<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/88141>88141</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LoopUnroll fails to unroll forced loop due to inlinable calls, leading to linker errors
        </td>
    </tr>

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

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

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

<pre>
    An AFDO profile roll broke Chromium's build with ThinLTO enabled by introducing linker warnings - which were promoted to errors - about `llvm.loop.unroll.enable`d loops not being unrolled:

```
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-optimization-reducer.h:685:11: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:1316:9: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:813:9: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:2124:9: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:2149:13: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:2154:20: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-lowering-reducer-inl.h:2185:20: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
ld.lld:  [0;31merror: [0m../../../../../../../home/chrome-bot/chrome_root/src/v8/src/compiler/turboshaft/machine-optimization-reducer.h:685:11: loop not unrolled: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
```

Unfortunately, I can't provide an 'easy' reproducer of the original breakage, as it only occurs with ThinLTO and a specific AutoFDO profile on a very large program.

What I _can_ provide is some information about investigating this locally:

1. The LoopUnroll pass ran on the loops that did not get unrolled.
2. The loops that did not get unrolled did have `llvm.loop.unroll.enable` metadata on them.
3. The loops that did not get unrolled had call(s) to functions that were deemed inline candidates by `UnrollCostEstimator`. Moreover, the following criteria were met for at least one call per loop (`PrepareForLTO` was `false`):

```
if (!Call->isNoInline() && TTI.isLoweredToCall(F) &&
     ((F->hasInternalLinkage() && F->hasOneLiveUse()) || PrepareForLTO)) {
  // ...
}
```

and unrolling is currently always skipped if that's detected in a loop:

```
  if (UCE.NumInlineCandidates != 0) {
    LLVM_DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
    return LoopUnrollResult::Unmodified;
  }
```

I can totally understand the desire to facilitate the inlining of calls like this, but if a user asks for forced unrolling, IMO that should be respected regardless.

I have a potential fix that I'll upload soon.
</pre>
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