<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/58109>58109</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            likely miscompile of code involving a usub.sat by AArch64 backend
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:AArch64,
            llvm:codegen,
            miscompilation
      </td>
    </tr>

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

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

<pre>
    consider this IR:
```llvm
define i64 @i(i8 signext %0) {
  %2 = add i8 %0, 1
  %3 = call i8 @llvm.usub.sat.i8(i8 %2, i8 1)
  %4 = zext i8 %3 to i64
  ret i64 %4
}

; Function Attrs: nocallback nofree nosync nounwind readnone speculatable willreturn
declare i8 @llvm.usub.sat.i8(i8, i8) #0

attributes #0 = { nocallback nofree nosync nounwind readnone speculatable willreturn }
```

if we generate aarch64 and link the result against this driver:
```c
#include <stdio.h>

unsigned long i(unsigned long);

int main(void) {
  printf("%lx\n", i(0xffffffffffffff81));
}
```

we get:
```console
$ ~/llvm-project/for-alive/bin/llc foo.ll
$ ~/llvm-project/for-alive/bin/clang foo.c foo.s
$ ./a.out 
ffffff81
$ 
```

I hope it's clear that (1) the chosen input respects the signext constraint on the argument and (2) a function returning a value that's a zext of an i8 should contain zeroes in all of the bits above the first byte

cc @nunoplopes @ryan-berger @nbushehri @Hatsunespica
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJytVNuOozgQ_Rp4sRoRCJA88JDuntHO6_6BMQV427EjX9Kd-fo5NskkvbM70koboWC7ijp1yqdqMOOlF0Y7OZJlfpGOffszqw9Z-ZqVh6wt10ep83E9GmmSmphstyzbljKrdnLHnJw1fXiWVU2ZVXuWdc-rN4tHFcvqV8bHkcF1dXlhmweHOjkIrlTy2Ca4IrgwFI77Qu5WlBgqforlBigPAbYpwPeYwupXM29ijjcfS35NGb5Xat3rdbH-18_sa9DCS6PZwXvrUASmTUxq4OINy8kS4eUuWuAV9LvUIwLzURsUxJ1IBMU9HxSxd6kUIIPVt6IJxS39jt1KLBWvqsvH1DiykUPw5JIpUUWB_4fk2L0It4t-BJYTeyc2kybLPTHOrVhQQ47ISuo3qIUA4YLyjM9caudXAY1Wnsn-KiJx3Ve11EKFkUDlxflRmmLJ6i-P0EEnSQHI6JlFlX06ibdfP3_KVXt2RA7wPBs5_k2EJwv7BFtWQUGN-siaF53WLyl4-TF9-u2Svh5BfluoVCX_D4TRV0bRjTbk133Jqq_x_p9O1vxFwmM7GfvEFUqG9RAZwEGwyZhCqf_8KXSGgsWP1xDuHqGAmRcmoE3T2U-qd4x_p_iNLeYEAQO1c0wo4nFa8NjyqVhJDGIxjjST-gQQKAOq8y5ZbgMiVsRbHm8LfRYt3M7hSNhHWSFYFYNxNt16cVWqBCnOzlwFSrApC752vJnwbWwtt5igxojhgQCjNegZrOJggVeEGyQy4oM5U9pO0kK1w8XTI1khYpvqoM1JgbWLO3vh-mkgO2NMRuMQ3EKLlXHzB_cuaPCVgufUb9p221XtftfkY1-P-3rPcy-9ol7JN1IXdpROmONJohmRljDoBKnPRp1XmrfRgLzY4bA2XWxz0mMerOoX709xPOE-8czSL_BHwKs-fpWJdC6Qw6LZbcp9vvS0a7bTpq2FICqnapzGrdgMXVPvd3U5dbtc8YGU67PmGV1yxQbiNZtr5-CV8OpDpIA5cT__yZDHS4znzWsu-6qsqk1ZVpu23G-bgnfUClGWxKuWum6PUhKaWBVpQho757ZPVIYwuzg4pfPubuQuyYpSmojPg1-M7S1SWWyeSPeJ8Q9AEBh9">