[Arm] Control forced unrolling of small loops (llvm#170127)

* Add flag to control cost threshold for forced unrolling of loops.
  Existing value preserved as default.

Author: VladiKrapp-Arm

llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp

@@ -66,6 +66,11 @@ extern cl::opt<bool> EnableMaskedGatherScatters;
 
 extern cl::opt<unsigned> MVEMaxSupportedInterleaveFactor;
 
+static cl::opt<int> ArmForceUnrollThreshold(
+    "arm-force-unroll-threshold", cl::init(12), cl::Hidden,
+    cl::desc(
+        "Threshold for forced unrolling of small loops in Arm architecture"));
+
 /// Convert a vector load intrinsic into a simple llvm load instruction.
 /// This is beneficial when the underlying object being addressed comes
 /// from a constant, since we get constant-folding for free.
@@ -2731,7 +2736,7 @@ void ARMTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
 
   // Force unrolling small loops can be very useful because of the branch
   // taken cost of the backedge.
-  if (Cost < 12)
+  if (Cost < ArmForceUnrollThreshold)
     UP.Force = true;
 }