[DebugInfo] [SelectionDAG] Fix handling of duplicate dbg values (#86598)

Before this fix, a duplicate llvm.dbg.value intrinsic referring to an
argument, after an alloca, would be generated with `$noreg`, losing
debug information. Instead, we silently drop the second debug info, so
it doesn't break the first one.

rdar://125375717

Author: Snowy1803

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -6007,12 +6007,15 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
     // incorrect hoisting of the DBG_VALUE to the function entry).
     // Notice that we allow one dbg.value per IR level argument, to accommodate
     // for the situation with fragments above.
+    // If there is no node for the value being handled, we return true to skip
+    // the normal generation of debug info, as it would kill existing debug
+    // info for the parameter in case of duplicates.
     if (VariableIsFunctionInputArg) {
       unsigned ArgNo = Arg->getArgNo();
       if (ArgNo >= FuncInfo.DescribedArgs.size())
         FuncInfo.DescribedArgs.resize(ArgNo + 1, false);
       else if (!IsInPrologue && FuncInfo.DescribedArgs.test(ArgNo))
-        return false;
+        return !NodeMap[V].getNode();
       FuncInfo.DescribedArgs.set(ArgNo);
     }
   }

llvm/test/DebugInfo/X86/dbg-value-funcarg-duplicates.ll

@@ -0,0 +1,65 @@
+; RUN: llc -mtriple=x86_64-unknown-linux-gnu -start-after=codegenprepare -stop-before=finalize-isel -o - %s -experimental-debug-variable-locations=false | FileCheck %s
+
+; Input to this test was created by reducing a Swift file using bugpoint
+
+; CHECK-DAG: ![[LHS:.*]] = !DILocalVariable(name: "lhs"
+
+define hidden i64 @"_wideDivide42"(ptr %0, ptr %1, ptr %2, i64 %3, i64 %4, i64 %5, i64 %6, i64 %7, i64 %8) local_unnamed_addr !dbg !16 {
+; CHECK-LABEL: name:            _wideDivide42
+; CHECK-NOT:  DBG_VALUE
+; CHECK:      DBG_VALUE $rcx, $noreg, ![[LHS]], !DIExpression(DW_OP_LLVM_fragment, 0, 64)
+; CHECK-NEXT: DBG_VALUE $r8, $noreg, ![[LHS]], !DIExpression(DW_OP_LLVM_fragment, 64, 64)
+; CHECK-NEXT: DBG_VALUE $r9, $noreg, ![[LHS]], !DIExpression(DW_OP_LLVM_fragment, 128, 64)
+; CHECK-NEXT: DBG_VALUE %fixed-stack.{{.+}}, ![[LHS]], !DIExpression(DW_OP_LLVM_fragment, 192, 64)
+; The duplicates should be removed:
+; CHECK-NOT:  DBG_VALUE
+
+entry:
+  %9 = alloca i64, align 8
+  call void @llvm.dbg.value(metadata i64 %3, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg !67
+  call void @llvm.dbg.value(metadata i64 %4, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg !67
+  call void @llvm.dbg.value(metadata i64 %3, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg !67
+  call void @llvm.dbg.value(metadata i64 %4, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg !67
+  call void @llvm.dbg.value(metadata i64 %5, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 128, 64)), !dbg !67
+  call void @llvm.dbg.value(metadata i64 %6, metadata !24, metadata !DIExpression(DW_OP_LLVM_fragment, 192, 64)), !dbg !67
+  br i1 poison, label %11, label %10, !dbg !68
+
+10:                                               ; preds = %entry
+  tail call void asm sideeffect "", "n"(i32 7) #7
+  unreachable
+
+11:                                               ; preds = %entry
+  tail call void @abort()
+  unreachable
+}
+
+declare void @abort()
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+attributes #7 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!13}
+!llvm.linker.options = !{!14, !15}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !1, producer: "Swift", isOptimized: true, runtimeVersion: 6, emissionKind: FullDebug)
+!1 = !DIFile(filename: "Int128.swift", directory: "")
+!13 = !{i32 2, !"Debug Info Version", i32 3}
+!14 = !{!"-lswiftCore"}
+!15 = !{!"-lobjc"}
+!16 = distinct !DISubprogram(name: "_wideDivide42", scope: !0, file: !1, line: 222, type: !17, scopeLine: 222, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !23)
+!17 = !DISubroutineType(types: !18)
+!18 = !{!19, !20, !20, !20, !20, !20, !20}
+!19 = !DICompositeType(tag: DW_TAG_structure_type, name: "4 x UInt64", flags: DIFlagFwdDecl, runtimeLang: DW_LANG_Swift)
+!20 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "UInt64", scope: !1, file: !1, size: 64, elements: !22, runtimeLang: DW_LANG_Swift)
+!22 = !{}
+!23 = !{!24, !27}
+!24 = !DILocalVariable(name: "lhs", arg: 1, scope: !16, file: !1, line: 223, type: !25, flags: DIFlagArtificial)
+!25 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !26)
+!26 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "2 x 2 x UInt64", file: !1, size: 256, elements: !22, runtimeLang: DW_LANG_Swift)
+!27 = !DILocalVariable(name: "rhs", arg: 2, scope: !16, file: !1, line: 223, type: !28, flags: DIFlagArtificial)
+!28 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !29)
+!29 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "2 x UInt64", file: !1, size: 128, elements: !22, runtimeLang: DW_LANG_Swift)
+!67 = !DILocation(line: 0, scope: !16)
+!68 = !DILocation(line: 225, column: 9, scope: !16)