<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/78281>78281</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [SimplifyCFG] eliminateDeadSwitchCases does not eliminate dead branches.
        </td>
    </tr>

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

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

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

<pre>
    I tried the following code, which I expect to be optimized.

```llvm
define i32 @src(i32 noundef %0) {
  %2 = icmp eq i32 %0, 0
  %3 = icmp eq i32 %0, 1
  %foo = zext i1 %3 to i32
 %bar = select i1 %2, i32 -1, i32 %foo
  switch i32 %bar, label %4 [
 i32 0, label %7
    i32 -1, label %7
  ]

4: ; preds = %1
  %5 = icmp eq i32 %0, 2
  %6 = zext i1 %5 to i32
  br label %7

7: ; preds = %4, %1, %1
  %8 = phi i32 [ %6, %4 ], [ 1, %1 ], [ 1, %1 ]
  ret i32 %8
}

define i32 @tgt(i32 noundef %0) {
 %.not = icmp ne i32 %0, 1
  %2 = zext i1 %.not to i32
  ret i32 %2
}
```

Alive2: https://alive2.llvm.org/ce/z/zyMuXt

Similar to #73446. But it's more complicated.
This should be caused by KnownBits. 

We know that this `%bar` can only be -1, 0 and 1. So we can transform the default branch to a branch with a specific value.
There are several variants of this issue, such as replacing -1 with some other value and increasing the number of cases.

From: https://github.com/rust-lang/rust/issues/119520.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJyEVUGPqzYQ_jXOZRQEBkJy4LBJlOrpqYdqK7VXYw_BrbGpbZKX_fWVDcmS3bettAvG_mbmmy8zY-acPGvEmpR7QulRMv3bd0IpKY8rNvrO2HraWzVG3Opv4K1EAb5DaI1S5ir1GbgRSOgBrp3kHXwD_DEg9-ANNAhm8LKXbygSkh5J-jI_N-n0p9Sln7YEtlIjyJwCKVJnOaHb8KHNqAW2QGiZEroDUu0nAwhbFEh-BMn7AfCfyTjiDpAuUPmXqGyBao2JuDf84UFmk6E3AT-jCC0bZiPIoQpJTjAaXAW36-y-mvzdvbur9Ly7HzTMBphiDarwXUCQf4IGSPp0Wt2dwCLE51NSHpcKFyR_AZLvYbAoXKRMaLlMt_xSFLpAbT5KUj5LAo39RGZ6Vj9lUIQAkcn9_R5rG0FDJyc25T4SmIFFzDCsyz08rL_enN1a9PfctjO76kmo57rzZ_-_dUdomWjj3-W7m_-kpuhH-aLls4ILivQjxXufLBm_KHlBGtTtvB8cyV8IPRF6YnE_CS2VGHsm9MSR0NNb-L_9Ov7pl05eZS8Vs4EKoXmVF8Umgf3oQXpCKwe9sQjc9IOSnPlH-_7eSQeuM6MSobs5Gx0KaG7wXZur3kvvEliG-QPhb22u4DvmwQfjkMzUApsUONNgtLoFX1Nhp8C0gCyBVwNXjABvmXatsX2cOwJbNioPjWWad4E_u6-v0nfAwA3IZSs5XJga8UEcLQKzCA4vaJmCC7OSae_AtBMz6dwYB5kbeQfMgcVBMR5G3DqbnDvTIxjfoZ2cR7JSc4vMBVwgqMe-QRu8cubQPc29kzX95x_uLH03Ngk3PaEnOzq_Vkyf5zWhp0jMEXrKsl1J02Ql6lzs8h1bYZ1VaVlk-bbIV10t6Gabs922KKpKNJs242W5obuKpdjSdkdXsqYpLdIs22RFWqR50tBqWzVsl-fNhnHckSLFnkn1qKJVDF5XW7rNVrHR3XxZxNGdv8wDnnlptAtXBz0QSnvpHIr18nC-VmwdDNfNeHakSJV03r0H89KreBm9ylB57e1w-oWUR0Ale6mZxyMy8RqH6SGIC8Kgg9BSDwQIZGKuCHTJarSq_g-5YxbTaz1Y8xfyJ8Vj3v8GAAD__-FLCPY">