Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1088,6 +1088,21 @@
     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,
Index: test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
@@ -0,0 +1,12 @@
+; RUN: opt -debugify -instcombine -S < %s | FileCheck %s
+
+define <2 x i64> @test3(<2 x i64> %A) {
+  %trunc = trunc <2 x i64> %A to <2 x i32>
+  %and = and <2 x i32> %trunc, <i32 23, i32 42>
+  %zext = zext <2 x i32> %and to <2 x i64>
+  ret <2 x i64> %zext
+}
+
+; CHECK: call void @llvm.dbg.value{{.*}}
+; CHECK: call void @llvm.dbg.value{{.*}}
+; CHECK: call void @llvm.dbg.value{{.*}}