<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/61425>61425</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [C++20] [constexpr] Should we cache the result of constexpr/consteval expression?
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c++20,
            consteval
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          ChuanqiXu9
      </td>
    </tr>
</table>

<pre>
    I found this one when I investigate the building time in modules.

The reproducer:

```
// a.cpp
constexpr bool f() {
        for (unsigned n = 0; n != 1'000'000; ++n) {
        }
        return true;
}

template<typename>
struct s {
        static constexpr auto a = f();
#ifndef NO_MULTIPLE_CALL
        static constexpr auto b = f();
        static constexpr auto c = f();
        static constexpr auto d = f();
#endif
};

template struct s<int>;
template struct s<long>;
```

Let's run it with:

```
set x
echo "Build time for duplicated"
time clang++ -std=c++20 a.cpp -c -o a.o
echo "Build time for not duplicated"
time clang++ -std=c++20 a.cpp -c -o a.o -DNO_MULTIPLE_CALL
```

The result in my machine shows:

```
Build time for duplicated

real    0m11.753s
user    0m11.611s
sys     0m0.016s
Build time for not duplicated

real    0m3.631s
user    0m3.576s
sys     0m0.013s
```

So now clang doesn't cache the result of constexpr/consteval clearly.  This is significant when we try to use some metaprogramming techniques. Is there any special reason? If not, I guess we may solve the problem by something like:

```
ExprResult ASTContext::EvaluateXXX(Expr *E) {
 ODRHash Hash;
     llvm::FoldingSetNodeID &ID;
 E->ProcessODRHash(ID, Hash);
     
     ExprResult Result = EvaluteCache.FindNodeOrInsert(Hash);
     if (!Result) {
 Evalute it actually...
     }

     return Result;
}
```
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVl1v6jgT_jXmZkSUOCSBCy6AgF6kvqdHp12pd0eOMyHedewcf7Tl36-cQKFd2t2VFiHHHk-e-fDM4zBrxUEhLkm2Jlk5Yd612iw3rWfql3jyi0ml6-NyD432qgbXCgtaIby0qGAPQj2jdeLAHIJrESovZC3UAZzoEISCTtdeoo1IXJJ4NY6PLYLB3ujaczQkXV1vkjw-_ccl3RG6Axbxvh8lXCvr8LU3UGktoSF0TugCSLE-wywabYDQuVdDbDUoIGkJMUnXYUqTsEoILeI4Po3pGghdE7pWH7FIUb7NDTpvFDjjkaRnlct-GB12vWQOSbpxxx4V65Ck23HPOuO5A_sO3zrmBIdLVMw7DWzw-BTbxRZNRaNqbODb_c___3b3uP9-t_25Wd3d_Q1cdRvuM3X-79Trz5xFVYvmkqYL0HWm4JwWkm6EciFbZ8VbKlKrw7XOx3oZxjt0hBYWjFcgHLwI135dZhYdvI5T5K0GQuk6lPJYx6Geat9LwZnDmlB6ci_sccnUYawdmFpXk7Tk45LGY9nClMNUA4v0lwaUdv-JEZiWn1THrUyNvWi9dEO3HqFjvBUKwbb6xX6dtM8zdPWOQSZJvIi7JImKLLWj1Fs0Z2meJCepPdpBGEdxktubRj5k6ZahNMrT5KOdNMqK_IaZs0M3k_OgQemXMftQa7SK0MIBZ7wd6e6UOd1cWoLQ3Th_ZhK4RGbkMQJ4DMQpLARGEo3gTLmRQ18QnDmC0-AtgtUdQoeO9UYfDOu6gUuRt0r88mgj2Ntg2CAwdQTbIxdMgkFmtSLpDvZNyBChG9jDwaO1Ab9jR7BaPo8-90ZXEjuojoM11wYTUvyBXx_29rU3P8ZwVw-PG60cvrrwSrraPjPpmcOnpydC50ERCF1t3zEp3Jc__sdsC2F4a14IPymfuxFop4fL4wHdN13jvgRC8_2FN2A7Jen2u9EcrT3hETrflyHecbF4D301vfL_9AikNbjucBNONNoJVQfD92avLBpH6Pwmqmhg4LpkBHof5wkx0A7jzjMpj1EUXfv0_sIYZKeL5YT316vlfBSTepnWi3TBJrhM8mI-z5M8mU3aJceiKbAokkWdxXXcJFWWsgWviqrivKE4EUsa0zROkyzOaZ7NohlveJFwNsMZW9DZnMxi7JiQUTiOSJvDRFjrcZknM5pNJKtQ2uEjgdI36gkkRTdBcq74IMnKiVkGlGnlD5bMYimssxdcJ5wcvjc2bzhZCSRbX3ooK-Gh1V7WoXz_ebsFAVorhl6YeCOXrXP9wGLDZ8RBuNZXEdcdobuh7MbHtDf6d-SO0N0QtCV0N8T9ZwAAAP__SrGvFQ">