[DAGCombiner] Fix an off-by-one error in vector logic

Without this, we could end up trying to get the Nth (0-indexed) element
from a subvector of size N.

Differential Revision: https://reviews.llvm.org/D37880

llvm-svn: 314380

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -14501,8 +14501,8 @@ SDValue DAGCombiner::reduceBuildVecToShuffle(SDNode *N) {
     }
 
     NearestPow2 = PowerOf2Ceil(MaxIndex);
-    if (InVT.isSimple() && (NearestPow2 > 2) &&
-        ((NumElems * 2) < NearestPow2)) {
+    if (InVT.isSimple() && NearestPow2 > 2 && MaxIndex < NearestPow2 &&
+        NumElems * 2 < NearestPow2) {
       unsigned SplitSize = NearestPow2 / 2;
       EVT SplitVT = EVT::getVectorVT(*DAG.getContext(),
                                      InVT.getVectorElementType(), SplitSize);

llvm/test/CodeGen/ARM/crash-on-pow2-shufflevector.ll

@@ -0,0 +1,24 @@
+; RUN: llc < %s -mtriple=armv7 | FileCheck %s
+;
+; Ensure that don't crash given a largeish power-of-two shufflevector index.
+
+%struct.desc = type { i32, [7 x i32] }
+
+define i32 @foo(%struct.desc* %descs, i32 %num, i32 %cw) local_unnamed_addr #0 {
+; CHECK-LABEL: foo:
+; CHECK:       @ BB#0: @ %entry
+; CHECK-NEXT:    mov r1, #32
+; CHECK-NEXT:    vld1.32 {d16, d17}, [r0], r1
+; CHECK-NEXT:    vld1.32 {d18, d19}, [r0]
+; CHECK-NEXT:    vtrn.32 q8, q9
+; CHECK-NEXT:    vadd.i32 d16, d16, d16
+; CHECK-NEXT:    vmov.32 r0, d16[1]
+; CHECK-NEXT:    bx lr
+entry:
+  %descs.vec = bitcast %struct.desc* %descs to <16 x i32>*
+  %wide.vec = load <16 x i32>, <16 x i32>* %descs.vec, align 4
+  %strided.vec = shufflevector <16 x i32> %wide.vec, <16 x i32> undef, <2 x i32> <i32 0, i32 8>
+  %bin.rdx20 = add <2 x i32> %strided.vec, %strided.vec
+  %0 = extractelement <2 x i32> %bin.rdx20, i32 1
+  ret i32 %0
+}