Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -3032,7 +3032,9 @@
       // If a subrange starts at the copy then an undefined value has been
       // copied and we must remove that subrange value as well.
       VNInfo *ValueOut = Q.valueOutOrDead();
-      if (ValueOut != nullptr && Q.valueIn() == nullptr) {
+      if (ValueOut != nullptr && (Q.valueIn() == nullptr ||
+                                  (V.Identical && V.Resolution == CR_Erase &&
+                                   ValueOut->def == Def))) {
         LLVM_DEBUG(dbgs() << "\t\tPrune sublane " << PrintLaneMask(S.LaneMask)
                           << " at " << Def << "\n");
         SmallVector<SlotIndex,8> EndPoints;
@@ -3041,7 +3043,7 @@
         // Mark value number as unused.
         ValueOut->markUnused();
 
-        if (V.Identical && S.Query(OtherDef).valueOut()) {
+        if (V.Identical && S.Query(OtherDef).valueOutOrDead()) {
           // If V is identical to V.OtherVNI (and S was live at OtherDef),
           // then we can't simply prune V from S. V needs to be replaced
           // with V.OtherVNI.
Index: test/CodeGen/AMDGPU/coalescer-subranges-prune-kill-copy.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/coalescer-subranges-prune-kill-copy.mir
@@ -0,0 +1,286 @@
+# RUN: llc -march=amdgcn -mcpu=gfx900 -run-pass=simple-register-coalescing -o - %s | FileCheck -check-prefix=GCN %s
+
+# Test used to crash with message:
+# JoinVals::ConflictResolution (anonymous namespace)::JoinVals::analyzeValue(unsigned int, (anonymous namespace)::JoinVals &): Assertion `DefMI != nullptr' failed
+
+# GCN:      bb.16:
+# GCN:      S_CMP_LT_I32 %1.sub0,
+# GCN-NEXT: vreg_256 = COPY %35
+# GCN-NEXT: S_CBRANCH_SCC1 %bb.23
+
+---
+name:            test
+tracksRegLiveness: true
+liveins:
+  - { reg: '$sgpr0_sgpr1', virtual-reg: '%16' }
+body:             |
+  bb.0:
+    successors: %bb.1(0x40000000), %bb.28(0x40000000)
+    liveins: $sgpr0_sgpr1
+
+    %16:sgpr_64 = COPY killed $sgpr0_sgpr1
+    S_CBRANCH_SCC1 %bb.28, implicit undef $scc
+    S_BRANCH %bb.1
+
+  bb.1:
+    successors: %bb.28(0x40000000), %bb.2(0x40000000)
+
+    S_CBRANCH_SCC1 %bb.28, implicit undef $scc
+    S_BRANCH %bb.2
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+
+    %19:sreg_128 = S_LOAD_DWORDX4_IMM killed %16, 36, 0, 0
+    %22:vgpr_32 = V_CVT_F32_U32_e32 %19.sub2, implicit $exec
+    %23:vgpr_32 = V_RCP_IFLAG_F32_e32 killed %22, implicit $exec
+    %25:vgpr_32 = V_MUL_F32_e32 1333788672, killed %23, implicit $exec
+    %26:vgpr_32 = V_CVT_U32_F32_e32 killed %25, implicit $exec
+    %133:vgpr_32 = V_MUL_HI_U32 %26, %19.sub2, implicit $exec
+    %134:vgpr_32 = V_MUL_LO_U32 %26, %19.sub2, implicit $exec
+    %135:vgpr_32 = V_SUB_U32_e32 0, %134, implicit $exec
+    %33:sreg_64_xexec = V_CMP_EQ_U32_e64 0, killed %133, implicit $exec
+    %35:vgpr_32 = V_CNDMASK_B32_e64 0, killed %134, 0, killed %135, %33, implicit $exec
+    %136:vgpr_32 = V_MUL_HI_U32 killed %35, %26, implicit $exec
+    %138:vgpr_32 = V_ADD_U32_e32 %26, %136, implicit $exec
+    %137:vgpr_32 = V_SUB_U32_e32 killed %26, killed %136, implicit $exec
+    %45:vgpr_32 = V_CNDMASK_B32_e64 0, killed %137, 0, killed %138, killed %33, implicit $exec
+    %139:vgpr_32 = V_MUL_HI_U32 killed %45, %19.sub1, implicit $exec
+    %142:vgpr_32 = V_MUL_LO_U32 %139, %19.sub2, implicit $exec
+    %143:vgpr_32 = V_SUB_U32_e32 %19.sub1, %142, implicit $exec
+    %52:sreg_64 = V_CMP_LE_U32_e64 %19.sub2, killed %143, implicit $exec
+    %54:sreg_64_xexec = V_CMP_GE_U32_e64 %19.sub1, killed %142, implicit $exec
