Use ScaledThreshold to calculate Unroll count

Earlier we were using partial threshold to calculate unroll count
for loop but for small loops it is giving more unroll count which
is degrading the performance.Using Scaled Threshold improves
the performance.

Author: chandmudda

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -574,6 +574,9 @@ class TargetTransformInfo {
     /// The cost threshold for the unrolled loop, like Threshold, but used
     /// for partial/runtime unrolling (set to UINT_MAX to disable).
     unsigned PartialThreshold;
+    /// The cost threshold for the unrolled loop, like Threshold, but used
+    /// for calculating unroll count for loop.
+    unsigned ScaledThreshold;
     /// The cost threshold for the unrolled loop when optimizing for size, like
     /// OptSizeThreshold, but used for partial/runtime unrolling (set to
     /// UINT_MAX to disable).

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -201,6 +201,7 @@ TargetTransformInfo::UnrollingPreferences llvm::gatherUnrollingPreferences(
   UP.MaxPercentThresholdBoost = 400;
   UP.OptSizeThreshold = UnrollOptSizeThreshold;
   UP.PartialThreshold = 150;
+  UP.ScaledThreshold = 100;
   UP.PartialOptSizeThreshold = UnrollOptSizeThreshold;
   UP.Count = 0;
   UP.DefaultUnrollRuntimeCount = 8;
@@ -884,8 +885,8 @@ shouldPartialUnroll(const unsigned LoopSize, const unsigned TripCount,
   if (UP.PartialThreshold != NoThreshold) {
     // Reduce unroll count to be modulo of TripCount for partial unrolling.
     if (UCE.getUnrolledLoopSize(UP, count) > UP.PartialThreshold)
-      count = (std::max(UP.PartialThreshold, UP.BEInsns + 1) - UP.BEInsns) /
-        (LoopSize - UP.BEInsns);
+      count = (std::max(UP.ScaledThreshold, UP.BEInsns + 1) - UP.BEInsns) /
+              (LoopSize - UP.BEInsns);
     if (count > UP.MaxCount)
       count = UP.MaxCount;
     while (count != 0 && TripCount % count != 0)