[LoopUnrolling] Re-prioritize Peeling and Partial unrolling

Summary:
In current implementation the loop peeling happens after trip-count based partial unrolling and may
sometimes not happen at all due to it (for example, if trip count is known, but UP.Partial = false). This
is generally bad, the more than there are some situations where peeling is profitable even if the partial
unrolling is disabled.

This patch is a NFC which reorders peeling and partial unrolling application and prepares the code for
implementation of the said optimizations.

Patch by Max Kazantsev!

Reviewers: sanjoy, anna, reames, apilipenko, igor-laevsky, mkuper

Reviewed By: mkuper

Subscribers: mkuper, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D30243

llvm-svn: 296897

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp b/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
index 3cb9e6e2e02..0227f8a5d13 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
@@ -61,7 +61,8 @@ static bool canPeel(Loop *L) {
 
 // Return the number of iterations we want to peel off.
 void llvm::computePeelCount(Loop *L, unsigned LoopSize,
-                            TargetTransformInfo::UnrollingPreferences &UP) {
+                            TargetTransformInfo::UnrollingPreferences &UP,
+                            unsigned &TripCount) {
   UP.PeelCount = 0;
   if (!canPeel(L))
     return;
@@ -70,6 +71,11 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
   if (!L->empty())
     return;
 
+  // Bail if we know the statically calculated trip count.
+  // In this case we rather prefer partial unrolling.
+  if (TripCount)
+    return;
+
   // If the user provided a peel count, use that.
   bool UserPeelCount = UnrollForcePeelCount.getNumOccurrences() > 0;
   if (UserPeelCount) {