Index: llvm/lib/CodeGen/LiveRangeEdit.cpp
===================================================================
--- llvm/lib/CodeGen/LiveRangeEdit.cpp
+++ llvm/lib/CodeGen/LiveRangeEdit.cpp
@@ -133,6 +133,22 @@
 
     if (OVNI != li.getVNInfoAt(UseIdx))
       return false;
+
+    // Check that subrange is live at UseIdx.
+    if (MO.getSubReg()) {
+      const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
+      LaneBitmask LM = TRI->getSubRegIndexLaneMask(MO.getSubReg());
+      for (LiveInterval::SubRange &SR : li.subranges()) {
+        if ((SR.LaneMask & LM).none())
+          continue;
+        if (!SR.liveAt(UseIdx))
+          return false;
+        // Early exit if all used lanes are checked. No need to continue.
+        LM &= ~SR.LaneMask;
+        if (LM.none())
+          break;
+      }
+    }
   }
   return true;
 }
Index: llvm/test/CodeGen/AMDGPU/remat-dead-subreg.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AMDGPU/remat-dead-subreg.mir
@@ -0,0 +1,81 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -start-before=greedy -stop-after=virtregrewriter -stress-regalloc=3 -verify-machineinstrs -o - %s | FileCheck -check-prefix=GCN %s
+
+# Check that subreg use is live at the point of materialization, not just the main range.
+# Do not rematerialize if used subreg is dead at a new index.
+
+---
+name:            dead_subreg
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    ; GCN-LABEL: name: dead_subreg
+    ; GCN: $m0 = IMPLICIT_DEF
+    ; GCN-NEXT: renamable $sgpr0_sgpr1 = S_MOV_B64 1, implicit $m0
+    ; GCN-NEXT: renamable $sgpr1 = S_MUL_I32 renamable $sgpr1, 3
+    ; GCN-NEXT: SI_SPILL_S32_SAVE killed renamable $sgpr1, %stack.0, implicit $exec, implicit $sp_reg :: (store (s32) into %stack.0, addrspace 5)
+    ; GCN-NEXT: renamable $sgpr2 = S_MOV_B32 2, implicit $m0
+    ; GCN-NEXT: renamable $sgpr1 = S_MOV_B32 3, implicit $m0
+    ; GCN-NEXT: dead %4:vgpr_32 = V_MOV_B32_e32 $sgpr0, implicit $exec, implicit killed $sgpr2, implicit killed $sgpr1
+    ; GCN-NEXT: renamable $sgpr1 = SI_SPILL_S32_RESTORE %stack.0, implicit $exec, implicit $sp_reg :: (load (s32) from %stack.0, addrspace 5)
+    ; GCN-NEXT: dead %5:vgpr_32 = V_MOV_B32_e32 killed $sgpr1, implicit $exec
+    ; GCN-NEXT: S_NOP 0, implicit killed renamable $sgpr0
+    $m0 = IMPLICIT_DEF
+    %0:sreg_64_xexec = S_MOV_B64 1, implicit $m0
+    %1:sreg_32 = S_MUL_I32 %0.sub1:sreg_64_xexec, 3
+    %2:sreg_32 = S_MOV_B32 2, implicit $m0
+    %3:sreg_32 = S_MOV_B32 3, implicit $m0
+    %4:vgpr_32 = V_MOV_B32_e32 %0.sub0:sreg_64_xexec, implicit $exec, implicit %2, implicit %3
+    %5:vgpr_32 = V_MOV_B32_e32 %1:sreg_32, implicit $exec
+    S_NOP 0, implicit %0.sub0
+...
