LoopUnroll fails to unroll forced loop due to inlinable calls, leading to linker errors

[gburgessiv]

An AFDO profile roll broke Chromium's build with ThinLTO enabled by introducing linker warnings - which were promoted to errors - about llvm.loop.unroll.enabled 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.