[lld] [lld-macho][NFC] Refactor ObjCSelRefsSection => ObjCSelRefsHelper (PR #86456)

[via llvm-commits]

https://github.com/alx32 created https://github.com/llvm/llvm-project/pull/86456

In a previous PR: https://github.com/llvm/llvm-project/pull/83878, the intent was to make no functional changes, just refactor out the code for reuse. 
However, by creating `ObjCSelRefsSection` as a `SyntheticSection` - this slightly changed the functionality of the application as the `SyntheticSection` constructor registers the `SyntheticSection` as a functional one - with an associated `SyntheticInputSection`. 

With this change we remove this unintended consequence by making the code not use a `SyntheticSection` as base, but just by having it be a static helper. 

>From 8df12dbaadfe81e36cfab02f45f196fa55ea0376 Mon Sep 17 00:00:00 2001
From: Alex B <alexborcan at meta.com>
Date: Sun, 24 Mar 2024 13:58:32 -0700
Subject: [PATCH] [lld-macho][NFC] Refactor ObjCSelRefsSection =>
 ObjCSelRefsHelper

---
 lld/MachO/Driver.cpp            |  1 +
 lld/MachO/SyntheticSections.cpp | 19 ++++++++++---------
 lld/MachO/SyntheticSections.h   | 21 ++++++---------------
 lld/MachO/Writer.cpp            |  3 +--
 4 files changed, 18 insertions(+), 26 deletions(-)

diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 919a14b8bcf08b..14c111ce9685c9 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1394,6 +1394,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     syntheticSections.clear();
     thunkMap.clear();
     unprocessedLCLinkerOptions.clear();
+    ObjCSelRefsHelper::cleanup();
 
     firstTLVDataSection = nullptr;
     tar = nullptr;
diff --git a/lld/MachO/SyntheticSections.cpp b/lld/MachO/SyntheticSections.cpp
index 1b3694528de1dd..0afbbd478bb9fd 100644
--- a/lld/MachO/SyntheticSections.cpp
+++ b/lld/MachO/SyntheticSections.cpp
@@ -806,10 +806,9 @@ void StubHelperSection::setUp() {
   dyldPrivate->used = true;
 }
 
-ObjCSelRefsSection::ObjCSelRefsSection()
-    : SyntheticSection(segment_names::data, section_names::objcSelrefs) {}
-
-void ObjCSelRefsSection::initialize() {
+llvm::DenseMap<llvm::CachedHashStringRef, ConcatInputSection *>
+    ObjCSelRefsHelper::methnameToSelref;
+void ObjCSelRefsHelper::initialize() {
   // Do not fold selrefs without ICF.
   if (config->icfLevel == ICFLevel::none)
     return;
@@ -836,7 +835,9 @@ void ObjCSelRefsSection::initialize() {
   }
 }
 
-ConcatInputSection *ObjCSelRefsSection::makeSelRef(StringRef methname) {
+void ObjCSelRefsHelper::cleanup() { methnameToSelref.clear(); }
+
+ConcatInputSection *ObjCSelRefsHelper::makeSelRef(StringRef methname) {
   auto methnameOffset =
       in.objcMethnameSection->getStringOffset(methname).outSecOff;
 
@@ -861,7 +862,7 @@ ConcatInputSection *ObjCSelRefsSection::makeSelRef(StringRef methname) {
   return objcSelref;
 }
 
