[DebugInfo] Prevent invalid fragments at ISel from dropping debug info

During SelectionDAG, if a value which is associated with a DBG_VALUE
needs to be split across multiple registers, the DBG_VALUE will be split
into a set of fragment expressions to recreate the original value.

If one or more of these fragments cannot be created, they would
previously be silently dropped, causing the old debug value to live past
its expiry date. This patch fixes this issue by keeping invalid
fragments while setting their value as Undef.

Differential revision: https://reviews.llvm.org/D70248

Author: SLTozer

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -5553,8 +5553,14 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
       for (auto RegAndSize : SplitRegs) {
         auto FragmentExpr = DIExpression::createFragmentExpression(
           Expr, Offset, RegAndSize.second);
-        if (!FragmentExpr)
+        // If a valid fragment expression cannot be created, the variable's
+        // correct value cannot be determined and so it is set as Undef.
+        if (!FragmentExpr) {
+          SDDbgValue *SDV = DAG.getConstantDbgValue(
+              Variable, Expr, UndefValue::get(V->getType()), DL, SDNodeOrder);
+          DAG.AddDbgValue(SDV, nullptr, false);
           continue;
+        }
         assert(!IsDbgDeclare && "DbgDeclare operand is not in memory?");
         FuncInfo.ArgDbgValues.push_back(
           BuildMI(MF, DL, TII->get(TargetOpcode::DBG_VALUE), false,

llvm/test/CodeGen/ARM/debuginfo-split-carryexpr.ll

@@ -0,0 +1,51 @@
+; RUN: llc < %s -mtriple=armv7-linux-gnueabihf -stop-before=finalize-isel | FileCheck %s
+
+; When splitting-up integers during CodeGen, if the debug info contains
+; an expression with carry operators (i.e. arithmetic and shift ops), the
+; debug information will be dropped as these operators cannot be correctly
+; expressed across different registers.
+
+; CHECK: [[HIGH:![0-9]+]] = !DILocalVariable(name: "high"
+; CHECK: [[LOW:![0-9]+]] = !DILocalVariable(name: "low"
+;
+; As the debug information for "high" contains an arithmetic shift, while for
+; "low" it does not, only the former should be undefined.
+; CHECK-LABEL: body:
+; CHECK: [[LOWR:%[0-9]+]]:gpr = COPY $r0
+; CHECK: DBG_VALUE [[LOWR]], $noreg, [[LOW]]
+; CHECK: DBG_VALUE $noreg, $noreg, [[HIGH]]
+; CHECK: DBG_VALUE $noreg, $noreg, [[HIGH]]
+
+define dso_local i64 @_Z2fnx(i64 returned %value) local_unnamed_addr !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata i64 %value, metadata !13, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i64 %value, metadata !14, metadata !DIExpression(DW_OP_constu, 32, DW_OP_shra, DW_OP_LLVM_convert, 64, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !17
+  call void @llvm.dbg.value(metadata i64 %value, metadata !16, metadata !DIExpression(DW_OP_LLVM_convert, 64, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !17
+  ret i64 %value, !dbg !18
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 10.0.0"}
+!7 = distinct !DISubprogram(name: "fn", linkageName: "_Z2fnx", scope: !8, file: !8, line: 2, type: !9, scopeLine: 2, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!8 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11}
+!11 = !DIBasicType(name: "long long int", size: 64, encoding: DW_ATE_signed)
+!12 = !{!13, !14, !16}
+!13 = !DILocalVariable(name: "value", arg: 1, scope: !7, file: !8, line: 2, type: !11)
+!14 = !DILocalVariable(name: "high", scope: !7, file: !8, line: 3, type: !15)
+!15 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!16 = !DILocalVariable(name: "low", scope: !7, file: !8, line: 4, type: !15)
+!17 = !DILocation(line: 0, scope: !7)
+!18 = !DILocation(line: 5, column: 3, scope: !7)