Enable unroll for constant bound loops when TripCount is not modulo of unroll factor, reducing it to maximum power-of-2 that satisfies threshold limit.

Commit for Evgeny Stupachenko (evstupac@gmail.com)

Differential Revision: http://reviews.llvm.org/D18290

llvm-svn: 265337

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
index c3761dcf4e8..fa6214f89f0 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -639,6 +639,16 @@ static bool tryToUnrollLoop(Loop *L, DominatorTree &DT, LoopInfo *LI,
       Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
       while (Count != 0 && TripCount % Count != 0)
         Count--;
+      if (Count <= 1) {
+        // If there is no Count that is modulo of TripCount, set Count to
+        // largest power-of-two factor that satisfies the threshold limit.
+        Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
+        UnrolledSize = (LoopSize - 2) * Count + 2;
+        while (Count != 0 && UnrolledSize > UP.PartialThreshold) {
+          Count >>= 1;
+          UnrolledSize = (LoopSize - 2) * Count + 2;
+        }
+      }
     }
   } else if (Unrolling == Runtime) {
     if (!AllowRuntime && !CountSetExplicitly) {