[DebugInfo][SelectionDAG] Add debug info salvaging for TRUNC nodes

This patch adds support for salvaging TRUNC nodes during SelectionDAG,
fixing LLVM issue #63076:
  #63076

Reviewed in: #66922

Author: Ben Mudd

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -13512,8 +13512,12 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
                         N0.getScalarValueSizeInBits(),
                         std::min(Op.getScalarValueSizeInBits(),
                                  VT.getScalarSizeInBits()));
-    if (TruncatedBits.isSubsetOf(Known.Zero))
-      return DAG.getZExtOrTrunc(Op, DL, VT);
+    if (TruncatedBits.isSubsetOf(Known.Zero)) {
+      SDValue ZExtOrTrunc = DAG.getZExtOrTrunc(Op, DL, VT);
+      DAG.salvageDebugInfo(*N0.getNode());
+
+      return ZExtOrTrunc;
+    }
   }
 
   // fold (zext (truncate x)) -> (and x, mask)

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -10778,7 +10778,7 @@ void SelectionDAG::salvageDebugInfo(SDNode &N) {
     switch (N.getOpcode()) {
     default:
       break;
-    case ISD::ADD:
+    case ISD::ADD: {
       SDValue N0 = N.getOperand(0);
       SDValue N1 = N.getOperand(1);
       if (!isa<ConstantSDNode>(N0) && isa<ConstantSDNode>(N1)) {
@@ -10819,6 +10819,40 @@ void SelectionDAG::salvageDebugInfo(SDNode &N) {
                    N0.getNode()->dumprFull(this);
                    dbgs() << " into " << *DIExpr << '\n');
       }
+      break;
+    }
+    case ISD::TRUNCATE: {
+      SDValue N0 = N.getOperand(0);
+      TypeSize FromSize = N0.getValueSizeInBits();
+      TypeSize ToSize = N.getValueSizeInBits(0);
+
+      DIExpression *DbgExpression = DV->getExpression();
+      auto ExtOps = DIExpression::getExtOps(FromSize, ToSize, false);
+      auto NewLocOps = DV->copyLocationOps();
+      bool Changed = false;
+      for (size_t i = 0; i < NewLocOps.size(); ++i) {
+        if (NewLocOps[i].getKind() != SDDbgOperand::SDNODE ||
+            NewLocOps[i].getSDNode() != &N)
+          continue;
+
+        NewLocOps[i] = SDDbgOperand::fromNode(N0.getNode(), N0.getResNo());
+        DbgExpression = DIExpression::appendOpsToArg(DbgExpression, ExtOps, i);
+        Changed = true;
+      }
+      assert(Changed && "Salvage target doesn't use N");
+
+      SDDbgValue *Clone =
+          getDbgValueList(DV->getVariable(), DbgExpression, NewLocOps,
+                          DV->getAdditionalDependencies(), DV->isIndirect(),
+                          DV->getDebugLoc(), DV->getOrder(), DV->isVariadic());
+
+      ClonedDVs.push_back(Clone);
+      DV->setIsInvalidated();
+      DV->setIsEmitted();
+      LLVM_DEBUG(dbgs() << "SALVAGE: Rewriting"; N0.getNode()->dumprFull(this);
+                 dbgs() << " into " << *DbgExpression << '\n');
+      break;
+    }
     }
   }
 

llvm/test/CodeGen/X86/selection-dag-salvagetrunc.ll

@@ -0,0 +1,52 @@
+; RUN: llc --stop-after=finalize-isel < %s | FileCheck %s
+;
+; Verify that we can correctly salvage truncate expressions during SelectionDAG.
+; Fixes LLVM issue #63076.
+
+; CHECK: body
+; CHECK: DBG_INSTR_REF !{{[0-9]+}}, !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_convert, 8, DW_ATE_unsigned, DW_OP_LLVM_convert, 1, DW_ATE_unsigned)
+
+source_filename = "repro.cpp"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define dso_local noundef i32 @_Z3funv() !dbg !10 {
+entry:
+  %call = tail call noundef zeroext i1 @_Z3getv(), !dbg !17
+  call void @llvm.dbg.value(metadata i1 %call, metadata !15, metadata !DIExpression()), !dbg !18
+  %conv = zext i1 %call to i32, !dbg !19
+  ret i32 %conv, !dbg !20
+}
+
+declare !dbg !21 noundef zeroext i1 @_Z3getv()
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !7, !8}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 18.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "repro.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 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
+!9 = !{!"clang version 18.0.0"}
+!10 = distinct !DISubprogram(name: "fun", linkageName: "_Z3funv", scope: !1, file: !1, line: 2, type: !11, scopeLine: 3, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !14)
+!11 = !DISubroutineType(types: !12)
+!12 = !{!13}
+!13 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!14 = !{!15}
+!15 = !DILocalVariable(name: "b", scope: !10, file: !1, line: 4, type: !16)
+!16 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
+!17 = !DILocation(line: 4, column: 11, scope: !10)
+!18 = !DILocation(line: 0, scope: !10)
+!19 = !DILocation(line: 5, column: 9, scope: !10)
+!20 = !DILocation(line: 5, column: 2, scope: !10)
+!21 = !DISubprogram(name: "get", linkageName: "_Z3getv", scope: !1, file: !1, line: 1, type: !22, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!22 = !DISubroutineType(types: !23)
+!23 = !{!16}