[CodeView] Avoid emitting empty debug globals subsection.

[akhuang]

Summary

In https://reviews.llvm.org/D89072 I added static const data members
to the debug subsection for globals. It skipped emitting an S_CONSTANT if it
didn't have a value, which meant the subsection could be empty.

This patch fixes the empty subsection issue.

[akhuang]

avoid calling isa on a null value

[rnk]

Looks good, thanks!

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

3140

LLVM likes to use llvm_unreachable instead for this kind of case

[rnk]

I forgot, you should add a test for this.

[rnk]

As a super bonus thing, can you make LLD emit a similar warning when it sees empty .debug$S sections? That way we won't run into this again in the future.

[akhuang]

Add test; add warning in lld when it sees empty subsection

[rnk]

Thanks, looks good

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

void CodeViewDebug::clear() {
   ScopeGlobals.clear();
 }
 
 void CodeViewDebug::collectMemberInfo(ClassInfo &Info,
                                       const DIDerivedType *DDTy) {
   if (!DDTy->getName().empty()) {
     Info.Members.push_back({DDTy, 0});
 
-    // Collect static const data members.
+    // Collect static const data members with values.
     if ((DDTy->getFlags() & DINode::FlagStaticMember) ==
-        DINode::FlagStaticMember)
+        DINode::FlagStaticMember) {
+      if (isa<ConstantInt>(DDTy->getConstant()) ||
+          isa<ConstantFP>(DDTy->getConstant()))
         StaticConstMembers.push_back(DDTy);
+    }
 
     return;
   }
 
   // An unnamed member may represent a nested struct or union. Attempt to
   // interpret the unnamed member as a DICompositeType possibly wrapped in
   // qualifier types. Add all the indirect fields to the current record if that
   // succeeds, and drop the member if that fails.
for (const DIDerivedType *DTy : StaticConstMembers) {
     if (const ConstantInt *CI =
             dyn_cast_or_null<ConstantInt>(DTy->getConstant()))
       Value = APSInt(CI->getValue(),
                      DebugHandlerBase::isUnsignedDIType(DTy->getBaseType()));
     else if (const ConstantFP *CFP =
                  dyn_cast_or_null<ConstantFP>(DTy->getConstant()))
       Value = APSInt(CFP->getValueAPF().bitcastToAPInt(), true);
     else
-      continue;
+      assert(false && "cannot emit a constant without a value");
 
     std::string QualifiedName = getFullyQualifiedName(Scope, DTy->getName());
 
     MCSymbol *SConstantEnd = beginSymbolRecord(SymbolKind::S_CONSTANT);
     OS.AddComment("Type");
     OS.emitInt32(getTypeIndex(DTy->getBaseType()).getIndex());
     OS.AddComment("Value");