Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1088,17 +1088,31 @@
     uint32_t SrcBitsKept = SrcTy->getScalarSizeInBits()-BitsToClear;
     uint32_t DestBitSize = DestTy->getScalarSizeInBits();
 
+    // Since the old instruction is merged, we preserve it's DI as
+    // a fragment in the resulting instruction
+    SmallVector<DbgInfoIntrinsic *, 1> SrcDbgInsts;
+    findDbgUsers(SrcDbgInsts, Src);
+    if (SrcDbgInsts.size()) {
+      DIBuilder DIB(*CI.getModule());
+      for (auto *DII : SrcDbgInsts) {
+        auto Fragment = DIExpression::createFragmentExpression(
+            DII->getExpression(), SrcBitsKept, DestBitSize);
+        DIB.insertDbgValueIntrinsic(
+            Res, DII->getVariable(), Fragment.getValue(),
+            DII->getDebugLoc().get(), &*std::next(CI.getIterator()));
+      }
+    }
+
     // If the high bits are already filled with zeros, just replace this
     // cast with the result.
-    if (MaskedValueIsZero(Res,
-                          APInt::getHighBitsSet(DestBitSize,
-                                                DestBitSize-SrcBitsKept),
-                             0, &CI))
+    if (MaskedValueIsZero(
+            Res, APInt::getHighBitsSet(DestBitSize, DestBitSize - SrcBitsKept),
+            0, &CI))
       return replaceInstUsesWith(CI, Res);
 
     // We need to emit an AND to clear the high bits.
-    Constant *C = ConstantInt::get(Res->getType(),
-                               APInt::getLowBitsSet(DestBitSize, SrcBitsKept));
+    Constant *C = ConstantInt::get(
+        Res->getType(), APInt::getLowBitsSet(DestBitSize, SrcBitsKept));
     return BinaryOperator::CreateAnd(Res, C);
   }
 
Index: test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
@@ -0,0 +1,43 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+define <2 x i64> @test3(<2 x i64> %A) !dbg !6 {
+  %trunc = trunc <2 x i64> %A to <2 x i32>, !dbg !14
+  call void @llvm.dbg.value(metadata <2 x i32> %trunc, metadata !9, metadata !DIExpression()), !dbg !14
+  %and = and <2 x i32> %trunc, <i32 23, i32 42>, !dbg !15
+  call void @llvm.dbg.value(metadata <2 x i32> %and, metadata !11, metadata !DIExpression()), !dbg !15
+  %zext = zext <2 x i32> %and to <2 x i64>, !dbg !16
+  call void @llvm.dbg.value(metadata <2 x i64> %zext, metadata !12, metadata !DIExpression()), !dbg !16
+  ret <2 x i64> %zext, !dbg !17
+}
+
+; CHECK: call void @llvm.dbg.value{{.*}}
+; CHECK: call void @llvm.dbg.value{{.*}}
+; CHECK: call void @llvm.dbg.value{{.*}}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "debuginfo-trunc-and-zext.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 4}
+!4 = !{i32 3}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "test3", linkageName: "test3", scope: null, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9, !11, !12}
+!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+!11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 2, type: !10)
+!12 = !DILocalVariable(name: "3", scope: !6, file: !1, line: 3, type: !13)
+!13 = !DIBasicType(name: "ty128", size: 128, encoding: DW_ATE_unsigned)
+!14 = !DILocation(line: 1, column: 1, scope: !6)
+!15 = !DILocation(line: 2, column: 1, scope: !6)
+!16 = !DILocation(line: 3, column: 1, scope: !6)
+!17 = !DILocation(line: 4, column: 1, scope: !6)