Index: lib/CodeGen/LiveIntervalAnalysis.cpp
===================================================================
--- lib/CodeGen/LiveIntervalAnalysis.cpp
+++ lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -1396,6 +1396,11 @@
                                         LaneBitmask LaneMask) {
   LiveInterval::iterator LII = LR.find(endIdx);
   SlotIndex lastUseIdx;
+  if (LII == LR.begin()) {
+    // This happens when the function is called for a subregister that only
+    // occurs _after_ the range that is to be repaired.
+    return;
+  }
   if (LII != LR.end() && LII->start < endIdx)
     lastUseIdx = LII->end;
   else
Index: test/CodeGen/AMDGPU/merge-store-crash.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/merge-store-crash.ll
@@ -0,0 +1,36 @@
+; RUN: llc -march=amdgcn -mcpu=verde -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs < %s | FileCheck %s
+
+; This is used to crash in LiveIntervalAnalysis via SILoadStoreOptimizer
+; while fixing up the merge of two ds_write instructions.
+
+@tess_lds = external addrspace(3) global [8192 x i32]
+
+; CHECK-LABEL: {{^}}main:
+; CHECK: ds_write2_b32
+; CHECK: v_mov_b32_e32 v1, v0
+; CHECK: tbuffer_store_format_xyzw v[0:3],
+define amdgpu_vs void @main(i32 inreg %arg) {
+main_body:
+  %tmp = load float, float addrspace(3)* undef, align 4
+  %tmp1 = load float, float addrspace(3)* undef, align 4
+  store float %tmp, float addrspace(3)* null, align 4
+  %tmp2 = bitcast float %tmp to i32
+  %tmp3 = add nuw nsw i32 0, 1
+  %tmp4 = zext i32 %tmp3 to i64
+  %tmp5 = getelementptr [8192 x i32], [8192 x i32] addrspace(3)* @tess_lds, i64 0, i64 %tmp4
+  %tmp6 = bitcast i32 addrspace(3)* %tmp5 to float addrspace(3)*
+  store float %tmp1, float addrspace(3)* %tmp6, align 4
+  %tmp7 = bitcast float %tmp1 to i32
+  %tmp8 = insertelement <4 x i32> undef, i32 %tmp2, i32 0
+  %tmp9 = insertelement <4 x i32> %tmp8, i32 %tmp7, i32 1
+  %tmp10 = insertelement <4 x i32> %tmp9, i32 undef, i32 2
+  %tmp11 = insertelement <4 x i32> %tmp10, i32 undef, i32 3
+  call void @llvm.SI.tbuffer.store.v4i32(<16 x i8> undef, <4 x i32> %tmp11, i32 4, i32 undef, i32 %arg, i32 0, i32 14, i32 4, i32 1, i32 0, i32 1, i32 1, i32 0)
+  ret void
+}
+
+; Function Attrs: nounwind
+declare void @llvm.SI.tbuffer.store.v4i32(<16 x i8>, <4 x i32>, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32) #0
+
+attributes #0 = { nounwind }