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[LoopUnroll] Honor '#pragma unroll' even with -fno-unroll-loops.
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Authored by Meinersbur on Dec 14 2018, 1:29 PM.

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Summary

When using clang with -fno-unroll-loops (implicitly added with -O1), the LoopUnrollPass is not not added to the (legacy) pass pipeline. This also means that it will not process any loop metadata such as llvm.loop.unroll.enable (which is generated by #pragma unroll or #pragma clang loop unroll(enable)) and remain in the IR until the WarnMissedTransformationsPass emits a warning that a forced transformation has not been applied (see https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181210/610833.html). Such explicit transformations should take precedence over disabling heuristics.

This patch unconditionally adds LoopUnrollPass to the optimizing pipeline (i.e. it is not added with -O0), but passes a flag indicating whether automatic unrolling is dis-/enabled. This is the same approach as LoopVectorize uses.

The new pass manager's pipeline builder has no option to disable unrolling, hence the problem does not apply.

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rL LLVM

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Meinersbur created this revision.Dec 14 2018, 1:29 PM
Meinersbur marked an inline comment as done.Dec 14 2018, 1:33 PM
Meinersbur added inline comments.
lib/Transforms/IPO/PassManagerBuilder.cpp
695 ↗(On Diff #178269)

Should these passes be added unconditionally as well?

Such explicit transformations should take precedence over disabling heuristics.

I agree. This was our consensus for the vectorization pragmas (that they should override the general enablement provided by command-line flags), and I see no reason why loop unrolling should be any different in this regard.

include/llvm/Transforms/Scalar/LoopUnrollPass.h
26 ↗(On Diff #178269)

I don't like this name "AlwaysUnroll" as it sounds like a forced action for all loops, and it's really a pass-enablement flag. I understand, however, that the corresponding flag in the loop vectorizer is named this:

/// If true, consider all loops for vectorization.
/// If false, only loops that explicitly request vectorization are
/// considered.
bool AlwaysVectorize = true;

which is also a suboptimal name, but as a comment which explains what it really means. You should have a similar comment here. Maybe a better name would be: OnlyWhenForced?

lib/Transforms/IPO/PassManagerBuilder.cpp
695 ↗(On Diff #178269)

In theory, you'd want these in the pipline for loops where unrolling is enabled. In practice, that's going to cause a larger variation from this change than the actually loop-unrolling changes, and so, if we want to do that, it should be a separate patch. For now, I'd leave these unchanged.

  • Rename AlwaysUnroll -> !OnlyWhenForced
  • Add comments describing OnlyWhenForced
This revision is now accepted and ready to land.Dec 17 2018, 12:17 PM
This revision was automatically updated to reflect the committed changes.