<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=http://email.email.llvm.org/c/eJy1Vk2T2jAM_TXmolkmOAmQQw7ALp1ettMZ7h0nUYhbE1PbWZb99ZUdwlJK2207nXE-bFnSk-SnpNDVMV8p0W7BIu4sOA0FQteKQqGf6L2TO_mCIKBF67ACYYw4gihLtBZ0Da5BqLXZAZtGe5YuJePrR5bejyvhRD9P_ZzEIFsyKcDK3Z7M0wrjcxobxheMZ3u6gkLYPIYNWV6BI7dQCosgLYvuWbTwev0IU-tMVzpYrZqu_QJstoSyEQZO7rm3Fi9p3T96jSCvyfGVKl8QhBVFb-W2pVAJehYMGnSdaWEIj1-Fd-njFkJ6KUOO7z5wn4ayMwZbp46wN7rqKJMsXtzUBNjpJ0XIUrTSY_Nv4nkQirbywuSO_1w2GURkYBBV1Ve_ZKrBpnnVI4cFhLqkJvg8iak2_V6hhq2Ulq83YRedg_dw0J2qKESs0YB0_jid4v1luC-F-sG_B5r9GONPAfT3BZkzCPjciM46-UTHifEljf5Q1VopfbDBWbp8p6tCK-dLyeeNc_tQFL6mse1FY222NHuhK36Y4LRMMQ6w6JA8cDaPWJbAu9WKTmt7SZwaDyeiWPQx-OwYuW0ctPoAPftIhfGZe1X7JNrjp7Pa-GaUDolHwlE6V3S-iI4bb55IBo8sfvieHQM5htQBbAaKPPbH9-Tigie_90Iv_dr7tsLnzXGPZ8d-g-dY8Ey6QcvjOvPsrHMm2iu4K8bdbCiXkG_ghWsEnvbkl3uIvuFcMF1pqoG_-XoSk6EvfKjmQbb2bZaT_2b5Bube3l8De5M6zf9PwnrDb8jX3wL8A-3fJOfMuVGVx1UWZ2IkOtdok3_suue96qxucdQZlV81DemarhiXekcTpZ6Gxx31wM9Ykue1tLaj3s_XaRxPZ6Mmj6KkrOsJZvOqnojZrEqo3dVxliTxDHE6HSlRoLI5sYBxjm0j2hJ36MM4fQF4-NAQiFJXuMXWC9L70b-DkzmPOI8mPJ5MEz6Jx_N5hlkU13w-L4t5HbEkwp2Qauzt-FY5MnkwWXRbS0IlrbOvQmoaPtUYYiGETjqF-ZoM2O_-O279dfyqK4wC7jyA_gbqyZU2>53367</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Fails to optimize nested array access p[i/N].data[i%N]
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            enhancement,
            clang:codegen
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          Quuxplusone
      </td>
    </tr>
</table>

<pre>
    Clang seems to be unable to optimize a nested array access of the form `p[i/N].data[i%N]` into a simple `((T*)p)[i]`. The C test case is
```
struct CChunk { char data[2]; };
char f(struct CChunk *p, unsigned i) { return p[i/2].data[i%2]; }
```
`clang -O2` currently produces:
```
  movl %esi, %eax
  andl $-2, %eax
  andl $1, %esi
  addq %rdi, %rax
  movb (%rsi,%rax), %al
  retq
```
but I would prefer it to produce:
```
  movzbl (%rsi,%rdi), %eax
  retq
```

A more exhaustive C++ test follows ([Godbolt](https://godbolt.org/z/3E1e6c5e3)) — GCC can optimize a few of these, but right now Clang can't optimize _any_ of these.
```
template<class T, int N>
struct Chunk {
    T data[N];
};

template<class T, int N, class IndexType>
int f(Chunk<T, N> *p, IndexType i) {
    return p[i/N].data[i%N];
}

template int f(Chunk<char,2>*, unsigned long long);  // GCC wins
template int f(Chunk<char,4>*, unsigned long long);  // GCC wins
template int f(Chunk<char,2>*, unsigned);
template int f(Chunk<char,4>*, unsigned);
template int f(Chunk<int,2>*, unsigned long long);  // GCC wins
template int f(Chunk<int,4>*, unsigned long long);
template int f(Chunk<int,2>*, unsigned);
template int f(Chunk<int,4>*, unsigned);
```
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy1VVuP2joQ_jXmZbQoOASShzwAXaq-bHUk3isnGYhPjU1tZ1n215-xQ1hKabunUiXnYo9n5pvLZ1emOZUrJfQOHOLegTdQIXRaVArDxBy83MtXBAEanccGhLXiBKKu0TkwW_AtwtbYPbBZcmDZUjK-fmLZh3EjvOjnWZiTGKQmkwKc3B_IPK0wntPYML5gvDjQExXi5jFsyPIKPLmFWjgE6VjygSWLoNePOHXedrWH1art9Fdg8yXUrbBwds-DtXRJ6-HTa0T5lhzfqPIFQVhR9E7uNIVK0Ito0KLvrIYhPH4T3rWPewjpp445fvjMQxrqzlrUXp3gYE3TUSZZurirCbA3z4qQZehkwBb-xMsgFLoJwukD_7lsMojIwCBqmm9hyTaDTfumRw4riHXJbPR5FlNt-r1CDVspLd_uwq46D5_gaDrVUIi4RQvSh3Y6x_vLcF8r9YP_ALT4McafAujfCzJnEfClFZ3z8pnaifEljb6ptkYpc3TRWbb8aJrKKB9KyfPW-0MsCl_T2PWisbE7mr3Skz5OcFZnmEZY1CSPnOUJK6bwcbWibtXXxNni8UwUhyGGkB0rd60HbY7Qs49UGJ_7N7UvQp--XNTGd6P0SDwSntK5ov4iOm6CeSIZPLH08Xt2DOQYUgewGSjy1Lfv2cUVT37vhX76tU-6wZfN6YAXx2FD4Fj0TLpRK-C68OyicyHaG7gbxt09UK4h38ELtwgC7ckvDxDDgXPFdGWoBuEV6klMhr7wsZpHqd37LE__muU7mHt7fwzsXeo0_zsJ6w2_I19_CvB_aP8mORfOjZoybYq0ECMvvcJyLaRy392R927IX3XwqLOqvDlqpG-7alybPU2Ueh4-D3Ry_os14V1L5zq6Mfg6S9PZfNSWdZpN06yYVlORN4jzpJ7Ps6poqnwm8tlkOlKiQuVK8sw4R90KXeMeQ_Dne4PH64lA1KbBHeogIHiy5AnnyYSnk9mUT9JxnhdYJOmW53ld5duETRPcUxrGAWI4Hke2jGirbudIqKTz7k1IB0VIL0YkZF90vjW2_KfrXg6qc0bjKAZXxsj-A_5If44">