From https://koji.fedoraproject.org/koji/taskinfo?taskID=20011859 , the failing part is not char array subscript issue but:


[659/811] cd ../../rewriter; ../out_linux/Release/gen_usage_rewriter_dictionary_main "--usage_data_file=../third_party/japanese_usage_dictionary/usage_dict.txt" "--cforms_file=../data/rules/cforms.def" "--output_base_conjugation_suffix=../out_linux/Release/gen/rewriter/usage_base_conj_suffix.data" "--output_conjugation_suffix=../out_linux/Release/gen/rewriter/usage_conj_suffix.data" "--output_conjugation_index=../out_linux/Release/gen/rewriter/usage_conj_index.data" "--output_usage_item_array=../out_linux/Release/gen/rewriter/usage_item_array.data" "--output_string_array=../out_linux/Release/gen/rewriter/usage_string_array.data"
FAILED: gen/rewriter/usage_base_conj_suffix.data gen/rewriter/usage_conj_index.data gen/rewriter/usage_conj_suffix.data gen/rewriter/usage_item_array.data gen/rewriter/usage_string_array.data 

And ./rewriter/gen_usage_rewriter_dictionary_main.cc has a comment:
    30  // * Usage
    31  // % gen_usage_rewriter_dictionary_main
    32  //    --usage_data_file=usage_data.txt
    33  //    --cforms_file=cforms.def
    34  //    --output_base_conjugation_suffix=base_conj_suffix.data
    35  //    --output_conjugation_suffix=conj_suffix.data
    36  //    --output_conjugation_index=conj_index.data
    37  //    --output_usage_item_array=usage_item_array.data
    38  //    --output_string_array=string_array.data
    39  //
    40  // * Prerequisite
    41  // Little endian is assumed. <=================

(In reply to Mamoru TASAKA from comment #1)
> And ./rewriter/gen_usage_rewriter_dictionary_main.cc has a comment:
>     30  // * Usage
>     31  // % gen_usage_rewriter_dictionary_main
>     32  //    --usage_data_file=usage_data.txt
>     33  //    --cforms_file=cforms.def
>     34  //    --output_base_conjugation_suffix=base_conj_suffix.data
>     35  //    --output_conjugation_suffix=conj_suffix.data
>     36  //    --output_conjugation_index=conj_index.data
>     37  //    --output_usage_item_array=usage_item_array.data
>     38  //    --output_string_array=string_array.data
>     39  //
>     40  // * Prerequisite
>     41  // Little endian is assumed. <=================

The line 251 has;
   250  void Convert() {
   251    CHECK(Util::IsLittleEndian());

(In reply to Mamoru TASAKA from comment #1)
> From https://koji.fedoraproject.org/koji/taskinfo?taskID=20011859 , the
> failing part is not char array subscript issue but:

Right. so reverted the change and added s390x to ExcludeArch then.