diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1212,7 +1212,10 @@
       }
       ++I;
     }
-    LIS->extendToIndices(SR, EndPoints);
+    SmallVector<SlotIndex, 8> Undefs;
+    IntB.computeSubRangeUndefs(Undefs, SR.LaneMask, *MRI,
+                               *LIS->getSlotIndexes());
+    LIS->extendToIndices(SR, EndPoints, Undefs);
   }
   // If any dead defs were extended, truncate them.
   shrinkToUses(&IntB);
diff --git a/llvm/test/CodeGen/AMDGPU/coalescer-removepartial-extend-undef-subrange.mir b/llvm/test/CodeGen/AMDGPU/coalescer-removepartial-extend-undef-subrange.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/coalescer-removepartial-extend-undef-subrange.mir
@@ -0,0 +1,310 @@
+# RUN: llc -march=amdgcn -run-pass simple-register-coalescing -verify-machineinstrs -o - %s | FileCheck %s
+#
+# CHECK-LABEL: bb.4:
+# CHECK-NOT:     COPY
+# CHECK-LABEL: bb.5:
+#
+# After some coalescing work, the copies in bb.4
+# change from:
+#     %12:vreg_512 = COPY killed %244
+#    %242:vreg_512 = COPY killed %12
+# to:
+#    %90:vreg_512 = COPY %38:vreg_512
+#
+# The failure occurs when processing this COPY in removePartialRedundency(). The
+# coalescer tries to prune and extend one subrange of %90, the offending subrange
+# has a def location in the predecessor path going through bb.26 into bb.4. But for
+# another predecessor path 0->1->27->4, the subrange has only one undef location in bb.0.
+# Then in findReachingDef(), it will search all the way back through the predecessor
+# path: 4->27->1->0 and failed to find a reaching-def (the undef in bb.0) in that
+# path and return error message:
+# "Use of $noreg does not have a corresponding definition on every path
+#  LLVM ERROR: Use not jointly dominated by defs"
+
+---
+name:            _amdgpu_ps_main
+alignment:       1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: $sgpr2, $sgpr3, $vgpr3
+
+    %0:vgpr_32 = COPY killed $vgpr3
+    %1:sgpr_32 = COPY killed $sgpr3
+    %2:sgpr_32 = COPY killed $sgpr2
+    %3:sreg_64 = S_GETPC_B64
+    %4:sreg_64 = COPY killed %3
+    %4.sub0:sreg_64 = COPY killed %2
+    %5:sgpr_128 = S_LOAD_DWORDX4_IMM killed %4, 0, 0, 0
+    %6:sgpr_32 = S_BUFFER_LOAD_DWORD_IMM killed %5, 1, 0, 0 :: (dereferenceable invariant load 4)
+    %7:vgpr_32 = V_MOV_B32_e32 1092616192, implicit $exec
+    %8:vgpr_32 = nnan nsz arcp contract afn reassoc nofpexcept V_MUL_F32_e32 %6, killed %7, implicit $mode, implicit $exec
+    %9:vgpr_32 = V_MOV_B32_e32 1065353216, implicit $exec
+    undef %10.sub0:vreg_512 = COPY killed %9
+    %10.sub2:vreg_512 = COPY %8
+    %10.sub3:vreg_512 = COPY %8
+    %11:sreg_64 = nofpexcept V_CMP_GT_F32_e64 0, 1065353216, 0, %0, 0, implicit $mode, implicit $exec
+    %12:sreg_64 = nofpexcept V_CMP_NGT_F32_e64 0, 1065353216, 0, %0, 0, implicit $mode, implicit $exec
