[WebAssembly] Remove duplicated line of code and unreachable check. NFC

[ncw]

Summary

Note - the line of code removed is duplicated at the end of createCtorFunction. It *used* to do something different to the line at the top, but now after some refactoring the line has in fact become useless.

Note 2 - the liveness check came to my eye because there's a clear typo in the line ("return" instead of "continue"). I tried to write a test to exercise the bug, but found that in fact it's not possible, since relocatable can't be used with GC, so it's not possible to have non-Live symbols in the symtab.

[sbc100]

wasm/Writer.cpp

697–699

But this isn't iterating the symbol table, its iterating all symbols in all the objects. Surely some of them can be non-live. I don't quite understand why we not hitting this case in some unit test.

[ncw]

wasm/Writer.cpp

697–699

The entire method Writer::assignSymtab has an early-exit for non-relocatable output. So all symbols are always Live when generating a symtab, since that's the relocatable case (and GC and relocatable are exclusive).

[sbc100]

lgtm, with commets.

wasm/Writer.cpp

697–699

I see. That was non-obvious to me, so perhaps at least comment?

This makes two assumptions:

1. liveness is only ever set by GC
2. GC never applies in relocatable mode.

I guess as long as the assert is here we will know if we ever violate those assumption, but I'm on the fence as to weather to just leave the check in. I'd be happy with a comment here explaining the assumptions I think.

[ncw]

wasm/Writer.cpp

697–699

I'll add a comment then, so we don't have a dead branch. As it is (with "return" rather than "continue") it needs to be changed somehow. Thanks!

wasm/Writer.cpp

if (!Config->Relocatable)
     return;
 
   unsigned SymbolIndex = SymtabEntries.size();
   for (ObjFile *File : Symtab->ObjectFiles) {
     DEBUG(dbgs() << "Symtab entries: " << File->getName() << "\n");
     for (Symbol *Sym : File->getSymbols()) {
       if (Sym->getFile() != File)
         continue;
-      if (!Sym->isLive())
-        return;
+      // (Since this is relocatable output, GC is not performed so symbols must
+      // be live.)
+      assert(Sym->isLive());
       Sym->setOutputSymbolIndex(SymbolIndex++);
       SymtabEntries.emplace_back(Sym);
     }
   }
 
   // For the moment, relocatable output doesn't contain any synthetic functions,
   // so no need to look through the Symtab for symbols not referenced by
   // Symtab->ObjectFiles.

 
 static const int OPCODE_CALL = 0x10;
 static const int OPCODE_END = 0xb;
 
 // Create synthetic "__wasm_call_ctors" function based on ctor functions
 // in input object.
 void Writer::createCtorFunction() {
   uint32_t FunctionIndex = NumImportedFunctions + InputFunctions.size();
-  WasmSym::CallCtors->setOutputIndex(FunctionIndex);
 
   // First write the body's contents to a string.
   std::string BodyContent;
   {
     raw_string_ostream OS(BodyContent);
     writeUleb128(OS, 0, "num locals");
     for (const WasmInitEntry &F : InitFunctions) {
       writeU8(OS, OPCODE_CALL, "CALL");