+    %56:sreg_64_xexec = S_AND_B64 killed %52, %54, implicit-def dead $scc
+    %141:vgpr_32 = V_ADD_U32_e32 1, %139, implicit $exec
+    %140:vgpr_32 = V_ADD_U32_e32 -1, %139, implicit $exec
+    %61:vgpr_32 = V_CNDMASK_B32_e64 0, killed %139, 0, killed %141, killed %56, implicit $exec
+    %64:vgpr_32 = V_CNDMASK_B32_e64 0, killed %140, 0, killed %61, killed %54, implicit $exec
+    %145:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    undef %144.sub0:vreg_64 = COPY killed %64
+    %144.sub1:vreg_64 = COPY killed %145
+
+  bb.3:
+    successors: %bb.5(0x40000000), %bb.4(0x40000000)
+
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 8, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.5, implicit killed $scc
+    S_BRANCH %bb.4
+
+  bb.4:
+    successors: %bb.28(0x40000000), %bb.6(0x40000000)
+
+    S_CMP_EQ_U32 %19.sub0, target-flags(amdgpu-gotprel) 8, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.28, implicit killed $scc
+    S_BRANCH %bb.6
+
+  bb.5:
+    successors: %bb.28(0x40000000), %bb.6(0x40000000)
+
+    S_CMP_LG_U32 %19.sub0, target-flags(amdgpu-gotprel) 7, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.6, implicit killed $scc
+    S_BRANCH %bb.28
+
+  bb.6:
+    successors: %bb.7(0x80000000)
+
+
+  bb.7:
+    successors: %bb.8(0x80000000)
+
+    %72:sreg_64 = V_CMP_NE_U64_e64 0, killed %144, implicit $exec
+    %109:sreg_32 = S_MOV_B32 0
+    %115:sreg_64 = S_AND_B64 $exec, killed %72, implicit-def dead $scc
+    %170:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+
+  bb.8:
+    successors: %bb.9(0x80000000)
+
+
+  bb.9:
+    successors: %bb.15(0x40000000), %bb.10(0x40000000)
+
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 4, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.15, implicit killed $scc
+    S_BRANCH %bb.10
+
+  bb.10:
+    successors: %bb.11(0x40000000), %bb.12(0x40000000)
+
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 5, implicit-def $scc
+    S_CBRANCH_SCC0 %bb.12, implicit killed $scc
+
+  bb.11:
+    successors: %bb.21(0x80000000)
+
+    undef %75.sub0:vreg_256 = COPY %170
+    %75.sub1:vreg_256 = COPY %170
+    %75.sub3:vreg_256 = COPY %170
+    %75.sub4:vreg_256 = COPY %170
+    %75.sub5:vreg_256 = COPY %170
+    %75.sub6:vreg_256 = COPY %170
+    %75.sub7:vreg_256 = COPY %170
+    %204:vreg_256 = COPY killed %75
+    S_BRANCH %bb.21
+
+  bb.12:
+    successors: %bb.13(0x40000000), %bb.14(0x40000000)
+
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 6, implicit-def $scc
+    S_CBRANCH_SCC0 %bb.14, implicit killed $scc
+
+  bb.13:
+    successors: %bb.20(0x80000000)
+
+    undef %79.sub0:vreg_256 = COPY %170
+    %79.sub1:vreg_256 = COPY %170
+    %79.sub3:vreg_256 = COPY %170
+    %79.sub4:vreg_256 = COPY %170
+    %79.sub5:vreg_256 = COPY %170
+    %79.sub6:vreg_256 = COPY %170
+    %79.sub7:vreg_256 = COPY %170
+    %203:vreg_256 = COPY killed %79
+    S_BRANCH %bb.20
+
+  bb.14:
+    successors: %bb.19(0x40000000), %bb.25(0x40000000)
+
+    S_CMP_EQ_U32 %19.sub0, target-flags(amdgpu-gotprel) 6, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.19, implicit killed $scc
+    S_BRANCH %bb.25
+
+  bb.15:
+    successors: %bb.17(0x40000000), %bb.16(0x40000000)
+
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 2, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.17, implicit killed $scc
+    S_BRANCH %bb.16
+
+  bb.16:
+    successors: %bb.23(0x40000000), %bb.22(0x40000000)
+
+    undef %103.sub0:vreg_256 = COPY %170
+    %103.sub1:vreg_256 = COPY %170
+    %103.sub2:vreg_256 = COPY %170
