Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ 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: test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
@@ -0,0 +1,21 @@
+; RUN: opt -debugify -instcombine -S < %s | FileCheck %s
+
+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"
+
+; Test that when zext is evaluated in different type
+; we preserve the debug information in the resulting
+; instruction.
+
+define i32 @test-scalar(i32 %x, i32 %y) {
+  %A = trunc i32 %x to i8
+  %B = trunc i32 %y to i8
+  %C = mul i8 %A, %B
+  %D = zext i8 %C to i32
+  ret i32 %D
+}
+
+; CHECK: define i32 @test-scalar
+; CHECK: {{.*}} = mul i32 {{.*}}
+; CHECK-NEXT: call void @llvm.dbg.value{{.*}}
+; CHECK: {{.*}} = and i32 {{.*}}
+; CHECK-NEXT: call void @llvm.dbg.value{{.*}}