<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62190>62190</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Incorrect instruction emission with `-mips1`
        </td>
    </tr>

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

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

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

<pre>
    I noticed Clang 16 is emitting invalid instructions when targeting MIPS I, specifically I noticed clang is emitting `ldc1` and `sdc1` instructions, which were added in MIPS II (see the bottom of the issue).

Minimal test case:
```c
double test = 1.0;

double getter (void) {
    return test;
}

void setter(double value) {
    test = value;
}
```

Assembly output, compiled with `-O0 -mips1 -EB`:
```mips
getter: # @getter
        addiu   $sp, $sp, -8
        sw $ra, 4($sp)                     # 4-byte Folded Spill
        sw      $fp, 0($sp)                     # 4-byte Folded Spill
        move    $fp, $sp
        lui     $1, %hi(test)
        ldc1    $f0, %lo(test)($1)
        move    $sp, $fp
        lw      $fp, 0($sp) # 4-byte Folded Reload
        lw      $ra, 4($sp) # 4-byte Folded Reload
        addiu   $sp, $sp, 8
 jr      $ra
        nop
setter:                                 # @setter
        addiu   $sp, $sp, -16
        sw      $ra, 12($sp) # 4-byte Folded Spill
        sw      $fp, 8($sp) # 4-byte Folded Spill
        move    $fp, $sp
        sdc1 $f12, 0($fp)
        ldc1    $f0, 0($fp)
        lui     $1, %hi(test)
        sdc1    $f0, %lo(test)($1)
        move    $sp, $fp
        lw      $fp, 8($sp)                     # 4-byte Folded Reload
        lw      $ra, 12($sp)                    # 4-byte Folded Reload
        addiu   $sp, $sp, 16
        jr      $ra
 nop
test:
        .8byte  0x3ff0000000000000              # double 1
```

godbolt link: https://godbolt.org/z/jro1oGfbz


Instead of emitting the `ldc1`
```mips
        lui     $1, %hi(test)
        ldc1    $f0, %lo(test)($1)
```
 Clang should use `lwc1` on `-mips1`:
```mips
 lui     $1, %hi(test)
        lwc1    $f1, %lo(test)($1)
 lui     $1, %hi(test + 0x4)
        lwc1    $f0, %lo(test + 0x4)($1)
```

It uses `$f0` and `$f1` because on MIPS1 doubles are composed of two 32bits registers

For little endian machines, the `$f0` and `$f1` registers should be swapped, ie:
```mips
        lui     $1, %hi(test)
        lwc1    $f0, %lo(test)($1)
        lui     $1, %hi(test + 0x4)
        lwc1    $f1, %lo(test + 0x4)($1)
```

As an optimization opportunity (but not required for this issue) would be to reuse the `lui` instruction
```mips
        lui     $1, %hi(test)
 lwc1    $f1, %lo(test)($1)
        lwc1    $f0, %lo(test + 0x4)($1)
```

This same logic applies to `sdc1`, clang should emit two `swc1` instead.

Finally, here's a screenshot of the MIPS ISA stating this instruction was introduced on MIPS II


</pre>
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