Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -2659,21 +2659,14 @@
                                     const MachineOperand &MOP2) {
   if (!MOP1.isReg() || !MOP2.isReg())
     return false;
-  MachineInstr *I1 = getDefIgnoringCopies(MOP1.getReg(), MRI);
-  if (!I1)
+  auto InstAndDef1 = getDefSrcRegIgnoringCopies(MOP1.getReg(), MRI);
+  if (!InstAndDef1)
     return false;
-  MachineInstr *I2 = getDefIgnoringCopies(MOP2.getReg(), MRI);
-  if (!I2)
+  auto InstAndDef2 = getDefSrcRegIgnoringCopies(MOP2.getReg(), MRI);
+  if (!InstAndDef2)
     return false;
-
-  // Handle a case like this:
-  //
-  // %0:_(s64), %1:_(s64) = G_UNMERGE_VALUES %2:_(<2 x s64>)
-  //
-  // Even though %0 and %1 are produced by the same instruction they are not
-  // the same values.
-  if (I1 == I2)
-    return MOP1.getReg() == MOP2.getReg();
+  MachineInstr *I1 = InstAndDef1->MI;
+  MachineInstr *I2 = InstAndDef2->MI;
 
   // If we have an instruction which loads or stores, we can't guarantee that
   // it is identical.
@@ -2724,7 +2717,22 @@
   //
   // On the off-chance that there's some target instruction feeding into the
   // instruction, let's use produceSameValue instead of isIdenticalTo.
-  return Builder.getTII().produceSameValue(*I1, *I2, &MRI);
+  if (Builder.getTII().produceSameValue(*I1, *I2, &MRI)) {
+    // Handle instructions with multiple defs that produce same values. Values
+    // are same for operands with same index.
+    //
+    // %0:_(s64), %1:_(s64) = G_UNMERGE_VALUES %2:_(<2 x s64>)
+    // Even though %0 and %1 are produced by the same instruction they are not
+    // the same values.
+    //
+    // %0:_(s8), %1:_(s8), %2:_(s8), %3:_(s8) = G_UNMERGE_VALUES %4:_(<4 x s8>)
+    // %5:_(s8), %6:_(s8), %7:_(s8), %8:_(s8) = G_UNMERGE_VALUES %4:_(<4 x s8>)
+    // I1 and I2 are different instructions but produce same values,
+    // %1 and %6 are same, %1 and %7 are not the same value.
+    return I1->findRegisterDefOperandIdx(InstAndDef1->Reg) ==
+           I2->findRegisterDefOperandIdx(InstAndDef2->Reg);
+  }
+  return false;
 }
 
 bool CombinerHelper::matchConstantOp(const MachineOperand &MOP, int64_t C) {
Index: llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizercombiner-select.mir
===================================================================
--- llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizercombiner-select.mir
+++ llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizercombiner-select.mir
@@ -50,3 +50,49 @@
     SI_RETURN_TO_EPILOG $vgpr0
 
 ...
+
+---
+name: select_different_result_from_different_unmerge_values_with_the_same_source
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1_vgpr2_vgpr3, $vgpr4
+
+    ; GCN-LABEL: name: select_different_result_from_different_unmerge_values_with_the_same_source
+    ; GCN: liveins: $vgpr0_vgpr1_vgpr2_vgpr3, $vgpr4
+    ; GCN: [[COPY:%[0-9]+]]:_(<4 x s32>) = COPY $vgpr0_vgpr1_vgpr2_vgpr3
+    ; GCN: [[COPY1:%[0-9]+]]:_(s32) = COPY $vgpr4
+    ; GCN: [[TRUNC:%[0-9]+]]:_(s1) = G_TRUNC [[COPY1]](s32)
+    ; GCN: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32), [[UV2:%[0-9]+]]:_(s32), [[UV3:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](<4 x s32>)
+    ; GCN: [[UV4:%[0-9]+]]:_(s32), [[UV5:%[0-9]+]]:_(s32), [[UV6:%[0-9]+]]:_(s32), [[UV7:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](<4 x s32>)
+    ; GCN: [[SELECT:%[0-9]+]]:_(s32) = G_SELECT [[TRUNC]](s1), [[UV1]], [[UV7]]
+    ; GCN: $vgpr0 = COPY [[SELECT]](s32)
+    %0:_(<4 x s32>) = COPY $vgpr0_vgpr1_vgpr2_vgpr3
+    %1:_(s32) = COPY $vgpr4
+    %2:_(s1) = G_TRUNC %1:_(s32)
+    %3:_(s32), %4:_(s32), %5:_(s32), %6:_(s32) = G_UNMERGE_VALUES %0:_(<4 x s32>)
+    %7:_(s32), %8:_(s32), %9:_(s32), %10:_(s32) = G_UNMERGE_VALUES %0:_(<4 x s32>)
+    %11:_(s32) = G_SELECT %2:_(s1), %4:_, %10:_
+    $vgpr0 = COPY %11
+...
+
+---
+name: select_same_result_from_different_unmerge_values_with_the_same_source
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1_vgpr2_vgpr3, $vgpr4
+
+    ; GCN-LABEL: name: select_same_result_from_different_unmerge_values_with_the_same_source
+    ; GCN: liveins: $vgpr0_vgpr1_vgpr2_vgpr3, $vgpr4
+    ; GCN: [[COPY:%[0-9]+]]:_(<4 x s32>) = COPY $vgpr0_vgpr1_vgpr2_vgpr3
+    ; GCN: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32), [[UV2:%[0-9]+]]:_(s32), [[UV3:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](<4 x s32>)
+    ; GCN: $vgpr0 = COPY [[UV1]](s32)
+    %0:_(<4 x s32>) = COPY $vgpr0_vgpr1_vgpr2_vgpr3
+    %1:_(s32) = COPY $vgpr4
+    %2:_(s1) = G_TRUNC %1:_(s32)
+    %3:_(s32), %4:_(s32), %5:_(s32), %6:_(s32) = G_UNMERGE_VALUES %0:_(<4 x s32>)
+    %7:_(s32), %8:_(s32), %9:_(s32), %10:_(s32) = G_UNMERGE_VALUES %0:_(<4 x s32>)
+    %11:_(s32) = G_SELECT %2:_(s1), %4:_, %8:_
+    $vgpr0 = COPY %11
+...