[WebAssembly] Ensure BasicSymbolRef.getRawDataRefImpl().p is non-null

[sbc100]

Summary

Store a non-zero value to ref.d.a and use ref.d.b to store the symbol
index. This means that ref.p is never null, which was confusing
llvm-nm.

Fixes PR40497

[sbc100]

• fix comment

[sbc100]

• update tests

[tlively]

I have no idea what Ref.d.a or Ref.d.b are, but that is just as true before this CL.

lib/Object/WasmObjectFile.cpp

 const uint8_t *WasmObjectFile::getPtr(size_t Offset) const {
   return reinterpret_cast<const uint8_t *>(getData().data() + Offset);
 }
 
 const wasm::WasmObjectHeader &WasmObjectFile::getHeader() const {
   return Header;
 }
 
-void WasmObjectFile::moveSymbolNext(DataRefImpl &Symb) const { Symb.d.a++; }
+void WasmObjectFile::moveSymbolNext(DataRefImpl &Symb) const { Symb.d.b++; }
 
 uint32_t WasmObjectFile::getSymbolFlags(DataRefImpl Symb) const {
   uint32_t Result = SymbolRef::SF_None;
   const WasmSymbol &Sym = getWasmSymbol(Symb);
 
   LLVM_DEBUG(dbgs() << "getSymbolFlags: ptr=" << &Sym << " " << Sym << "\n");
   if (Sym.isBindingWeak())
     Result |= SymbolRef::SF_Weak;
   if (!Sym.isBindingLocal())
     Result |= SymbolRef::SF_Global;
   if (Sym.isHidden())
     Result |= SymbolRef::SF_Hidden;
   if (!Sym.isDefined())
     Result |= SymbolRef::SF_Undefined;
   if (Sym.isTypeFunction())
     Result |= SymbolRef::SF_Executable;
   return Result;
 }
 
 basic_symbol_iterator WasmObjectFile::symbol_begin() const {
   DataRefImpl Ref;
-  Ref.d.a = 0;
+  Ref.d.a = 1; // Arbitrary non-zero value so that Ref.p is non-null
+  Ref.d.b = 0; // Symbol index
   return BasicSymbolRef(Ref, this);
 }
 
 basic_symbol_iterator WasmObjectFile::symbol_end() const {
   DataRefImpl Ref;
-  Ref.d.a = Symbols.size();
+  Ref.d.a = 1; // Arbitrary non-zero value so that Ref.p is non-null
+  Ref.d.b = Symbols.size(); // Symbol index
   return BasicSymbolRef(Ref, this);
 }
 
 const WasmSymbol &WasmObjectFile::getWasmSymbol(const DataRefImpl &Symb) const {
-  return Symbols[Symb.d.a];
+  return Symbols[Symb.d.b];
 }
 
 const WasmSymbol &WasmObjectFile::getWasmSymbol(const SymbolRef &Symb) const {
   return getWasmSymbol(Symb.getRawDataRefImpl());
 }
 
 Expected<StringRef> WasmObjectFile::getSymbolName(DataRefImpl Symb) const {
   return getWasmSymbol(Symb).Info.Name;

test/tools/llvm-nm/lit.local.cfg

 if not 'X86' in config.root.targets:
     config.unsupported = True
 
-config.suffixes = ['.s', '.test', '.yaml']
+config.suffixes = ['.ll', '.s', '.test', '.yaml']

test/tools/llvm-nm/wasm/extern-only.ll

+; RUN: llc -filetype=obj -mtriple=wasm32-unknown-unknown -o %t.o %s
+; RUN: llvm-nm --extern-only %t.o | FileCheck %s
+
+; Verity that hidden symbols are listed even when --extern-only is passed
+
+define hidden i32 @foo() {
+entry:
+  ret i32 42
+}
+
+define i32 @bar() {
+entry:
+  ret i32 43
+}
+
+define internal i32 @baz() {
+entry:
+  ret i32 44
+}
+
+; CHECK: 00000006 T bar
+; CHECK-NOT: baz
+; CHECK: 00000001 T foo