+    %13:sreg_64_xexec = nofpexcept V_CMP_LT_F32_e64 0, 1065353216, 0, killed %1, 0, implicit $mode, implicit $exec
+    %14:sreg_64 = nofpexcept V_CMP_GT_F32_e64 0, 0, 0, %0, 0, implicit $mode, implicit $exec
+    %15:sreg_64 = nofpexcept V_CMP_NGT_F32_e64 0, 0, 0, killed %0, 0, implicit $mode, implicit $exec
+    %16:sreg_64 = COPY %11
+    %17:vreg_512 = COPY %10
+    %18:sreg_64 = COPY $exec, implicit-def $exec
+    %19:sreg_64 = S_AND_B64 %18, %12, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %19
+    S_CBRANCH_EXECZ %bb.2, implicit $exec
+    S_BRANCH %bb.1
+
+  bb.1:
+    %20:sreg_64 = nofpexcept V_CMP_NGT_F32_e64 0, 1065353216, 0, killed %6, 0, implicit $mode, implicit $exec
+    %21:sreg_64 = S_MOV_B64 -1
+    %22:sreg_64 = S_AND_B64 $exec, killed %20, implicit-def dead $scc
+    $vcc = COPY killed %22
+    S_CBRANCH_VCCNZ %bb.3, implicit killed $vcc
+    S_BRANCH %bb.27
+
+  bb.2:
+    successors: %bb.13, %bb.17
+
+    $exec = S_OR_B64 $exec, killed %18, implicit-def $scc
+    %23:vreg_512 = COPY killed %17
+    %24:sreg_64 = COPY killed %16
+    %25:sreg_64 = COPY $exec, implicit-def $exec
+    %26:sreg_64 = S_AND_B64 %25, killed %24, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %26
+    S_CBRANCH_EXECZ %bb.17, implicit $exec
+    S_BRANCH %bb.13
+
+  bb.3:
+    %27:sreg_64 = S_MOV_B64 0
+    %28:sreg_64 = IMPLICIT_DEF
+    %29:sreg_64 = IMPLICIT_DEF
+    %30:sreg_64 = COPY killed %27
+    %31:vreg_512 = COPY killed %10
+    S_BRANCH %bb.5
+
+  bb.4:
+    %32:sreg_64 = COPY killed %33
+    %34:sreg_64 = S_ANDN2_B64 killed %11, $exec, implicit-def dead $scc
+    %35:sreg_64 = S_AND_B64 killed %32, $exec, implicit-def dead $scc
+    %36:sreg_64 = S_OR_B64 killed %34, killed %35, implicit-def dead $scc
+    %16:sreg_64 = COPY killed %36
+    %37:vreg_512 = COPY killed %38
+    %17:vreg_512 = COPY killed %37
+    S_BRANCH %bb.2
+
+  bb.5:
+    successors: %bb.6, %bb.7
+
+    %39:vreg_512 = COPY killed %31
+    %40:sreg_64 = COPY killed %30
+    %41:sreg_64 = COPY killed %29
+    %42:sreg_64 = COPY killed %28
+    %43:sreg_64 = S_MOV_B64 -1
+    %44:vgpr_32 = V_CNDMASK_B32_e64 0, 0, 0, 1, %13, implicit $exec
+    V_CMP_NE_U32_e32 1, killed %44, implicit-def $vcc, implicit $exec
+    $vcc = S_AND_B64 $exec, killed $vcc, implicit-def dead $scc
+    %45:sreg_64 = COPY %43
+    %46:vreg_512 = COPY %39
+    S_CBRANCH_VCCNZ %bb.7, implicit killed $vcc
+    S_BRANCH %bb.6
+
+  bb.6:
+    successors: %bb.8, %bb.9
+
+    %47:sreg_64 = S_MOV_B64 0
+    %48:sreg_64 = COPY killed %47
+    %49:vreg_512 = COPY %39
+    %50:sreg_64 = COPY $exec, implicit-def $exec
+    %51:sreg_64 = S_AND_B64 %50, %15, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %51
+    S_CBRANCH_EXECZ %bb.9, implicit $exec
+    S_BRANCH %bb.8
+
+  bb.7:
+    successors: %bb.10, %bb.11
+
+    %52:vreg_512 = COPY killed %46
+    %53:sreg_64 = COPY killed %45
