Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1079,6 +1079,17 @@
     Value *Res = EvaluateInDifferentType(Src, DestTy, false);
     assert(Res->getType() == DestTy);
 
+    // When DestTy is integer, try to preserve any debug values referring
+    // to the zext being replaced.
+    // TODO: This should work for vectors as well, possibly via the use
+    // of DWARF fragments.
+    if (DestTy->isIntegerTy()) {
+      insertReplacementDbgValues(
+          *Src, *Res, CI, [](DbgInfoIntrinsic &OldDII) -> DIExpression * {
+            return OldDII.getExpression();
+          });
+    }
+
     uint32_t SrcBitsKept = SrcTy->getScalarSizeInBits()-BitsToClear;
     uint32_t DestBitSize = DestTy->getScalarSizeInBits();
 
Index: llvm/trunk/test/Transforms/InstCombine/cast-mul-select.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/cast-mul-select.ll
+++ llvm/trunk/test/Transforms/InstCombine/cast-mul-select.ll
@@ -1,5 +1,5 @@
-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instcombine -S | FileCheck %s
+; RUN: opt -debugify -instcombine -S < %s | FileCheck %s -check-prefix DBGINFO
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32"
 
@@ -8,7 +8,16 @@
 ; CHECK-NEXT:    [[C:%.*]] = mul i32 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    [[D:%.*]] = and i32 [[C]], 255
 ; CHECK-NEXT:    ret i32 [[D]]
-;
+
+; Test that when zext is evaluated in different type
+; we preserve the debug information in the resulting
+; instruction.
+; DBGINFO-LABEL: @mul(
+; DBGINFO-NEXT:    [[C:%.*]] = mul i32 {{.*}}
+; DBGINFO-NEXT:    call void @llvm.dbg.value(metadata i32 [[C]]
+; DBGINFO-NEXT:    [[D:%.*]] = and i32 {{.*}}
+; DBGINFO-NEXT:    call void @llvm.dbg.value(metadata i32 [[D]]
+
   %A = trunc i32 %x to i8
   %B = trunc i32 %y to i8
   %C = mul i8 %A, %B