<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/141863>141863</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Bad codegen for reversing bits in bytes without swapping bytes
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          tom-rein
      </td>
    </tr>
</table>

<pre>
    The following code on RISC-V target with Zbkb extension ([godbolt](https://godbolt.org/z/cTqEYsbaz)):
```C++
#include <cstdint>
uint8_t brev8(uint8_t x) {
    return __builtin_bitreverse8(x);
}
```
Results in:
```
brev8(unsigned char):
 rev8    a0, a0
        brev8   a0, a0
        srli    a0, a0, 56
 ret
```
The expected result would be just `brev8 a0, a0` since it doesn't swap bytes.

On armv8 target (and similarly on armv7) bit reversing bytes in `uint16_t` uses `rev16` which results in an extra shift:
```C++
uint16_t brev8(uint16_t x) {
    return __builtin_bswap16(__builtin_bitreverse16(x));
}
```
```
brev8(unsigned short):
        rbit    w8, w0
        lsr     w8, w8, #16
        rev16 w0, w8
        ret
```
This could instead be:
```
brev8(unsigned short):
        rbit    w0, w0
        rev     w0, w0
 ret
```
(Also the register allocation somehow uses w8 instead of reusing w0, which I find weird, since that is done on other targets, including armv7)

When doing `bswap16` first and then `bitreverse16`, it doesn't even use `rev16`, but adds two shifts:
```C++
uint16_t brev8_2(uint16_t x) {
 return __builtin_bitreverse16(__builtin_bswap16(x));
}
```
```
brev8_2(unsigned short):
        rev     w8, w0
        lsr     w8, w8, #16
        rbit w8, w8
        lsr     w0, w8, #16
        ret
```
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJyclcFu4zgPx59GuRAtZDl2nIMPaTsB5vQB8w12sXsJZIuJNatIXYmO23n6Be0kk6addnYNw0FEUfxL_InUKdmdR6xFcSeKh5nuqQuxprC_iWj9rAnmuf7aIWyDc2GwfgdtMAjBw5fP_7-_-Q1Ixx0SDJY6-LP5qwF8IvTJBg9CVaK42wXTBEeieBCq6ogek8hXQq2FWh9NtyHuhFp_F2rdfv370x-p0d-FWvKbr4RciVJO771Qd_zKlVC59a3rDYLI79tExnoS-SchV731VG0ImoiHSqjq9P9JqCWIBXsDAESkPnrYbJreOrJ-01iKeMCYkL2exuhjqMXDpQYhV18w9Y4SWH-lT8jVOaofD9ZA2-l43gmwlaNrKdQ9fycx_DRH2xumFJ194aXuoSin9ehKACcLnx6xJTQQR6UwhN4ZaBC-9YlAlHKKdV6ulJCsbxEsgQmYvFALgjToR2ieCdMtx5Cr_3nQcX-oTjkXqtLeQLJ763R0z0wFT1jwSTeWYDpPhmZcBqzn4JyQrNwQh-0TJh6LeMhKHhg623ZH3aOD9kxU1JA6u6WfAnFa9EXax4EP8877zEqhqrdYGA1PJxrf5uHd_KcuRPoBwPGJfDwAMFScguEy2S5FuDCNX6HyrLx05-Nit2nGpeE1DzZBOwJgfSLUDMIvcPuBbvlKd8QDXJteyxGqWrkUgDqEiDubCCNo50KriWtGCnvswjCBMVRnzWELEfuRpWOAEZTPsLXewIA2Gh6dMKZOE9gEJvixVAXqMB6pTTxtqh282InXifDfO_RgAhv4lhzJKCVsbUwETDvxFDZeIlKOkl7cHjyg511c0s2Tmp5AG5OAhjAxnX4R6o36Gdbv1LJrsM-w_1ump-gf0HFC4D9SzXCdJ7zhK9-7ES9Im5k6N8t8qWdYZ4t5NS-rqsxmXd2UplF6u0CVmfm2ksuqKNpto7DayrZZLma2VlIVslCVyuaLeXZbNDKfZ6oocinbal6IucS9tu7WucOeW9fMptRjnc2zqsxnTjfo0thPlfI4wGgVSnF7jTU73TT9Lom5dDZR-rEMWXJY32kzttgdetiGeFlD7VQRp1rKDTf0U5V-PJfYWR9dfdVkLXV9c9uGvVBrDnb8uXmM4Ru2JNR6lJiEWh_3cKjVPwEAAP__GU5iow">