+    %54:sreg_64 = S_OR_B64 killed %41, $exec, implicit-def dead $scc
+    %55:sreg_64 = COPY killed %43
+    %56:sreg_64 = COPY %54
+    %57:sreg_64 = COPY $exec, implicit-def $exec
+    %58:sreg_64 = S_AND_B64 %57, killed %53, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %58
+    S_CBRANCH_EXECZ %bb.11, implicit $exec
+    S_BRANCH %bb.10
+
+  bb.8:
+    undef %59.sub0:vreg_512 = COPY %8
+    %59.sub2:vreg_512 = COPY %39.sub2
+    %59.sub3:vreg_512 = COPY %39.sub3
+    %60:vreg_512 = COPY killed %59
+    %61:sreg_64 = COPY $exec
+    %48:sreg_64 = COPY killed %61
+    %49:vreg_512 = COPY killed %60
+
+  bb.9:
+    $exec = S_OR_B64 $exec, killed %50, implicit-def $scc
+    %62:vreg_512 = COPY killed %49
+    %63:sreg_64 = COPY killed %48
+    %45:sreg_64 = COPY killed %63
+    %46:vreg_512 = COPY killed %62
+    S_BRANCH %bb.7
+
+  bb.10:
+    successors: %bb.12, %bb.25
+
+    %64:sreg_64 = S_MOV_B64 -1
+    %65:sreg_64 = COPY killed %64
+    %66:sreg_64 = COPY $exec, implicit-def $exec
+    %67:sreg_64 = S_AND_B64 %66, %15, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %67
+    S_CBRANCH_EXECZ %bb.25, implicit $exec
+    S_BRANCH %bb.12
+
+  bb.11:
+    successors: %bb.26(0x04000000), %bb.5(0x7c000000)
+
+    $exec = S_OR_B64 $exec, killed %57, implicit-def $scc
+    %68:sreg_64 = COPY killed %56
+    %69:sreg_64 = COPY killed %55
+    %70:sreg_64 = S_AND_B64 $exec, killed %69, implicit-def $scc
+    %71:sreg_64 = S_OR_B64 %70, killed %40, implicit-def $scc
+    %72:sreg_64 = S_ANDN2_B64 killed %42, $exec, implicit-def dead $scc
+    %73:sreg_64 = S_AND_B64 %68, $exec, implicit-def dead $scc
+    %74:sreg_64 = S_OR_B64 killed %72, killed %73, implicit-def dead $scc
+    %28:sreg_64 = COPY %74
+    %29:sreg_64 = COPY killed %68
+    %30:sreg_64 = COPY %71
+    %31:vreg_512 = COPY killed %52
+    $exec = S_ANDN2_B64_term $exec, %71, implicit-def $scc
+    S_CBRANCH_EXECNZ %bb.5, implicit $exec
+    S_BRANCH %bb.26
+
+  bb.12:
+    %75:sreg_64 = S_XOR_B64 $exec, -1, implicit-def dead $scc
+    %65:sreg_64 = COPY killed %75
+    S_BRANCH %bb.25
+
+  bb.13:
+    successors: %bb.14, %bb.18
+
+    %76:sreg_64 = S_MOV_B64 -1
+    %77:sreg_64 = COPY killed %76
+    %78:vreg_512 = COPY %23
+    %79:sreg_64 = COPY $exec, implicit-def $exec
+    %80:sreg_64 = S_AND_B64 %79, %12, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %80
+    S_CBRANCH_EXECZ %bb.18, implicit $exec
+    S_BRANCH %bb.14
+
+  bb.14:
+    successors: %bb.19, %bb.20
+
+    %81:sreg_64 = S_MOV_B64 0
+    %82:sreg_64 = COPY killed %81
+    %83:vreg_512 = COPY %23
+    %84:sreg_64 = COPY $exec, implicit-def $exec
+    %85:sreg_64 = S_AND_B64 %84, killed %15, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %85
+    S_CBRANCH_EXECZ %bb.20, implicit $exec
+    S_BRANCH %bb.19
+
+  bb.15:
+    S_BRANCH %bb.23
+
+  bb.16:
+
+  bb.17:
