMerger {
   mergeCategoriesIntoSingleCategory(std::vector<InfoInputCategory> &categories);
 
   void eraseISec(ConcatInputSection *isec);
-  void removeRefsToErasedIsecs();
   void eraseMergedCategories();
 
   void generateCatListForNonErasedCategories(
@@ -519,8 +518,6 @@ class ObjcCategoryMerger {
   std::vector<ConcatInputSection *> &allInputSections;
   // Map of base class Symbol to list of InfoInputCategory's for it
   MapVector<const Symbol *, std::vector<InfoInputCategory>> categoryMap;
-  // Set for tracking InputSection erased via eraseISec
-  DenseSet<InputSection *> erasedIsecs;
 
   // Normally, the binary data comes from the input files, but since we're
   // generating binary data ourselves, we use the below array to store it in.
@@ -1272,8 +1269,6 @@ void ObjcCategoryMerger::generateCatListForNonErasedCategories(
 }
 
 void ObjcCategoryMerger::eraseISec(ConcatInputSection *isec) {
-  erasedIsecs.insert(isec);
-
   isec->live = false;
   for (auto &sym : isec->symbols)
     sym->used = false;
@@ -1326,33 +1321,6 @@ void ObjcCategoryMerger::eraseMergedCategories() {
                                   catLayout.instancePropsOffset);
     }
   }
-
-  removeRefsToErasedIsecs();
-}
-
-// The compiler may generate references to categories inside the addrsig
-// section. This function will erase these references.
-void ObjcCategoryMerger::removeRefsToErasedIsecs() {
-  for (InputSection *isec : inputSections) {
-    if (isec->getName() != section_names::addrSig)
-      continue;
-
-    auto removeRelocs = [this](Reloc &r) {
-      auto *isec = dyn_cast_or_null<ConcatInputSection>(
-          r.referent.dyn_cast<InputSection *>());
-      if (!isec) {
-        Defined *sym =
-            dyn_cast_or_null<Defined>(r.referent.dyn_cast<Symbol *>());
-        if (sym)
-          isec = dyn_cast<ConcatInputSection>(sym->isec());
-      }
-      if (!isec)
-        return false;
-      return erasedIsecs.count(isec) > 0;
-    };
-
-    llvm::erase_if(isec->relocs, removeRelocs);
-  }
 }
 
 void ObjcCategoryMerger::doMerge() {

lld/test/MachO/dead-strip.s

@@ -329,6 +329,17 @@
 # LIT-NEXT: Contents of (__TEXT,__literals) section
 # LIT-NEXT: ef be ad de {{$}}
 
+## Ensure that addrsig metadata does not keep unreferenced functions alive.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
+# RUN:     %t/addrsig.s -o %t/addrsig.o
+# RUN: %lld -lSystem -dead_strip --icf=safe %t/addrsig.o -o %t/addrsig
+# RUN: llvm-objdump --syms %t/addrsig | \
+# RUN:     FileCheck --check-prefix=ADDSIG --implicit-check-not _addrsig %s
+# ADDSIG-LABEL: SYMBOL TABLE:
+# ADDSIG-NEXT:   g F __TEXT,__text _main
+# ADDSIG-NEXT:   g F __TEXT,__text __mh_execute_header
+# ADDSIG-NEXT:   *UND* dyld_stub_binder
+
 ## Duplicate symbols that will be dead stripped later should not fail when using
 ## the --dead-stripped-duplicates flag
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
@@ -988,3 +999,16 @@ _more_data:
 _main:
   callq _ref_undef_fun
 .subsections_via_symbols
+
+#--- addrsig.s
+.globl _main, _addrsig
+_main:
+  retq
+
+_addrsig:
+  retq
+
+.subsections_via_symbols
+
+.addrsig
+.addrsig_sym _addrsig

lld/test/MachO/icf-safe.ll

@@ -5,14 +5,19 @@
 ; RUN: llc -filetype=obj %s -O3 -o %t/icf-obj.o -enable-machine-outliner=never -mtriple arm64-apple-macos -addrsig
 ; RUN: %lld -arch arm64 -lSystem --icf=safe -dylib -o %t/icf-safe.dylib %t/icf-obj.o
 ; RUN: %lld -arch arm64 -lSystem --icf=all  -dylib -o %t/icf-all.dylib  %t/icf-obj.o
-; RUN: llvm-objdump %t/icf-safe.dylib -d --macho | FileCheck %s --check-prefix=ICFSAFE
-; RUN: llvm-objdump %t/icf-all.dylib  -d --macho | FileCheck %s --check-prefix=ICFALL
+; RUN: llvm-objdump %t/icf-safe.dylib -d -h --macho | FileCheck %s --check-prefixes=ICFSAFE,CHECK
+; RUN: llvm-objdump %t/icf-all.dylib  -d -h --macho | FileCheck %s --check-prefixes=ICFALL,CHECK
 
 ; RUN: llvm-as %s -o %t/icf-bitcode.o
 ; RUN: %lld -arch arm64 -lSystem --icf=safe -dylib -o %t/icf-safe-bitcode.dylib %t/icf-bitcode.o
 ; RUN: %lld -arch arm64 -lSystem --icf=all  -dylib -o %t/icf-all-bitcode.dylib %t/icf-bitcode.o
-; RUN: llvm-objdump %t/icf-safe-bitcode.dylib -d --macho | FileCheck %s --check-prefix=ICFSAFE
-; RUN: llvm-objdump %t/icf-all-bitcode.dylib  -d --macho | FileCheck %s --check-prefix=ICFALL
+; RUN: llvm-objdump %t/icf-safe-bitcode.dylib -d -h --macho | FileCheck %s --check-prefixes=ICFSAFE,CHECK
+; RUN: llvm-objdump %t/icf-all-bitcode.dylib  -d -h --macho | FileCheck %s --check-prefixes=ICFALL,CHECK
+
+;; Regression test: if we tried writing __llvm_addrsig to the output, -fixup_chains would fail with a "fixups overlap"
+;;                  error, as the relocations (which reference the address-taken functions) are all at offset 0.
+; RUN: %lld -arch arm64 -lSystem --icf=safe -fixup_chains -dylib -o %t/icf-safe-chained.dylib %t/icf-obj.o
+; RUN: llvm-objdump %t/icf-safe-chained.dylib -d -h --macho | FileCheck %s --check-prefixes=ICFSAFE,CHECK
 
 ; ICFSAFE-LABEL:  _callAllFunctions
 ; ICFSAFE:        bl _func02
@@ -24,6 +29,9 @@
 ; ICFALL-NEXT:    bl _func03_takeaddr
 ; ICFALL-NEXT:    bl _func03_takeaddr
 
+; CHECK-LABEL: Sections:
+; CHECK-NOT:   __llvm_addrsig
+
 target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
 target triple = "arm64-apple-macos11.0"