[VectorUtils] Do not try to add indices matching tombstone/empty values.

Keys matching the tombstone/empty special values cannot be inserted in a
DenseMap. Under some circumstances, LV tries to add members to an
interleave group that match the special values. Skip adding such
members. This is unlikely to have any impact in practice, because
interleave groups with such indices are very likely to not be
vectorized, due to gaps.

This issue has been surfaced by fuzzing, see
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11638

Author: fhahn

llvm/include/llvm/Analysis/VectorUtils.h

@@ -620,6 +620,11 @@ template <typename InstTy> class InterleaveGroup {
       return false;
     int32_t Key = *MaybeKey;
 
+    // Skip if the key is used for either the tombstone or empty special values.
+    if (DenseMapInfo<int32_t>::getTombstoneKey() == Key ||
+        DenseMapInfo<int32_t>::getEmptyKey() == Key)
+      return false;
+
     // Skip if there is already a member with the same index.
     if (Members.find(Key) != Members.end())
       return false;

llvm/test/Transforms/LoopVectorize/X86/interleaved-accesses-large-gap.ll

@@ -38,3 +38,63 @@ for.body:                                         ; preds = %for.body, %entry
 for.cond.cleanup:                                 ; preds = %for.body
   ret void
 }
+
+; Make sure interleave groups with a key being the special 'empty' value for
+; the map do not cause a crash.
+define void @test_gap_empty_key() {
+; CHECK-LABEL: @test_gap_empty_key()
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label %for.body
+
+; CHECK-LABEL: for.body:
+; CHECK: store i32
+; CHECK: store i32
+; CHECK-NOT: store
+;
+entry:
+  br label %for.body
+
+for.body:
+  %iv = phi i64 [ 1, %entry ], [ %iv.next, %for.body ]
+  %iv.next = add nsw i64 %iv, 1
+  %arrayidx = getelementptr inbounds [3 x i32], [3 x i32]* undef, i64 0, i64 %iv.next
+  %G2 = getelementptr i32, i32* %arrayidx, i64 %iv.next
+  %G9 = getelementptr i32, i32* %G2, i32 -2147483647
+  store i32 0, i32* %G2
+  store i32 1, i32* %G9
+  %cmp = icmp ule i64 %iv, 1000
+  br i1 false, label %for.body, label %exit
+
+exit:
+  ret void
+}
+
+; Make sure interleave groups with a key being the special 'tombstone' value for
+; the map do not cause a crash.
+define void @test_tombstone_key() {
+; CHECK-LABEL: @test_tombstone_key()
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label %for.body
+
+; CHECK-LABEL: for.body:
+; CHECK: store i32
+; CHECK: store i32
+; CHECK-NOT: store
+;
+entry:
+  br label %for.body
+
+for.body:
+  %iv = phi i64 [ 1, %entry ], [ %iv.next, %for.body ]
+  %iv.next = add nsw i64 %iv, 1
+  %arrayidx = getelementptr inbounds [3 x i32], [3 x i32]* undef, i64 0, i64 %iv.next
+  %G2 = getelementptr i32, i32* %arrayidx, i64 %iv.next
+  %G9 = getelementptr i32, i32* %G2, i32 -2147483648
+  store i32 0, i32* %G2
+  store i32 1, i32* %G9
+  %cmp = icmp ule i64 %iv, 1000
+  br i1 false, label %for.body, label %exit
+
+exit:
+  ret void
+}