<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/96267>96267</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            IndVarSimplifyPass miscompiles if/else with underlying-type-not-specified enum
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          luke-jr
      </td>
    </tr>
</table>

<pre>
    CI caught an incorrect result in Bitcoin Knots which appears to be a bug in the IndVarSimplifyPass optimisation as of Clang 14.0.0-1ubuntu1.1 (Ubuntu 22.04.4) when targeting i686-pc-linux-gnu.

The test case has been somewhat (but only barely) minimised in this branch: https://github.com/bitcoinknots/bitcoin/tree/tmp2

Compilation flow:
```
apt install build-essential cmake llvm clang g++-multilib autoconf pkg-config libtool  # not sure all these are needed
./autogen.sh
( cd depends && make HOST=i686-pc-linux-gnu DEBUG=1 MULTIPROCESS=1 NO_QT=1; )
CONFIG_SITE=$PWD/depends/i686-pc-linux-gnu/share/config.site ./configure --enable-debug CC='clang -m32' CXX='clang++ -m32' CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE' --disable-wallet
make -C src test/test_bitcoin
src/test/test_bitcoin -t script_tests/script_DataCarrierBytes  # the test fails due to the bug
```

The bug can be "fixed" by:

* Disabling optimizations. (-O0)
* Removing the `else` part of the `else if` statement-chain.
* Printing the `last_opcode` variable at the end, or inside the loop.
* Specifying `enum opcodetype` to have an underlying type of `unsigned int` (which appears to be its default) or `unsigned char` (other types not tested).
* Removing the `&& last_opcode <= OP_PUSHDATA4` condition from the `else if` clause.

I bisected the issue to occur at exactly `-opt-bisect-limit=11829` (out of 35046, if that matters).
</pre>
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