<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/143778>143778</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Unnecessary `and x,C` introduced due to assume on trunc+zext
        </td>
    </tr>

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

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

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

<pre>
    The code:
```c
#include<stdint.h>

uint64_t foo(uint64_t a) {
    uint32_t b = a;
    if (b >= 100) __builtin_unreachable();
    return b;
}
```
with `-O3` compiles to:
```asm
foo:
        mov rax, rdi
        and eax, 127
 ret
```
but it could be:
```asm
foo:
        mov eax, edi
 ret
```
[compiler explorer](https://godbolt.org/z/5v3dY9nTr) on the instcombine pass that introduces the `and`; namely, it transforms:
```llvm
define dso_local range(i64 0, 4294967296) i64 @foo(i64 noundef %a) local_unnamed_addr {
entry:
  %conv = trunc i64 %a to i32
 %cmp = icmp uge i32 %conv, 100
  %0 = xor i1 %cmp, true
  call void @llvm.assume(i1 %0)
  %conv2 = zext i32 %conv to i64
  ret i64 %conv2
}
```
to:
```llvm
define dso_local range(i64 0, 4294967296) i64 @foo(i64 noundef %a) local_unnamed_addr {
entry:
  %conv = trunc i64 %a to i32
  %cmp = icmp ult i32 %conv, 100
  call void @llvm.assume(i1 %cmp)
 %conv2 = and i64 %a, 127
  ret i64 %conv2
}
```
reduced from [this Rust code](https://godbolt.org/z/Kcq1qjoas)
</pre>
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