+    $exec = S_OR_B64 $exec, killed %25, implicit-def $scc
+    %86:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    EXP_DONE 0, killed %86, undef %87:vgpr_32, undef %88:vgpr_32, undef %89:vgpr_32, -1, 0, 1, implicit $exec
+    S_ENDPGM 0
+
+  bb.18:
+    successors: %bb.21, %bb.16
+
+    $exec = S_OR_B64 $exec, killed %79, implicit-def $scc
+    %90:vreg_512 = COPY killed %78
+    %91:sreg_64 = COPY killed %77
+    %92:sreg_64 = COPY $exec, implicit-def $exec
+    %93:sreg_64 = S_AND_B64 %92, killed %91, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %93
+    S_CBRANCH_EXECZ %bb.16, implicit $exec
+    S_BRANCH %bb.21
+
+  bb.19:
+    undef %94.sub0:vreg_512 = COPY %23.sub2
+    %94.sub3:vreg_512 = COPY %23.sub3
+    %95:vreg_512 = COPY killed %94
+    %96:sreg_64 = COPY $exec
+    %82:sreg_64 = COPY killed %96
+    %83:vreg_512 = COPY killed %95
+
+  bb.20:
+    $exec = S_OR_B64 $exec, killed %84, implicit-def $scc
+    %97:vreg_512 = COPY killed %83
+    %98:sreg_64 = COPY killed %82
+    %99:sreg_64 = S_ORN2_B64 killed %98, $exec, implicit-def dead $scc
+    %77:sreg_64 = COPY killed %99
+    %78:vreg_512 = COPY killed %97
+    S_BRANCH %bb.18
+
+  bb.21:
+    successors: %bb.22, %bb.23
+
+    %100:sreg_64 = COPY $exec, implicit-def $exec
+    %101:sreg_64 = S_AND_B64 %100, killed %12, implicit-def dead $scc
+    $exec = S_MOV_B64_term killed %101
+    S_CBRANCH_EXECZ %bb.23, implicit $exec
+    S_BRANCH %bb.22
+
+  bb.22:
+    %102:vgpr_32 = COPY %90.sub0
+    %103:sreg_64 = S_MOV_B64 0
+    %104:sreg_64 = COPY killed %103
+    %105:vgpr_32 = COPY killed %102
+    S_BRANCH %bb.24
+
+  bb.23:
+    S_BRANCH %bb.16
+
+  bb.24:
+    successors: %bb.15(0x04000000), %bb.24(0x7c000000)
+
+    %106:sreg_64 = COPY killed %104
+    %107:sreg_64 = S_AND_B64 $exec, %14, implicit-def $scc
+    %108:sreg_64 = S_OR_B64 %107, killed %106, implicit-def $scc
+    %109:vgpr_32 = COPY killed %105
+    %110:vgpr_32 = V_CNDMASK_B32_e64 0, killed %109, 0, %90.sub3, %13, implicit $exec
+    %104:sreg_64 = COPY %108
+    %105:vgpr_32 = COPY killed %110
+    $exec = S_ANDN2_B64_term $exec, %108, implicit-def $scc
+    S_CBRANCH_EXECNZ %bb.24, implicit $exec
+    S_BRANCH %bb.15
+
+  bb.25:
+    $exec = S_OR_B64 $exec, killed %66, implicit-def $scc
+    %111:sreg_64 = COPY killed %65
+    %112:sreg_64 = S_ANDN2_B64 killed %54, $exec, implicit-def dead $scc
+    %113:sreg_64 = COPY killed %112
+    %114:sreg_64 = S_ORN2_B64 killed %111, $exec, implicit-def dead $scc
+    %55:sreg_64 = COPY killed %114
+    %56:sreg_64 = COPY killed %113
+    S_BRANCH %bb.11
+
+  bb.26:
+    $exec = S_OR_B64 $exec, killed %71, implicit-def $scc
+    %33:sreg_64 = COPY killed %74
+    %38:vreg_512 = COPY killed %39
+    S_BRANCH %bb.4
+
+  bb.27:
+    %33:sreg_64 = COPY killed %21
+    %38:vreg_512 = COPY killed %10
+    S_BRANCH %bb.4
+
+...