diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index e624088a0964e..ebf50bc366cf1 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -507,6 +507,15 @@ bool CoalescerPair::setRegisters(const MachineInstr *MI) {
       if (Src == Dst && SrcSub != DstSub)
         return false;
 
+      // The implicit-def of the super register is zero extended.
+      for (unsigned I = MI->getDesc().getNumOperands(),
+                    E = MI->getNumOperands();
+           I != E; ++I) {
+        const MachineOperand &MO = MI->getOperand(I);
+        if (MO.isReg() && MO.isDef() && MO.getReg() == Dst)
+          return false;
+      }
+
       NewRC = TRI.getCommonSuperRegClass(SrcRC, SrcSub, DstRC, DstSub, SrcIdx,
                                          DstIdx);
       if (!NewRC)
diff --git a/llvm/test/CodeGen/X86/coalesce-implicit-def.mir b/llvm/test/CodeGen/X86/coalesce-implicit-def.mir
new file mode 100644
index 0000000000000..bd9de3b933394
--- /dev/null
+++ b/llvm/test/CodeGen/X86/coalesce-implicit-def.mir
@@ -0,0 +1,24 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -run-pass register-coalescer -mtriple x86_64-unknown-linux-gnu -o - %s | FileCheck %s
+
+# Checks that we do not merge %1 into %0.
+# `undef %1.sub_32bit = COPY %0.sub_32bit, implicit-def %1` is zero extended.
+
+---
+name:            fn1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $rdi
+
+    ; CHECK-LABEL: name: fn1
+    ; CHECK: liveins: $rdi
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64_with_sub_8bit = COPY $rdi
+    ; CHECK-NEXT: undef [[COPY1:%[0-9]+]].sub_32bit:gr64_with_sub_8bit = COPY [[COPY]].sub_32bit, implicit-def [[COPY1]]
+    ; CHECK-NEXT: $rax = COPY [[COPY1]]
+    %0:gr64_with_sub_8bit = COPY $rdi
+    undef %1.sub_32bit:gr64_with_sub_8bit = COPY %0.sub_32bit, implicit-def %1
+    $rax = COPY %1
+...
+
diff --git a/llvm/test/CodeGen/X86/pr169996.ll b/llvm/test/CodeGen/X86/pr169996.ll
new file mode 100644
index 0000000000000..71e50489afd4d
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr169996.ll
@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+; FIXME: The first instruction should be `movl %edi, %eax`.
+
+define i64 @fn1(i64 %arg, ptr %arg1) {
+; CHECK-LABEL: fn1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movq %rdi, %rax
+; CHECK-NEXT:    movb (%rsi), %al
+; CHECK-NEXT:    retq
+  %i = trunc i64 %arg to i32
+  %i4 = load i8, ptr %arg1
+  %i5 = zext i8 %i4 to i32
+  %i6 = and i32 %i, -256
+  %i7 = or i32 %i6, %i5
+  %i12 = zext i32 %i7 to i64
+  ret i64 %i12
+}