[CodeView] Avoid emitting empty debug globals subsection.

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.

Differential Revision: https://reviews.llvm.org/D92049

lld/COFF/PDB.cpp

@@ -500,6 +500,9 @@ void PDBLinker::mergeSymbolRecords(TpiSource *source,
   cantFail(symData.readBytes(0, symData.getLength(), symsBuffer));
   SmallVector<SymbolScope, 4> scopes;
 
+  if (symsBuffer.empty())
+    warn("empty symbols subsection in " + file->getName());
+
   // Iterate every symbol to check if any need to be realigned, and if so, how
   // much space we need to allocate for them.
   bool needsRealignment = false;

lld/test/COFF/empty-subsection.s

@@ -0,0 +1,20 @@
+# REQUIRES: x86
+# RUN: llvm-mc -triple=x86_64-windows-msvc -filetype=obj -o %t.obj %s
+# RUN: lld-link /entry:main /debug /out:%t.exe %t.obj 2>&1 | FileCheck %s
+
+# CHECK: warning: empty symbols subsection
+
+.globl main
+.Lfunc_begin0:
+main:
+  xorl  %eax, %eax
+  retq
+.Lfunc_end0:
+
+.section .debug$S,"dr"
+	.p2align	2
+	.long	4                               # Debug section magic
+	.long	241                             # Symbol subsection for globals
+  .long .Ltmp5-.Ltmp4                   # Subsection size
+.Ltmp4:
+.Ltmp5:

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

@@ -2150,10 +2150,13 @@ void CodeViewDebug::collectMemberInfo(ClassInfo &Info,
   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) {
-      StaticConstMembers.push_back(DDTy);
+      if (DDTy->getConstant() && (isa<ConstantInt>(DDTy->getConstant()) ||
+                                  isa<ConstantFP>(DDTy->getConstant())))
+        StaticConstMembers.push_back(DDTy);
+    }
 
     return;
   }
@@ -3134,7 +3137,7 @@ void CodeViewDebug::emitStaticConstMemberList() {
                  dyn_cast_or_null<ConstantFP>(DTy->getConstant()))
       Value = APSInt(CFP->getValueAPF().bitcastToAPInt(), true);
     else
-      continue;
+      llvm_unreachable("cannot emit a constant without a value");
 
     std::string QualifiedName = getFullyQualifiedName(Scope, DTy->getName());
 

llvm/test/DebugInfo/COFF/empty-globals-subsection.ll

@@ -0,0 +1,29 @@
+; RUN: llc < %s | FileCheck %s
+
+; The debug info contains a struct with a non-const static member.
+; Check that we don't emit a symbol subsection for globals.
+
+; CHECK-NOT:  Symbol subsection for globals
+
+; ModuleID = 'a.cpp'
+source_filename = "a.cpp"
+target triple = "x86_64-pc-windows-msvc19.25.28614"
+
+define dso_local i32 @main() {
+entry:
+  ret i32 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!8, !9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, nameTableKind: None)
+!1 = !DIFile(filename: "a.cpp", directory: "C:/path/to/test", checksumkind: CSK_MD5, checksum: "a1dbf3aabea9e8f9d1be48f60287942f")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !1, line: 5, size: 32, flags: DIFlagTypePassByValue | DIFlagNonTrivial, elements: !5, identifier: ".?AU?$A@J@@")
+!5 = !{!6}
+!6 = !DIDerivedType(tag: DW_TAG_member, name: "x", scope: !4, file: !1, line: 6, baseType: !7, size: 32, flags: DIFlagStaticMember)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!8 = !{i32 2, !"CodeView", i32 1}
+!9 = !{i32 2, !"Debug Info Version", i32 3}