+---
+name:            live_subreg
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    ; GCN-LABEL: name: live_subreg
+    ; GCN: $m0 = IMPLICIT_DEF
+    ; GCN-NEXT: renamable $sgpr4_sgpr5 = S_MOV_B64 1, implicit $m0
+    ; GCN-NEXT: renamable $sgpr2 = S_MOV_B32 2, implicit $m0
+    ; GCN-NEXT: SI_SPILL_S32_SAVE killed renamable $sgpr2, %stack.0, implicit $exec, implicit $sp_reg :: (store (s32) into %stack.0, addrspace 5)
+    ; GCN-NEXT: renamable $sgpr2 = S_MOV_B32 3, implicit $m0
+    ; GCN-NEXT: renamable $sgpr0 = SI_SPILL_S32_RESTORE %stack.0, implicit $exec, implicit $sp_reg :: (load (s32) from %stack.0, addrspace 5)
+    ; GCN-NEXT: dead %4:vgpr_32 = V_MOV_B32_e32 $sgpr4, implicit $exec, implicit killed $sgpr0, implicit killed $sgpr2
+    ; GCN-NEXT: renamable $sgpr0 = S_MUL_I32 renamable $sgpr5, 3
+    ; GCN-NEXT: dead %5:vgpr_32 = V_MOV_B32_e32 killed $sgpr0, implicit $exec
+    ; GCN-NEXT: S_NOP 0, implicit killed renamable $sgpr5
+    $m0 = IMPLICIT_DEF
+    %0:sreg_64_xexec = S_MOV_B64 1, implicit $m0
+    %1:sreg_32 = S_MUL_I32 %0.sub1:sreg_64_xexec, 3
+    %2:sreg_32 = S_MOV_B32 2, implicit $m0
+    %3:sreg_32 = S_MOV_B32 3, implicit $m0
+    %4:vgpr_32 = V_MOV_B32_e32 %0.sub0:sreg_64_xexec, implicit $exec, implicit %2, implicit %3
+    %5:vgpr_32 = V_MOV_B32_e32 %1:sreg_32, implicit $exec
+    S_NOP 0, implicit %0.sub1
+...
+---
+name:            partially_live_wide_subreg
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    ; GCN-LABEL: name: partially_live_wide_subreg
+    ; GCN: renamable $sgpr0_sgpr1 = IMPLICIT_DEF
+    ; GCN-NEXT: renamable $sgpr8_sgpr9_sgpr10_sgpr11 = S_LOAD_DWORDX4_IMM killed renamable $sgpr0_sgpr1, 1, 0
+    ; GCN-NEXT: renamable $sgpr4_sgpr5 = S_MOV_B64 renamable $sgpr10_sgpr11
+    ; GCN-NEXT: SI_SPILL_S64_SAVE killed renamable $sgpr4_sgpr5, %stack.0, implicit $exec, implicit $sp_reg :: (store (s64) into %stack.0, align 4, addrspace 5)
+    ; GCN-NEXT: renamable $sgpr4_sgpr5 = S_MOV_B64 3, implicit $m0
+    ; GCN-NEXT: renamable $sgpr0_sgpr1 = S_MOV_B64 2, implicit $m0
+    ; GCN-NEXT: dead %5:vgpr_32 = V_MOV_B32_e32 $sgpr8, implicit $exec, implicit killed $sgpr0_sgpr1, implicit killed $sgpr4_sgpr5
+    ; GCN-NEXT: renamable $sgpr4_sgpr5 = SI_SPILL_S64_RESTORE %stack.0, implicit $exec, implicit $sp_reg :: (load (s64) from %stack.0, align 4, addrspace 5)
+    ; GCN-NEXT: dead %6:vreg_64 = V_MOV_B64_PSEUDO killed $sgpr4_sgpr5, implicit $exec
+    ; GCN-NEXT: S_NOP 0, implicit killed renamable $sgpr8, implicit renamable $sgpr11
+    %0:sreg_64 = IMPLICIT_DEF
+    %1:sgpr_128 = S_LOAD_DWORDX4_IMM %0, 1, 0
+    %2:sreg_64 = S_MOV_B64 %1.sub2_sub3
+    %3:sreg_64 = S_MOV_B64 2, implicit $m0
+    %4:sreg_64 = S_MOV_B64 3, implicit $m0
+    %5:vgpr_32 = V_MOV_B32_e32 %1.sub0, implicit $exec, implicit %3, implicit %4
+    %6:vreg_64 = V_MOV_B64_PSEUDO %2, implicit $exec
+    S_NOP 0, implicit %1.sub0, implicit %1.sub3
+...