[WebAssembly] Unify concepts of disgarded and non-live input chunks. NFC.

[sbc100]

Summary

It seems redundant to store this information twice. None of the
locations where this bit is checks cares about the distinction.

This simplifies the code and avoids having to reason about two
different concepts of aliveness.

[sbc100]

• update comment

[ruiu]

LGTM

[ncw]

Cool, looks like it should work.

NB. Discarded/Live were distinguished based on the ELF and COFF implementations, so maybe we'll find a reason to bring the distinction back later - or maybe those can be simplified too, just to use the Live bit?

wasm/InputChunks.h

public:
 
   uint32_t getOutputOffset() const { return OutputOffset; }
   ArrayRef<WasmRelocation> getRelocations() const { return Relocations; }
   StringRef getFileName() const { return File->getName(); }
 
   virtual StringRef getComdat() const = 0;
   virtual StringRef getName() const = 0;
 
-  bool Discarded = false;
   std::vector<OutputRelocation> OutRelocations;
   ObjFile *File;
 
-  // The garbage collector sets sections' Live bits.
-  // If GC is disabled, all sections are considered live by default.
+  // Signals that the section is part of the output.  The garbage collector,
+  // and COMDAT handling can set a sections' Live bit.
+  // If GC is disabled, all sections start out as live by default.
   unsigned Live : 1;
 
 protected:
   InputChunk(ObjFile *F, Kind K)
       : File(F), Live(!Config->GcSections), SectionKind(K) {}
   virtual ~InputChunk() = default;
   void calcRelocations();
   virtual ArrayRef<uint8_t> data() const = 0;

wasm/InputFiles.cpp

for (const SymbolRef &Sym : WasmObj->symbols()) {
     Symbol *S;
     switch (WasmSym.Type) {
     case WasmSymbol::SymbolType::FUNCTION_EXPORT: {
       InputFunction *Function = getFunction(WasmSym);
       if (!isExcludedByComdat(Function)) {
         S = createDefined(WasmSym, Symbol::Kind::DefinedFunctionKind, Function);
         break;
       } else {
-        Function->Discarded = true;
+        Function->Live = false;
         LLVM_FALLTHROUGH; // Exclude function, and add the symbol as undefined
       }
     }
     case WasmSymbol::SymbolType::FUNCTION_IMPORT:
       S = createUndefined(WasmSym, Symbol::Kind::UndefinedFunctionKind,
                           getFunctionSig(WasmSym));
       break;
     case WasmSymbol::SymbolType::GLOBAL_EXPORT: {
       InputSegment *Segment = getSegment(WasmSym);
       if (!isExcludedByComdat(Segment)) {
         S = createDefined(WasmSym, Symbol::Kind::DefinedGlobalKind, Segment,
                           getGlobalValue(WasmSym));
         break;
       } else {
-        Segment->Discarded = true;
+        Segment->Live = false;
         LLVM_FALLTHROUGH; // Exclude global, and add the symbol as undefined
       }
     }
     case WasmSymbol::SymbolType::GLOBAL_IMPORT:
       S = createUndefined(WasmSym, Symbol::Kind::UndefinedGlobalKind);
       break;
     }
 

wasm/Writer.cpp

ExportedSymbols.emplace_back(
         WasmExportEntry{WasmSym::CallCtors, WasmSym::CallCtors->getName()});
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     for (Symbol *Sym : File->getSymbols()) {
       if (!Sym->isDefined() || File != Sym->getFile())
         continue;
       if (Sym->isGlobal())
         continue;
-      if (Sym->getChunk()->Discarded)
+      if (!Sym->getChunk()->Live)
         continue;
 
       if ((Sym->isHidden() || Sym->isLocal()) && !ExportHidden)
         continue;
-
-      // We should never be exporting a non-live symbol
-      assert(Sym->getChunk()->Live);
       ExportedSymbols.emplace_back(WasmExportEntry{Sym, BudgeLocalName(Sym)});
     }
   }
 
   for (const Symbol *Sym : DefinedGlobals) {
     // Can't export the SP right now because its mutable, and mutuable globals
     // are yet supported in the official binary format.
     // TODO(sbc): Remove this if/when the "mutable global" proposal is accepted.
for (ObjFile *File : Symtab->ObjectFiles) {
         AddDefinedGlobal(Sym);
       }
     }
   }
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     DEBUG(dbgs() << "Functions: " << File->getName() << "\n");
     for (InputFunction *Func : File->Functions) {
-      if (Func->Discarded || !Func->Live)
+      if (!Func->Live)
         continue;
       DefinedFunctions.emplace_back(Func);
       Func->setOutputIndex(FunctionIndex++);
     }
   }
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     DEBUG(dbgs() << "Handle relocs: " << File->getName() << "\n");
     auto HandleRelocs = [&](InputChunk *Chunk) {
-      if (Chunk->Discarded)
+      if (!Chunk->Live)
         return;
       ArrayRef<WasmSignature> Types = File->getWasmObj()->types();
       for (const WasmRelocation& Reloc : Chunk->getRelocations()) {
         if (Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_I32 ||
             Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_SLEB) {
           Symbol *Sym = File->getFunctionSymbol(Reloc.Index);
           if (Sym->hasTableIndex() || !Sym->hasOutputIndex())
             continue;
static StringRef getOutputDataSegmentName(StringRef Name) {
   }
 
   return Name;
 }
 
 void Writer::createOutputSegments() {
   for (ObjFile *File : Symtab->ObjectFiles) {
     for (InputSegment *Segment : File->Segments) {
-      if (Segment->Discarded || !Segment->Live)
+      if (!Segment->Live)
         continue;
       StringRef Name = getOutputDataSegmentName(Segment->getName());
       OutputSegment *&S = SegmentMap[Name];
       if (S == nullptr) {
         DEBUG(dbgs() << "new segment: " << Name << "\n");
         S = make<OutputSegment>(Name);
         Segments.push_back(S);
       }