[DebugInfo] Treat empty metadata operands the same as undef operands in SelectionDAG

[Orlando]

Summary

Without this patch SelectionDAG silently drops dbg.values using !{} operands.

Related to https://discourse.llvm.org/t/auto-undef-debug-uses-of-a-deleted-value

[StephenTozer]

LGTM!

[Orlando]

note to self: this could use convertToUndefExpression from D133929 when it lands

[goncharov]

Heads up: this seems to make compilation non-determenistic (I am still trying to confirm).

[goncharov]

In D140990#4298798, @goncharov wrote:

Heads up: this seems to make compilation non-determenistic (I am still trying to confirm).

Likely fixed by https://reviews.llvm.org/D140901

[Orlando]

Most curious.... I'm not sure off the top of my head why that'd be the case (either the issue or the fact that D140901 fixies it). SelectionDAG has a load of weird edge cases with debug info handling though. Thanks for looking into this, is there anything I can do to help?

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

public:
   void salvageUnresolvedDbgValue(DanglingDebugInfo &DDI);
 
   /// For a given list of Values, attempt to create and record a SDDbgValue in
   /// the SelectionDAG.
   bool handleDebugValue(ArrayRef<const Value *> Values, DILocalVariable *Var,
                         DIExpression *Expr, DebugLoc DbgLoc, unsigned Order,
                         bool IsVariadic);
 
+  /// Create a record for a kill location debug intrinsic.
+  void handleKillDebugValue(DILocalVariable *Var, DIExpression *Expr,
+                            DebugLoc DbgLoc, unsigned Order);
+
   /// Evict any dangling debug information, attempting to salvage it first.
   void resolveOrClearDbgInfo();
 
   SDValue getValue(const Value *V);
 
   SDValue getNonRegisterValue(const Value *V);
   SDValue getValueImpl(const Value *V);
 

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

void SelectionDAGBuilder::salvageUnresolvedDbgValue(DanglingDebugInfo &DDI) {
   assert(OrigV && "V shouldn't be null");
   auto *Undef = UndefValue::get(OrigV->getType());
   auto *SDV = DAG.getConstantDbgValue(Var, Expr, Undef, DL, SDNodeOrder);
   DAG.AddDbgValue(SDV, false);
   LLVM_DEBUG(dbgs() << "Dropping debug value info for:\n  " << printDDI(DDI)
                     << "\n");
 }
 
+void SelectionDAGBuilder::handleKillDebugValue(DILocalVariable *Var,
+                                               DIExpression *Expr,
+                                               DebugLoc DbgLoc,
+                                               unsigned Order) {
+  Value *Poison = PoisonValue::get(Type::getInt1Ty(*Context));
+  DIExpression *NewExpr = DIExpression::get(*Context, std::nullopt);
+  if (auto Frag = Expr->getFragmentInfo())
+    NewExpr = *DIExpression::createFragmentExpression(
+        NewExpr, Frag->OffsetInBits, Frag->SizeInBits);
+  handleDebugValue(Poison, Var, NewExpr, DbgLoc, Order,
+                   /*IsVariadic*/ false);
+}
+
 bool SelectionDAGBuilder::handleDebugValue(ArrayRef<const Value *> Values,
                                            DILocalVariable *Var,
                                            DIExpression *Expr, DebugLoc DbgLoc,
                                            unsigned Order, bool IsVariadic) {
   if (Values.empty())
     return true;
   SmallVector<SDDbgOperand> LocationOps;
   SmallVector<SDNode *> Dependencies;
case Intrinsic::dbg_value: {
     if (getEnableAssignmentTracking())
       return;
     const DbgValueInst &DI = cast<DbgValueInst>(I);
     assert(DI.getVariable() && "Missing variable");
 
     DILocalVariable *Variable = DI.getVariable();
     DIExpression *Expression = DI.getExpression();
     dropDanglingDebugInfo(Variable, Expression);
-    SmallVector<Value *, 4> Values(DI.getValues());
-    if (Values.empty())
+
+    if (DI.isKillLocation()) {
+      handleKillDebugValue(Variable, Expression, DI.getDebugLoc(), SDNodeOrder);
       return;
+    }
 
-    if (llvm::is_contained(Values, nullptr))
+    SmallVector<Value *, 4> Values(DI.getValues());
+    if (Values.empty())
       return;
 
     bool IsVariadic = DI.hasArgList();
     if (!handleDebugValue(Values, Variable, Expression, DI.getDebugLoc(),
                           SDNodeOrder, IsVariadic))
       addDanglingDebugInfo(&DI, SDNodeOrder);
     return;
   }

llvm/test/DebugInfo/X86/dbg-empty-metadata-lowering.ll

+; RUN: llc %s -stop-after=finalize-isel -o - | FileCheck %s --implicit-check-not=DBG
+
+;; Check that dbg.values with empty metadata are treated as kills (i.e. become
+;; DBG_VALUE $noreg, ...).
+
+; CHECK: DBG_VALUE float 5.000000e+00
+; CHECK: @ext
+; CHECK: DBG_VALUE $noreg
+; CHECK: @ext
+
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: mustprogress uwtable
+define dso_local void @fun() local_unnamed_addr #0 !dbg !9 {
+entry:
+  call void @llvm.dbg.value(metadata float 5.000000e+00, metadata !13, metadata !DIExpression()), !dbg !15
+  tail call void @ext(), !dbg !16
+  call void @llvm.dbg.value(metadata !{}, metadata !13, metadata !DIExpression()), !dbg !15
+  tail call void @ext(), !dbg !16
+  ret void, !dbg !17
+}
+
+declare !dbg !18 void @ext() local_unnamed_addr
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 16.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 8, !"PIC Level", i32 2}
+!6 = !{i32 7, !"PIE Level", i32 2}
+!7 = !{i32 7, !"uwtable", i32 2}
+!8 = !{!"clang version 16.0.0"}
+!9 = distinct !DISubprogram(name: "fun", linkageName: "fun", scope: !1, file: !1, line: 2, type: !10, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !{!13}
+!13 = !DILocalVariable(name: "f", scope: !9, file: !1, line: 3, type: !14)
+!14 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!15 = !DILocation(line: 0, scope: !9)
+!16 = !DILocation(line: 4, column: 3, scope: !9)
+!17 = !DILocation(line: 5, column: 1, scope: !9)
+!18 = !DISubprogram(name: "ext", linkageName: "ext", scope: !1, file: !1, line: 1, type: !10, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !19)
+!19 = !{}