Globalopt pass produces invalid debug info (#100654)

This patch fixes an issue in the GlobalOpt pass where deleting a global
variable fails to update the corresponding dbg.value and it references
an empty metadata entry. The SalvageDebugInfo() function has been
updated to handle dbg.value intrinsic when globals are deleted.

Author: ykhatav

llvm/lib/IR/Metadata.cpp

@@ -346,6 +346,12 @@ void ReplaceableMetadataImpl::SalvageDebugInfo(const Constant &C) {
     MetadataTracking::OwnerTy Owner = Pair.second.first;
     if (!Owner)
       continue;
+    // Check for MetadataAsValue.
+    if (isa<MetadataAsValue *>(Owner)) {
+      cast<MetadataAsValue *>(Owner)->handleChangedMetadata(
+          ValueAsMetadata::get(UndefValue::get(C.getType())));
+      continue;
+    }
     if (!isa<Metadata *>(Owner))
       continue;
     auto *OwnerMD = dyn_cast_if_present<MDNode>(cast<Metadata *>(Owner));

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -1338,6 +1338,7 @@ deleteIfDead(GlobalValue &GV,
     if (DeleteFnCallback)
       DeleteFnCallback(*F);
   }
+  ReplaceableMetadataImpl::SalvageDebugInfo(GV);
   GV.eraseFromParent();
   ++NumDeleted;
   return true;

llvm/test/DebugInfo/X86/undef-dbg-val.ll

@@ -0,0 +1,34 @@
+; RUN:  opt -S -passes=globalopt --experimental-debuginfo-iterators=false <%s | FileCheck %s
+; CHECK: #dbg_value(ptr undef, 
+; CHECK-SAME:    [[VAR:![0-9]+]],
+; CHECK-SAME:    !DIExpression()
+; CHECK: [[VAR]] = !DILocalVariable(name: "_format"
+
+
+; ModuleID = '<stdin>'
+source_filename = "test.cpp"
+
+@_ZZZZ4main_format = internal constant [24 x i8] c"Result1: Hello, World!\0A\00", align 16, !dbg !9
+
+define void  @foo() align 2 !dbg !5 {
+entry:
+  call void @llvm.dbg.value(metadata ptr @_ZZZZ4main_format, metadata !11, metadata !DIExpression()), !dbg !12
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang", isOptimized: true, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test.cpp", directory: "/path/to")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 27, type: !6, scopeLine: 27, spFlags: DISPFlagDefinition, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{null}
+!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!9 = !DIGlobalVariableExpression(var: !10, expr: !DIExpression())
+!10 = distinct !DIGlobalVariable(name: "_format", scope: !5, file: !1, line: 49, type: !8, isLocal: true, isDefinition: true)
+!11 = !DILocalVariable(name: "_format", arg: 1, scope: !5, file: !1, line: 79, type: !8)
+!12 = !DILocation(line: 0, scope: !5)