+    %103.sub3:vreg_256 = COPY %170
+    %103.sub4:vreg_256 = COPY %170
+    %103.sub5:vreg_256 = COPY %170
+    %103.sub6:vreg_256 = COPY %170
+    %103.sub7:vreg_256 = COPY %170
+    S_CMP_LT_I32 %19.sub0, target-flags(amdgpu-gotprel) 3, implicit-def $scc
+    %205:vreg_256 = COPY %103
+    %206:vreg_256 = COPY killed %103
+    S_CBRANCH_SCC1 %bb.23, implicit killed $scc
+    S_BRANCH %bb.22
+
+  bb.17:
+    successors: %bb.18(0x40000000), %bb.25(0x40000000)
+
+    S_CMP_EQ_U32 %19.sub0, target-flags(amdgpu-gotprel) 1, implicit-def $scc
+    S_CBRANCH_SCC0 %bb.25, implicit killed $scc
+
+  bb.18:
+    successors: %bb.24(0x80000000)
+
+    undef %108.sub0:vreg_256 = COPY %170
+    %108.sub1:vreg_256 = COPY %170
+    %108.sub2:vreg_256 = COPY %170
+    %108.sub3:vreg_256 = COPY %170
+    %108.sub4:vreg_256 = COPY %170
+    %108.sub5:vreg_256 = COPY %170
+    %108.sub6:vreg_256 = COPY %170
+    %108.sub7:vreg_256 = COPY %170
+    %207:vreg_256 = COPY killed %108
+    S_BRANCH %bb.24
+
+  bb.19:
+    successors: %bb.20(0x80000000)
+
+    %84:vgpr_32 = GLOBAL_LOAD_DWORD undef %86:vreg_64, 0, 0, 0, 0, implicit $exec
+    undef %178.sub5:vreg_256 = COPY killed %84
+    %3:vreg_256 = COPY killed %178
+    %203:vreg_256 = COPY killed %3
+
+  bb.20:
+    successors: %bb.21(0x80000000)
+
+    %4:vreg_256 = COPY killed %203
+    %5:vreg_256 = COPY killed %4
+    %204:vreg_256 = COPY killed %5
+
+  bb.21:
+    successors: %bb.22(0x80000000)
+
+    %6:vreg_256 = COPY killed %204
+    %7:vreg_256 = COPY killed %6
+    %205:vreg_256 = COPY killed %7
+
+  bb.22:
+    successors: %bb.23(0x80000000)
+
+    %8:vreg_256 = COPY killed %205
+    %9:vreg_256 = COPY killed %8
+    %9.sub2:vreg_256 = COPY %109
+    %206:vreg_256 = COPY killed %9
+
+  bb.23:
+    successors: %bb.24(0x80000000)
+
+    %10:vreg_256 = COPY killed %206
+    %11:vreg_256 = COPY killed %10
+    %207:vreg_256 = COPY killed %11
+
+  bb.24:
+    successors: %bb.26(0x80000000)
+
+    %12:vreg_256 = COPY killed %207
+    %13:vreg_256 = COPY killed %12
+    %208:vreg_256 = COPY killed %13
+    S_BRANCH %bb.26
+
+  bb.25:
+    successors: %bb.26(0x80000000)
+
+    undef %112.sub0:vreg_256 = COPY %170
+    %112.sub1:vreg_256 = COPY %170
+    %112.sub2:vreg_256 = COPY %170
+    %112.sub3:vreg_256 = COPY %170
+    %112.sub4:vreg_256 = COPY %170
+    %112.sub5:vreg_256 = COPY %170
+    %112.sub6:vreg_256 = COPY %170
+    %112.sub7:vreg_256 = COPY %170
+    %208:vreg_256 = COPY killed %112
+
+  bb.26:
+    successors: %bb.8(0x7c000000), %bb.27(0x04000000)
+
+    %14:vreg_256 = COPY killed %208
+    $vcc = COPY %115
+    S_CBRANCH_VCCNZ %bb.8, implicit killed $vcc
+    S_BRANCH %bb.27
+
+  bb.27:
+    successors: %bb.28(0x80000000)
+
+    %187:vgpr_32 = COPY %14.sub7
+    %192:vgpr_32 = COPY %14.sub6
+    %193:vgpr_32 = COPY %14.sub5
+    %194:vgpr_32 = COPY %14.sub4
+    undef %188.sub0:vreg_128 = COPY killed %194
+    %188.sub1:vreg_128 = COPY killed %193
+    %188.sub2:vreg_128 = COPY killed %192
+    %188.sub3:vreg_128 = COPY killed %187
+    GLOBAL_STORE_DWORDX4 undef %123:vreg_64, killed %188, 0, 0, 0, 0, implicit $exec
+    %195:vgpr_32 = COPY %14.sub3
+    %200:vgpr_32 = COPY %14.sub2
+    %201:vgpr_32 = COPY %14.sub1
+    %202:vgpr_32 = COPY killed %14.sub0
+    undef %196.sub0:vreg_128 = COPY killed %202
+    %196.sub1:vreg_128 = COPY killed %201
+    %196.sub2:vreg_128 = COPY killed %200
+    %196.sub3:vreg_128 = COPY killed %195
+    GLOBAL_STORE_DWORDX4 undef %131:vreg_64, killed %196, 0, 0, 0, 0, implicit $exec
+
+  bb.28:
+    S_ENDPGM 0
+
+...