<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/131168>131168</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [AA] Return-only captures can regress results
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            llvm:optimizations,
            missed-optimization
      </td>
    </tr>

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

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

<pre>
    ```llvm
; RUN: opt -passes=aa-eval -print-all-alias-modref-info < %s
declare ptr @callee(ptr)

define void @test1(ptr noalias %a, ptr noalias %b) {
  %p = call ptr @callee(ptr writeonly captures(ret: address, provenance) %a, ptr captures(none) %b) memory(argmem: readwrite)
  load i32, ptr %p
  ret void
}

define void @test2(ptr noalias %a, ptr noalias %b) {
  %p = call ptr @callee(ptr writeonly %a, ptr captures(none) %b) memory(argmem: readwrite)
  load i32, ptr %p
  ret void
}
```
Results in:
```
Function: test1: 1 pointers, 1 call sites
  Both ModRef:  Ptr: i32* %p    <->  %p = call ptr @callee(ptr writeonly captures(ret: address, provenance) %a, ptr captures(none) %b) #0
Function: test2: 1 pointers, 1 call sites
  Just Mod:  Ptr: i32* %p       <->  %p = call ptr @callee(ptr writeonly %a, ptr captures(none) %b) #0
```

The problem here is that without the `captures(ret: address, provenance)`, the call result is an escape source, so we know it does not alias with the non-escaping `%b` and thus only the Mod effect on the `%a` parameter is relevant.

With the return-only capture it's no longer an escape source, and we currently are no longer able to determine it cannot alias with `%b`.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzEVU2L6zYX_jXHm4ODLcWJs_DCk7xZvHBLGVq6lu3jWL2yZKTjhOmvL1Im3DCdwi20XAiY5Hw8H34iqRD0xRI1UL1AdcrUypPzjdVfdZ91bnhrYFfcP8ZcZyhakC_4-utPIFt0C2O-qBAogDwpldNVGcwXry3nyphcGa1CPrvB05hrOzoEeUQQVYCiHag3yhMu7BG2Ra-MIQJRL-xBHCJS7Bm1Jbw6PcQepsDlvQWtS9vjNgXiiB9-60AcEPYvULQYvy8I8oQR5DNAvHnN5Kx5w14tvHoKIGpPHGWqYfAUQgLx7kpW2Z7S-ifopzHr7KOcWMw0O_8Golb-MtMcV3pSQ4K8K0U0Tg2opXisi4xTwRMn-dGP_elvXRH_sSs_RuojelC0rxRWwwG1Bdl-qJ1X27N2sYL3iMgWS1yctkw-vbjyLjJoppDAXhxP-MUNrzTGbvyZfXwmXu07pwPIYw7yfz86QCDkZzLFd8j8_xo4yvyXNf4z4s_vCor2l4miD52hGSfyhDogT4rxpnlyKyNPhLArvt_IuFoc01wi71NY4l5lkUKvFsLgVh-bjxgc3gi_WndDzTg4Cmgd4_1PEjmkRdbZPI1qe8EEUHWwK1DZAXlaAyYjYucXNyCNI_WMzj7IJ4d2BS7Kq5mYfGTjydBVWd7cjfjtgeWJV2_z5_igZhD7yAyNsxfyn0qJZG6E_eo9WTZvGA_Up5HOELLDIRKY45mhGXtlP8j9pm6TDY0cDvKgMmrK_bas621RFdnUVOPQd_W23-93SlG53dVVURzqvuyr_SjLXaYbUYiqkKUsd3FmQ11Xi1HKeuzLbV3UsC1oVtps4kWycf6S6RBWakpZlrs6M6ojE9I9JES6a2TrFtaz_kPF1AcQ8cgAIWYdAg35czHWqlPmmziYd-slwLYwOnD4hsaaTbrn2haqE77-1fMQvUFPlxiy9xCFbPWmmZiXEA8ecQZxvmie1m7TuxnEOVG9P_LFu9-pZxDnJC2AOL-ruzbizwAAAP__Uv9Rcw">