[LoopUnroll] Enable PGO-based loop peeling by default.

This enables peeling of loops with low dynamic iteration count by default,
when profile information is available.

Differential Revision: https://reviews.llvm.org/D27734

llvm-svn: 295796

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -110,7 +110,7 @@ static cl::opt<unsigned> FlatLoopTripCountThreshold(
              "aggressively unrolled."));
 
 static cl::opt<bool>
-    UnrollAllowPeeling("unroll-allow-peeling", cl::Hidden,
+    UnrollAllowPeeling("unroll-allow-peeling", cl::init(true), cl::Hidden,
                        cl::desc("Allows loops to be peeled when the dynamic "
                                 "trip count is known to be low."));
 
@@ -155,7 +155,7 @@ static TargetTransformInfo::UnrollingPreferences gatherUnrollingPreferences(
   UP.AllowExpensiveTripCount = false;
   UP.Force = false;
   UP.UpperBound = false;
-  UP.AllowPeeling = false;
+  UP.AllowPeeling = true;
 
   // Override with any target specific settings
   TTI.getUnrollingPreferences(L, UP);

llvm/test/Transforms/LoopUnroll/peel-loop-pgo.ll

@@ -1,4 +1,4 @@
-; RUN: opt < %s -S -debug-only=loop-unroll -loop-unroll -unroll-allow-peeling 2>&1 | FileCheck %s
+; RUN: opt < %s -S -debug-only=loop-unroll -loop-unroll 2>&1 | FileCheck %s
 ; REQUIRES: asserts
 
 ; Make sure we use the profile information correctly to peel-off 3 iterations