[DebugInfo][SelectionDAG] Do not drop dbg intrinsics with empty metadata locs

[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

This causes assignment-tracking to behaviour to match non-assignment-tracking behaviour after a recent change (see D140990).

[jmorse]

LGTM, what's the behaviour with AT at the moment, they're just ignored?

[Orlando]

Thanks. Yep they are dropped, causing locations terminated by the dbg.value to no longer be terminated; incorrectly extending coverage of the preceding variable location. (EDIT: That's the behaviour without this patch, to be clear).

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

void SelectionDAGBuilder::visit(const Instruction &I) {
   // SDNodeOrder, as this mapping is {Inst -> Locs BEFORE Inst}.
   if (FunctionVarLocs const *FnVarLocs = DAG.getFunctionVarLocs()) {
     // Add SDDbgValue nodes for any var locs here. Do so before updating
     // SDNodeOrder, as this mapping is {Inst -> Locs BEFORE Inst}.
     for (auto It = FnVarLocs->locs_begin(&I), End = FnVarLocs->locs_end(&I);
          It != End; ++It) {
       auto *Var = FnVarLocs->getDILocalVariable(It->VariableID);
       dropDanglingDebugInfo(Var, It->Expr);
+      if (It->Values.isKillLocation(It->Expr)) {
+        handleKillDebugValue(Var, It->Expr, It->DL, SDNodeOrder);
+        continue;
+      }
       SmallVector<Value *> Values(It->Values.location_ops());
       if (!handleDebugValue(Values, Var, It->Expr, It->DL, SDNodeOrder,
                             It->Values.hasArgList()))
         addDanglingDebugInfo(It, SDNodeOrder);
     }
   }
 
   // Increase the SDNodeOrder if dealing with a non-debug instruction.

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

-; RUN: llc %s -stop-after=finalize-isel -o - | FileCheck %s --implicit-check-not=DBG
-
+; RUN: llc %s -stop-after=finalize-isel -o - \
+; RUN: | FileCheck %s --implicit-check-not=DBG --check-prefixes=AT-DISABLED,BOTH
 ;; Check that dbg.values with empty metadata are treated as kills (i.e. become
 ;; DBG_VALUE $noreg, ...). dbg.declares with empty metadata location operands
 ;; should be ignored.
 
-; CHECK: stack: []
-; CHECK: DBG_VALUE float 5.000000e+00
-; CHECK: @ext
-; CHECK: DBG_VALUE $noreg
-; CHECK: @ext
+; RUN: sed 's/;Uncomment-with-sed//g' < %s \
+; RUN: | llc -stop-after=finalize-isel -o - \
+; RUN: | FileCheck %s --implicit-check-not=DBG --check-prefixes=AT-ENABLED,BOTH
+;; Check the same behaviour occurs with assignment tracking enabled.
+
+;; First ensure assignment tracking is truly unset/set.
+; AT-DISABLED: !llvm.module.flags = !{![[DWARFv:[0-9]+]], ![[DEBUGv:[0-9]+]]}
+; AT-DISABLED-NOT: !{i32 7, !"debug-info-assignment-tracking", i1 true}
+; AT-ENABLED: !llvm.module.flags = !{![[DWARFv:[0-9]+]], ![[DEBUGv:[0-9]+]], ![[AT:[0-9]+]]}
+; AT-ENABLED-DAG: ![[AT]] = !{i32 7, !"debug-info-assignment-tracking", i1 true}
+; BOTH-DAG: ![[DWARFv]] = !{i32 7, !"Dwarf Version", i32 5}
+; BOTH-DAG: ![[DEBUGv]] = !{i32 2, !"Debug Info Version", i32 3}
+
+; BOTH: stack: []
+; BOTH: DBG_VALUE float 5.000000e+00
+; BOTH: @ext
+; BOTH: DBG_VALUE $noreg
+; BOTH: @ext
 
 target triple = "x86_64-unknown-linux-gnu"
 
 define dso_local void @fun() local_unnamed_addr #0 !dbg !9 {
 entry:
   call void @llvm.dbg.declare(metadata !{}, metadata !20, metadata !DIExpression()), !dbg !15
   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)
 declare void @llvm.dbg.declare(metadata, metadata, metadata)
 
 !llvm.dbg.cu = !{!0}
-!llvm.module.flags = !{!2, !3}
+!llvm.module.flags = !{!2, !3
+;Uncomment-with-sed , !21
+}
 !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}
 !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 = !{}
 !20 = !DILocalVariable(name: "g", scope: !9, file: !1, line: 3, type: !14)
+!21 = !{i32 7, !"debug-info-assignment-tracking", i1 true}