-ConcatInputSection *ObjCSelRefsSection::getSelRef(StringRef methname) {
+ConcatInputSection *ObjCSelRefsHelper::getSelRef(StringRef methname) {
   auto it = methnameToSelref.find(CachedHashStringRef(methname));
   if (it == methnameToSelref.end())
     return nullptr;
@@ -890,8 +891,8 @@ StringRef ObjCStubsSection::getMethname(Symbol *sym) {
 void ObjCStubsSection::addEntry(Symbol *sym) {
   StringRef methname = getMethname(sym);
   // We create a selref entry for each unique methname.
-  if (!in.objcSelRefs->getSelRef(methname))
-    in.objcSelRefs->makeSelRef(methname);
+  if (!ObjCSelRefsHelper::getSelRef(methname))
+    ObjCSelRefsHelper::makeSelRef(methname);
 
   auto stubSize = config->objcStubsMode == ObjCStubsMode::fast
                       ? target->objcStubsFastSize
@@ -940,7 +941,7 @@ void ObjCStubsSection::writeTo(uint8_t *buf) const {
     Defined *sym = symbols[i];
 
     auto methname = getMethname(sym);
-    InputSection *selRef = in.objcSelRefs->getSelRef(methname);
+    InputSection *selRef = ObjCSelRefsHelper::getSelRef(methname);
     assert(selRef != nullptr && "no selref for methname");
     auto selrefAddr = selRef->getVA(0);
     target->writeObjCMsgSendStub(buf + stubOffset, sym, in.objcStubs->addr,
diff --git a/lld/MachO/SyntheticSections.h b/lld/MachO/SyntheticSections.h
index 6d85f0aea8e04c..4586a4a0bf4361 100644
--- a/lld/MachO/SyntheticSections.h
+++ b/lld/MachO/SyntheticSections.h
@@ -315,24 +315,16 @@ class StubHelperSection final : public SyntheticSection {
   Defined *dyldPrivate = nullptr;
 };
 
-class ObjCSelRefsSection final : public SyntheticSection {
+class ObjCSelRefsHelper {
 public:
-  ObjCSelRefsSection();
-  void initialize();
-
-  // This SyntheticSection does not do directly write data to the output, it is
-  // just a placeholder for easily creating SyntheticInputSection's which will
-  // be inserted into inputSections and handeled by the default writing
-  // mechanism.
-  uint64_t getSize() const override { return 0; }
-  bool isNeeded() const override { return false; }
-  void writeTo(uint8_t *buf) const override {}
+  static void initialize();
+  static void cleanup();
 
-  ConcatInputSection *getSelRef(StringRef methname);
-  ConcatInputSection *makeSelRef(StringRef methname);
+  static ConcatInputSection *getSelRef(StringRef methname);
+  static ConcatInputSection *makeSelRef(StringRef methname);
 
 private:
-  llvm::DenseMap<llvm::CachedHashStringRef, ConcatInputSection *>
+  static llvm::DenseMap<llvm::CachedHashStringRef, ConcatInputSection *>
       methnameToSelref;
 };
 
@@ -813,7 +805,6 @@ struct InStruct {
   LazyPointerSection *lazyPointers = nullptr;
   StubsSection *stubs = nullptr;
   StubHelperSection *stubHelper = nullptr;
-  ObjCSelRefsSection *objcSelRefs = nullptr;
   ObjCStubsSection *objcStubs = nullptr;
   UnwindInfoSection *unwindInfo = nullptr;
   ObjCImageInfoSection *objCImageInfo = nullptr;
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
index 8f335188e12c7b..a18b5268fd42aa 100644
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -720,7 +720,7 @@ static void addNonWeakDefinition(const Defined *defined) {
 
 void Writer::scanSymbols() {
   TimeTraceScope timeScope("Scan symbols");
-  in.objcSelRefs->initialize();
+  ObjCSelRefsHelper::initialize();
   for (Symbol *sym : symtab->getSymbols()) {
     if (auto *defined = dyn_cast<Defined>(sym)) {
       if (!defined->isLive())
@@ -1359,7 +1359,6 @@ void macho::createSyntheticSections() {
   in.got = make<GotSection>();
   in.tlvPointers = make<TlvPointerSection>();
   in.stubs = make<StubsSection>();
-  in.objcSelRefs = make<ObjCSelRefsSection>();
   in.objcStubs = make<ObjCStubsSection>();
   in.unwindInfo = makeUnwindInfoSection();
   in.objCImageInfo = make<ObjCImageInfoSection>();