<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/175717>175717</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            wasm-exceptions in clang 21 vs clang 20
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          anutosh491
      </td>
    </tr>
</table>

<pre>
    Hey @sbc100 @aheejin 

I would like some help from your end. I maintain clang-repl and the clang-repl in the browser through wasm usecase (which allows writing C++ in the browser in a repl format)

Can be tried out here : https://compiler-research.org/xeus-cpp/lab/index.html

The above link is already using `-fwasm-exceptions` & `-mllvm -wasm-enable-sjlj` with the CompilerBuilder as can be seen through the clang cc1 command. This is clang 20.1.8 based.

`
"" -cc1 -triple wasm32-unknown-emscripten ....... -target-feature +exception-handling -target-feature +multivalue -target-feature +reference-types -exception-model=wasm -mllvm -wasm-enable-eh ....... -fcxx-exceptions -fexceptions -exception-model=wasm -iwithsysroot/include/compat -fincremental-extensions -mllvm -wasm-enable-sjlj -o "<<< inputs >>>.o" -x c++ "<<< inputs >>>"
`

And hence you can do exception handling using the same

<img width="1073" height="316" alt="Image" src="https://github.com/user-attachments/assets/85b59f3f-b7c6-4070-b702-b99ba25d6400" />


**But what we notice is as we're migrating to clang 21.1.8, wasm-exceptions doesn't work the same.**

Let me take this out of Jupyter and demonstrate it through a toy project. A standard clang-repl in the browser workflow looks like this 

code in cell x -> PTU (partial translation unit as code in a repl can be incomplete -> LLVM IR -> incr_module_x.o where x is cell number -> incr_module_x.wasm which is built as a standlone side module and loaded on top of a main module (from where we can parse and execute code)

So it's one of these steps, where things can go wrong. In our case I suspect the LLVM IR being built is wrong and hence the wasm generated is also wrong. 

Taking the minimalistic example of 
```
try { throw 1; } catch (...) { 0; }
```
note that we're using only `-fwasm-exceptions`. our compiler builder args look something like
```
 CB.SetCompilerArgs({
      "-v",
      "-xc++",
 "-std=c++23",
      "-Xclang", "-iwithsysroot/include/compat",
 "-fwasm-exceptions"
  });
```
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJyEVl1v47oR_TX0y0CCTNmW_eAHO9mgKbZA0bst-nZBkWOJG4oUyFFk__uCpOLsptm9hpBI_JiPwzNnKELQnUU8su2ZbR9XYqLe-aOwE7nQbw7rVevU7fg3vAHbVKGV66qKb6JH_K4tsOrEqtMzzG4yCox-QQhuQOjRjHDxboCbmzygVSU8wyC0JaEtSCNsV3gcDQirgHr8cUjbNNJ6Nwf0QL13U9fDLMIAU0ApAgLj-7nXsgdhjJsDzF6Tth08MH5m_PzRhrYgIBm_OD8IYvyQQ38QFloE8hoVuImgR4_A6hP0RGNg9YnxJ8afpBtGbdAXHgMKL_vS-Y7xpytOoZDjyPiTES3jT9oqvJY9DSY7-NYjiNa9IhhtX0AHEMajUDeYQgyY7ariElMr8CpxJO1sYLsKGN-lucGY1wGKvMKK1mARvpvvccmsqU9ZPizBnSdtFHoQAWTOKyDaO4B3mEHKNUg3DCKey7dehxhXnuJVuS730IqAqswpsF0V_3LOOIci7i3I69FgOpKaF5N9sW62BQ5Bej0SWijzDwoSvkMqLihoisjy8z3PohdWmQjCJ6uGyZB-FWbCz2Y9XtCjlVjQbcQA7-AVg1NoWP2Y6PIZfNi_R3eR1-sPwENx-fHjV0Z1RD7cgneO0pFLMylcWCIIiou20uOAloQp8EpoQ7b4i9OEwkHEt37ID2g7ThSA1V_yU7qE_RXkwu_fro6T92OrTieroI9gxWJMzFAO7rnB_RQyISNLghhwOfv6QQ8dzFpRz-pHxvm6auoYTI-66ymP1etdHBJm-X4eRIdxJHiZR36upk5TP7WldAPjT1NAXwgiIfuIWGD8SYSA6WW_bbeHS30p2kbuik3VVEXbVLxoD4dW8K3abaoq-olm6y9LyJGr8TlPBHMvCGYE60hLTOUXYEbGG48w6M6LJBvk3ui_jvRn_AE-1CQoh8Ey3hDMzr_cYSqzr-z3KxIMCCReECiWVVQUd4G_T-ONYmFaBQoHZwN5QQia7sUpgNwNRu--o6QSThBIWCW8-o0yxkAuxs1gnHsJWX6T2xyNdArjFonGwBUKVn-Bf377d9TOUXjSwgB5YYMRiQeT1ZSkY9m2COaiJNpGchskzIa-fv3PP-D5X_kj0v3PwanJ4J_X0sGcVPSaVCU6t9PQov9sbSqorOQ6QDtpk2IQOX3jLELQCiFvSAAaJ1QUawvkxoiuSI3lbQnj-9R4cgwzpgRG4UPejVeUE2HK8t4F_nCgifEmQHToLhHlgBAIx5C4kGxRr22XpbVzMHtnuxKeLcQOl5rSM4QpjCgpndIbQi1GhuXUdMj7Uii5JuPShEKHFiMrVG4S4e5iaSTi5a08B231IIwOpCXgVcRziVEvRZ-f6kT-Bqw5J4rNsGb1GVjzCFKQ7CNMZVkyfkhLqmXygwXrKAaYSiiXTNYIZ83tV52rzHgsLSnlnXqS70LiabogJCgTYT94hIdz-QfSW0c7-S4wvmfNOc6lH-O8eE3N6OGnsesije9TcTSQYvXjMsXrT_b9N9VXnkgDf6HuP9v_PwCS9ELCkh9Yff4xv5U61upQH8QKj-tmt-f7ZtvsV_3xoLbygFXdtrjbVxts-Fq1VY2HzW7fKnVZ6SOv-K5ar-tqtz3U-_Ky31co1a7acCH5HtmmwkFoU8YGE-8mKx3CFN1sm3WzMqJFE9Itj_N7wtvHlT_GDUU7dYFtqkip8G6CNBk8fhTCt_sb8DW8vl8bVpM3x9_IfLS6_CsWmYvoxiij0i-Bvh75_wIAAP__e1NqcA">