Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -1927,6 +1927,28 @@
   // Update regalloc hint.
   TRI->updateRegAllocHint(CP.getSrcReg(), CP.getDstReg(), *MF);
 
+  if (!CP.isPhys()) {
+    // Fix up the case that some subreg was undefined at a use, and thus the
+    // use was not in the live range, but due to merging it has now become
+    // defined.
+    LiveInterval &LI = LIS->getInterval(CP.getDstReg());
+    LLVM_DEBUG(dbgs() << "\tBefore extending subranges: " << LI << "\n");
+    for (auto &S : LI.subranges()) {
+      SmallVector<SlotIndex, 8> Uses;
+      for (auto &MO : MRI->reg_operands(CP.getDstReg())) {
+        if (!MO.isUse() || MO.isUndef() || MO.isDebug())
+          continue;
+        auto OperandMask = TRI->getSubRegIndexLaneMask(MO.getSubReg());
+        if ((OperandMask & S.LaneMask).none())
+          continue;
+        Uses.push_back(LIS->getInstructionIndex(*MO.getParent()).getRegSlot());
+      }
+      SmallVector<SlotIndex, 8> Undefs;
+      LI.computeSubRangeUndefs(Undefs, S.LaneMask, *MRI, *LIS->getSlotIndexes());
+      LIS->extendToIndices(S, Uses, Undefs);
+    }
+  }
+
   LLVM_DEBUG({
     dbgs() << "\tSuccess: " << printReg(CP.getSrcReg(), TRI, CP.getSrcIdx())
            << " -> " << printReg(CP.getDstReg(), TRI, CP.getDstIdx()) << '\n';
Index: test/CodeGen/AMDGPU/coalescing-subreg-was-undef-but-became-def.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/coalescing-subreg-was-undef-but-became-def.mir
@@ -0,0 +1,183 @@
+# RUN: llc -march=amdgcn -mcpu=gfx803 -run-pass simple-register-coalescing -verify-machineinstrs -o - %s | FileCheck --check-prefix=GCN %s
+#
+# This is another example of a test giving "Couldn't join subrange!"
+#
+# This shows a case where a whole reg use point of the whole register was
+# undefined in one subreg, but after coalescing it became defined but the
+# subrange was not updated to reflect that.
+#
+# GCN: {{^body}}
+
+---
+name:            _amdgpu_cs_main
+alignment:       0
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+liveins:         
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  maxCallFrameSize: 4294967295
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      
+stack:           
+constants:       
+body:             |
+  bb.0:
+    successors: %bb.21(0x40000000), %bb.1(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.21, implicit undef $scc
+    S_BRANCH %bb.1
+  
+  bb.1:
+    successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.3, implicit undef $scc
+    S_BRANCH %bb.2
+  
+  bb.2:
+    successors: %bb.18(0x80000000)
+  
+    %36:vreg_128 = IMPLICIT_DEF
+    S_BRANCH %bb.18
+  
+  bb.3:
+    successors: %bb.5(0x40000000), %bb.4(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.5, implicit undef $scc
+    S_BRANCH %bb.4
+  
+  bb.4:
+    successors: %bb.5(0x80000000)
+  
+  
+  bb.5:
+    successors: %bb.7(0x40000000), %bb.6(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.7, implicit undef $scc
+    S_BRANCH %bb.6
+  
+  bb.6:
+    successors: %bb.7(0x80000000)
+  
+  
+  bb.7:
+    successors: %bb.9(0x40000000), %bb.8(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.9, implicit undef $scc
+    S_BRANCH %bb.8
+  
+  bb.8:
+    successors: %bb.9(0x80000000)
+  
+  
+  bb.9:
+    successors: %bb.10(0x40000000), %bb.14(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.14, implicit undef $scc
+    S_BRANCH %bb.10
+  
+  bb.10:
+    successors: %bb.12(0x40000000), %bb.11(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.12, implicit undef $scc
+    S_BRANCH %bb.11
+  
+  bb.11:
+    successors: %bb.12(0x80000000)
+  
+  
+  bb.12:
+    successors: %bb.14(0x40000000), %bb.13(0x40000000)
+  
+    S_CBRANCH_SCC1 %bb.14, implicit undef $scc
+    S_BRANCH %bb.13
+  
+  bb.13:
+    successors: %bb.14(0x80000000)
+  
+  
+  bb.14:
+    successors: %bb.15(0x40000000), %bb.16(0x40000000)
+  
+    %6:sreg_32_xm0 = S_MOV_B32 0
+    undef %7.sub2:sreg_128 = COPY %6
+    S_CBRANCH_SCC0 %bb.16, implicit undef $scc
+  
+  bb.15:
+    successors: %bb.17(0x80000000)
+  
+    undef %8.sub0:sreg_128 = COPY killed %6
+    %5:sreg_128 = COPY killed %7
+    %34:sreg_128 = COPY killed %5
+    %35:sreg_128 = COPY killed %8
+    S_BRANCH %bb.17
+  
+  bb.16:
+    successors: %bb.17(0x80000000)
+  
+    undef %18.sub0:sreg_128 = COPY %6
+    %18.sub1:sreg_128 = COPY %6
+    %18.sub2:sreg_128 = COPY killed %6
+    %16:sreg_128 = COPY killed %18
+    %15:sreg_128 = COPY killed %7
+    %34:sreg_128 = COPY killed %15
+    %35:sreg_128 = COPY killed %16
+  
+  bb.17:
+    successors: %bb.18(0x40000000), %bb.22(0x40000000)
+  
+    %2:sreg_128 = COPY killed %35
+    %1:sreg_128 = COPY killed %34
+    %24:sreg_32_xm0 = S_ADD_I32 killed %1.sub2, target-flags(amdgpu-gotprel32-hi) 1, implicit-def dead $scc
+    S_CMP_LT_I32 killed %24, 8, implicit-def $scc
+    %30:vreg_128 = COPY %2
+    %36:vreg_128 = COPY killed %30
+    S_CBRANCH_SCC1 %bb.22, implicit killed $scc
+    S_BRANCH %bb.18
+  
+  bb.18:
+    successors: %bb.19(0x30000000), %bb.20(0x50000000)
+  
+    %29:vreg_128 = COPY killed %36
+    V_CMP_NE_U32_e32 0, killed %29.sub3, implicit-def $vcc, implicit $exec
+    $vcc = S_AND_B64 $exec, killed $vcc, implicit-def dead $scc
+    S_CBRANCH_VCCNZ %bb.20, implicit killed $vcc
+    S_BRANCH %bb.19
+  
+  bb.19:
+    successors: %bb.20(0x80000000)
+  
+  
+  bb.20:
+    successors: %bb.21(0x80000000)
+  
+  
+  bb.21:
+    S_ENDPGM
+  
+  bb.22:
+    successors: %bb.18(0x80000000)
+  
+    %31:vreg_128 = COPY killed %2
+    %36:vreg_128 = COPY killed %31
+    S_BRANCH %bb.18
+
+...