[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

include/llvm/Transforms/Utils/UnrollLoop.h

@@ -53,7 +53,8 @@ bool UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
                                 bool PreserveLCSSA);
 
 void computePeelCount(Loop *L, unsigned LoopSize,
-                      TargetTransformInfo::UnrollingPreferences &UP);
+                      TargetTransformInfo::UnrollingPreferences &UP,
+                      unsigned &TripCount);
 
 bool peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE,
               DominatorTree *DT, AssumptionCache *AC, bool PreserveLCSSA);

lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -784,7 +784,15 @@ static bool computeUnrollCount(
     }
   }
 
-  // 4rd priority is partial unrolling.
+  // 4th priority is loop peeling
+  computePeelCount(L, LoopSize, UP, TripCount);
+  if (UP.PeelCount) {
+    UP.Runtime = false;
+    UP.Count = 1;
+    return ExplicitUnroll;
+  }
+
+  // 5th priority is partial unrolling.
   // Try partial unroll only when TripCount could be staticaly calculated.
   if (TripCount) {
     UP.Partial |= ExplicitUnroll;
@@ -847,14 +855,6 @@ static bool computeUnrollCount(
         << "Unable to fully unroll loop as directed by unroll(full) pragma "
            "because loop has a runtime trip count.");
 
-  // 5th priority is loop peeling
-  computePeelCount(L, LoopSize, UP);
-  if (UP.PeelCount) {
-    UP.Runtime = false;
-    UP.Count = 1;
-    return ExplicitUnroll;
-  }
-
   // 6th priority is runtime unrolling.
   // Don't unroll a runtime trip count loop when it is disabled.
   if (HasRuntimeUnrollDisablePragma(L)) {

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) {