<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62124>62124</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Collapse `x/N*N` → `x` when the multiplication is inside a GEP and the division exact
        </td>
    </tr>

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

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

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

<pre>
    This is, of course, already handed for normal `udiv`+`mul`.

But it looks like it's *not* handled when the "multiply by N" is because of a GEP over `[N x T]`.

Alive2 says it's legal: <https://alive2.llvm.org/ce/z/6azqo5>
```llvm
define ptr @src(ptr nonnull noundef %0, i64 noundef %1) {
%start:
  %d = udiv exact i64 noundef %1, 3
  %p = gep inbounds ptr nonnull noundef %0, 3 x i64 %d
  ret ptr %p
}
=>
define ptr @tgt(ptr nonnull noundef %0, i64 noundef %1) {
%start:
  %p = gep inbounds ptr nonnull noundef %0, 1 x i64 noundef %1
  ret ptr %p
}
Transformation seems to be correct!
```

But opt doesn't do it: <https://llvm.godbolt.org/z/KGfPMnbh6>

And thus the resulting code ends up with a division followed by a LEA multiply:
```nasm
src:                                    # @src
        mov     rax, rsi
        movabs  rcx, -6148914691236517205
        mul     rcx
        shr     rdx
        lea     rax, [rdx + 2*rdx]
        add     rax, rdi
        ret
```

This should probably happen also for when the denominator of the `udiv exact` is just a factor of the array length in the `GEP`, like in <https://alive2.llvm.org/ce/z/aPjgyB>
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVdty4jgTfprmpmtSchub-IILCJCL_99ULvICstVgZYTklWQC8_Rbsp0D7NTsTtW6KHzog7pb3_dJhqAPlnkJxRqKzUz2sXV-GRoX47E5zmqnLsuXVgfUAegB3R4b1_vA6UUaz1JdsJVWscK982idP0qDUIpe6ROUAmgNpTj2BkpxB2IDYjX-r_uIOqJx7ntAo78z6gi0CAi0si4CrYa8hhW-tWwxtoxAdOxN1J25YH3BJyBCHbDmRvaBU3ESH7fP6E7sUw1QrJ_wjC9QbG6XXxl9YsIgL-F9YcMHaSBfIeQPbYxdgHwFtAPaycH5zpjT8c75A9CuYaDdD6BdKX_86QrIt1PyUoy_5Dt-UrzXlrGLHmEugm-A7tOLddb2xqB1vVW8R6BCpKnqcv71WwZUISzWU34qQpQ-ptKGD5h8FEK-wTRx5LNs4k9yPGD-JaAbAg7cobZ18gv4q5JyPA8p01LvWTzHsScquqm2xWZ6yDcfA7nuPh7if9_9bzaTTc1crfLPTb14acM-wTtqZzEwHwNGhzVj47znJgJlNyC4xbvrIirHwQIt0lNC3s_wNgDt4FTtTJwAl7D2v8f98x-2bstPuI1Ytgpj24eBI55Doog9YOMUI6dx9B2-6diiRKVPOqT6984Y98Yq8Uji_7crfGfWx3A_2rAyTFhO6M1X-C8uoPwd7tNsx-voTsPdy3PaDB_03-yyDoi-Gezfymx-X2XzssooL4tsQaK4CejNmLA5XxtC60eDujEYll9LgGLt1RmB1khAq-RebK4jpFJXRauboj3HX-z8IJ-hdb1R2HlXy9okzew6tihNcINwfmicYuuO2srofBK0QfZGNR25DaVImvfah4gS97L54ii9lxc0bA-xRW3fgx-3z4MSP0w6a39L4eTz6-Gyhnw7U8tcVXklZ7zMyvtsXoi8ErN2SZKzmu8X81IuKKe8ygqRFU3RKFEWTVHN9JIE5WKezUWR55m424tqUVYLlTWlKPYVwVzwUWrzUcBMh9DzsqSM5jMjazZhOKKILL_hYASidGL5ZYr5VveHAHNhdIjhM0vU0fDywRkjuzDM4gy0ewJaPaU5wpbgvoSKRkspPrdhYoNuRrqnA9AGrXg6YeRAOf7k07A3s96b5fVgDzq2fX3XuOPE6-n2rfPudRCN3dBNSGdJ6vavAAAA__8G2jdZ">