Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15553,6 +15553,9 @@
   // Attempt to match a '*_extend_vector_inreg' shuffle, we just search for
   // power-of-2 extensions as they are the most likely.
   for (unsigned Scale = 2; Scale < NumElts; Scale *= 2) {
+    // Check for non power of 2 vector sizes
+    if (NumElts % Scale != 0)
+      continue;
     if (!isAnyExtend(Scale))
       continue;
 
Index: test/CodeGen/AMDGPU/dagcomb-shuffle-vecextend-non2.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/dagcomb-shuffle-vecextend-non2.ll
@@ -0,0 +1,32 @@
+; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
+
+; We are only checking that instruction selection can succeed in this case. This
+; cut down test results in no instructions, but that's fine.
+;
+; See https://llvm.org/PR33743 for details of the bug being addressed
+;
+; Checking that shufflevector with 3-vec mask is handled in
+; combineShuffleToVectorExtend
+;
+; GCN: s_endpgm
+
+define amdgpu_ps void @main(i32 %in1) local_unnamed_addr {
+.entry:
+  br i1 undef, label %bb12, label %bb
+
+bb:
+  %__llpc_global_proxy_r5.12.vec.insert = insertelement <4 x i32> undef, i32 %in1, i32 3
+  %tmp3 = shufflevector <4 x i32> %__llpc_global_proxy_r5.12.vec.insert, <4 x i32> undef, <3 x i32> <i32 undef, i32 undef, i32 1>
+  %tmp4 = bitcast <3 x i32> %tmp3 to <3 x float>
+  %a2.i123 = extractelement <3 x float> %tmp4, i32 2
+  %tmp5 = bitcast float %a2.i123 to i32
+  %__llpc_global_proxy_r2.0.vec.insert196 = insertelement <4 x i32> undef, i32 %tmp5, i32 0
+  br label %bb12
+
+bb12:
+  %__llpc_global_proxy_r2.0 = phi <4 x i32> [ %__llpc_global_proxy_r2.0.vec.insert196, %bb ], [ undef, %.entry ]
+  %tmp6 = shufflevector <4 x i32> %__llpc_global_proxy_r2.0, <4 x i32> undef, <3 x i32> <i32 1, i32 2, i32 3>
+  %tmp7 = bitcast <3 x i32> %tmp6 to <3 x float>
+  %a0.i = extractelement <3 x float> %tmp7, i32 0
+  ret void
+}