<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/84089>84089</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Missed Optimization: Block Placement Reveals Missed Constant Propagation in Many Archs
        </td>
    </tr>

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

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

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

<pre>
    Example code:
```cpp
#include <stdint.h>
typedef uint32_t u32;

u32 fib(u32 n) {
    u32 a = 0;
    u32 b = 1;
    u32 c = 1;

    for (u32 i = 0; i < n; i++) {
        a = b;
        b = c;
        c += a;
    }

 return a;
}
```
On compilation, Basic Block Placement will for many archs end up separating:
```
early_exit:
  $a = MOVi 0
exit:
 $return_register = COPY $a
  ret
```
into:
```
early_exit:
 $a = MOVi 0
  $return_register = COPY $a
  ret
exit:
 $return_register = COPY $a
  ret
```
Instead of just propagating the constant to the returning register!!

List of tested archs which are affected on O3 (unless specified otherwise):
- Aarch64
- Many 32-bit ARM archs: v4T, v5T, v5TE, v6, v6T2, v7, v7A (Outliers: v6M, v7M due to loop unrolling, both of which are affected at -O1)
- LoongArch
- RISC-V (both 32-bit and 64-bit)
- XCore

Tested on Godbolt Clang Trunk
Godbolt link: https://godbolt.org/z/5roWa3z71
</pre>
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