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, 36 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 824bc14..d3817ee 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1067,3 +1067,39 @@ bool llvm::isGuaranteedToExecute(const Instruction &Inst,
   // just a special case of this.)
   return true;
 }
+
+Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) {
+  // Only support loops with a unique exiting block, and a latch.
+  if (!L->getExitingBlock())
+    return None;
+
+  // Get the branch weights for the the loop's backedge.
+  BranchInst *LatchBR =
+      dyn_cast<BranchInst>(L->getLoopLatch()->getTerminator());
+  if (!LatchBR || LatchBR->getNumSuccessors() != 2)
+    return None;
+
+  assert((LatchBR->getSuccessor(0) == L->getHeader() ||
+          LatchBR->getSuccessor(1) == L->getHeader()) &&
+         "At least one edge out of the latch must go to the header");
+
+  // To estimate the number of times the loop body was executed, we want to
+  // know the number of times the backedge was taken, vs. the number of times
+  // we exited the loop.
+  // The branch weights give us almost what we want, since they were adjusted
+  // from the raw counts to provide a better probability estimate. Remove
+  // the adjustment by subtracting 1 from both weights.
+  uint64_t TrueVal, FalseVal;
+  if (!LatchBR->extractProfMetadata(TrueVal, FalseVal) || (TrueVal <= 1) ||
+      (FalseVal <= 1))
+    return None;
+
+  TrueVal -= 1;
+  FalseVal -= 1;
+
+  // Divide the count of the backedge by the count of the edge exiting the loop.
+  if (LatchBR->getSuccessor(0) == L->getHeader())
+    return TrueVal / FalseVal;
+  else
+    return FalseVal / TrueVal;
+}