[LLD][COFF][NFC] Store live flag in ImportThunkChunk. (#108459)

Instead of ImportFile. This is a preparation for ARM64EC support, which
has both x86 and ARM64EC thunks and each of them needs a separate flag.

Author: cjacek

lld/COFF/Chunks.cpp

@@ -774,6 +774,10 @@ void StringChunk::writeTo(uint8_t *buf) const {
   buf[str.size()] = '\0';
 }
 
+ImportThunkChunk::ImportThunkChunk(COFFLinkerContext &ctx, Defined *s)
+    : NonSectionCodeChunk(ImportThunkKind), live(!ctx.config.doGC),
+      impSymbol(s), ctx(ctx) {}
+
 ImportThunkChunkX64::ImportThunkChunkX64(COFFLinkerContext &ctx, Defined *s)
     : ImportThunkChunk(ctx, s) {
   // Intel Optimization Manual says that all branch targets

lld/COFF/Chunks.h

@@ -557,10 +557,13 @@ static const uint8_t importThunkARM64EC[] = {
 // contents will be a JMP instruction to some __imp_ symbol.
 class ImportThunkChunk : public NonSectionCodeChunk {
 public:
-  ImportThunkChunk(COFFLinkerContext &ctx, Defined *s)
-      : NonSectionCodeChunk(ImportThunkKind), impSymbol(s), ctx(ctx) {}
+  ImportThunkChunk(COFFLinkerContext &ctx, Defined *s);
   static bool classof(const Chunk *c) { return c->kind() == ImportThunkKind; }
 
+  // We track the usage of the thunk symbol separately from the import file
+  // to avoid generating unnecessary thunks.
+  bool live;
+
 protected:
   Defined *impSymbol;
   COFFLinkerContext &ctx;

lld/COFF/InputFiles.cpp

@@ -1002,7 +1002,7 @@ void ObjFile::enqueuePdbFile(StringRef path, ObjFile *fromFile) {
 }
 
 ImportFile::ImportFile(COFFLinkerContext &ctx, MemoryBufferRef m)
-    : InputFile(ctx, ImportKind, m), live(!ctx.config.doGC), thunkLive(live) {}
+    : InputFile(ctx, ImportKind, m), live(!ctx.config.doGC) {}
 
 MachineTypes ImportFile::getMachineType() const {
   uint16_t machine =

lld/COFF/InputFiles.h

@@ -371,11 +371,8 @@ class ImportFile : public InputFile {
   // are actually in use.
   //
   // If the Live bit is turned off by MarkLive, Writer will ignore dllimported
-  // symbols provided by this import library member. We also track whether the
-  // imported symbol is used separately from whether the thunk is used in order
-  // to avoid creating unnecessary thunks.
+  // symbols provided by this import library member.
   bool live;
-  bool thunkLive;
 };
 
 // Used for LTO.

lld/COFF/MapFile.cpp

@@ -125,7 +125,7 @@ static void getSymbols(const COFFLinkerContext &ctx,
     if (!file->thunkSym)
       continue;
 
-    if (!file->thunkLive)
+    if (!file->thunkSym->isLive())
       continue;
 
     if (auto *thunkSym = dyn_cast<Defined>(file->thunkSym))

lld/COFF/MarkLive.cpp

@@ -58,7 +58,7 @@ void markLive(COFFLinkerContext &ctx) {
       addImportFile(sym->file);
     } else if (auto *sym = dyn_cast<DefinedImportThunk>(b)) {
       addImportFile(sym->wrappedSym->file);
-      sym->wrappedSym->file->thunkLive = true;
+      sym->getChunk()->live = true;
     }
   };
 

lld/COFF/PDB.cpp

@@ -1527,8 +1527,8 @@ void PDBLinker::addImportFilesToPDB() {
     if (!file->thunkSym)
       continue;
 
-    if (!file->thunkLive)
-        continue;
+    if (!file->thunkSym->isLive())
+      continue;
 
     std::string dll = StringRef(file->dllName).lower();
     llvm::pdb::DbiModuleDescriptorBuilder *&mod = dllToModuleDbi[dll];

lld/COFF/Symbols.cpp

@@ -84,7 +84,7 @@ bool Symbol::isLive() const {
   if (auto *imp = dyn_cast<DefinedImportData>(this))
     return imp->file->live;
   if (auto *imp = dyn_cast<DefinedImportThunk>(this))
-    return imp->wrappedSym->file->thunkLive;
+    return imp->getChunk()->live;
   // Assume any other kind of symbol is live.
   return true;
 }

lld/COFF/Symbols.h

@@ -395,12 +395,12 @@ class DefinedImportThunk : public Defined {
   }
 
   uint64_t getRVA() { return data->getRVA(); }
-  Chunk *getChunk() { return data; }
+  ImportThunkChunk *getChunk() const { return data; }
 
   DefinedImportData *wrappedSym;
 
 private:
-  Chunk *data;
+  ImportThunkChunk *data;
 };
 
 // If you have a symbol "foo" in your object file, a symbol name

lld/COFF/Writer.cpp

@@ -1258,7 +1258,7 @@ void Writer::appendImportThunks() {
     if (!isa<DefinedImportThunk>(file->thunkSym))
       fatal(toString(ctx, *file->thunkSym) + " was replaced");
     DefinedImportThunk *thunk = cast<DefinedImportThunk>(file->thunkSym);
-    if (file->thunkLive)
+    if (thunk->getChunk()->live)
       textSec->addChunk(thunk->getChunk());
     if (file->impchkThunk)
       textSec->addChunk(file->impchkThunk);