Use profile info to adjust loop unroll threshold.

Summary:
For flat loop, even if it is hot, it is not a good idea to unroll in runtime, thus we set a lower partial unroll threshold.
For hot loop, we set a higher unroll threshold and allows expensive tripcount computation to allow more aggressive unrolling.

Reviewers: davidxl, mzolotukhin

Subscribers: sanjoy, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D26527

llvm-svn: 287186

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
index 9c41475f352..307fb424b5e 100644
--- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
@@ -461,6 +461,11 @@ Optional<const MDOperand *> findStringMetadataForLoop(Loop *TheLoop,
 void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
                              unsigned V = 0);
 
+/// \brief Get a loop's estimated trip count based on branch weight metadata.
+/// Returns 0 when the count is estimated to be 0, or None when a meaningful
+/// estimate can not be made.
+Optional<unsigned> getLoopEstimatedTripCount(Loop *L);
+
 /// Helper to consistently add the set of standard passes to a loop pass's \c
 /// AnalysisUsage.
 ///