peel loops with runtime small trip counts

For Hexagon, peeling loops with small runtime trip count is beneficial for our
benchmarks. We set PeelCount in HexagonTargetInfo.cpp and we use PeelCount set
by the target for computing the desired peel count.

Differential Revision: https://reviews.llvm.org/D44880

llvm-svn: 329042

diff --git a/llvm/include/llvm/Transforms/Utils/UnrollLoop.h b/llvm/include/llvm/Transforms/Utils/UnrollLoop.h
index 3983637..d96c658 100644 (file)
--- a/llvm/include/llvm/Transforms/Utils/UnrollLoop.h
+++ b/llvm/include/llvm/Transforms/Utils/UnrollLoop.h
@@ -73,6 +73,8 @@ void computePeelCount(Loop *L, unsigned LoopSize,
                       TargetTransformInfo::UnrollingPreferences &UP,
                       unsigned &TripCount, ScalarEvolution &SE);
 
+bool canPeel(Loop *L);
+
 bool peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE,
               DominatorTree *DT, AssumptionCache *AC, bool PreserveLCSSA);
 

diff --git a/llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.cpp b/llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.cpp
index 3290b59..786f8eb 100644 (file)
--- a/llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.cpp
@@ -44,6 +44,13 @@ HexagonTTIImpl::getPopcntSupport(unsigned IntTyWidthInBit) const {
 void HexagonTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
                                              TTI::UnrollingPreferences &UP) {
   UP.Runtime = UP.Partial = true;
+  // Only try to peel innermost loops with small runtime trip counts.
+  if (L && L->empty() &&
+      SE.getSmallConstantTripCount(L) == 0 &&
+      SE.getSmallConstantMaxTripCount(L) > 0 &&
+      SE.getSmallConstantMaxTripCount(L) <= 5) {
+    UP.PeelCount = 2;
+  }
 }
 
 bool HexagonTTIImpl::shouldFavorPostInc() const {

diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
index a1b25a2..ad454d7 100644 (file)
--- a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -978,6 +978,9 @@ static LoopUnrollResult tryToUnrollLoop(
   if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0))
     return LoopUnrollResult::Unmodified;
 
+  if (UP.PeelCount && !canPeel(L))
+    UP.PeelCount = 0;
+
   SmallPtrSet<const Value *, 32> EphValues;
   CodeMetrics::collectEphemeralValues(L, &AC, EphValues);
 

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp b/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
index 5f465f3..f47744d 100644 (file)
--- a/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp
@@ -69,7 +69,7 @@ static const unsigned InfiniteIterationsToInvariance =
     std::numeric_limits<unsigned>::max();
 
 // Check whether we are capable of peeling this loop.
-static bool canPeel(Loop *L) {
+bool llvm::canPeel(Loop *L) {
   // Make sure the loop is in simplified form
   if (!L->isLoopSimplifyForm())
     return false;
@@ -221,6 +221,9 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
                             TargetTransformInfo::UnrollingPreferences &UP,
                             unsigned &TripCount, ScalarEvolution &SE) {
   assert(LoopSize > 0 && "Zero loop size is not allowed!");
+  // Save the UP.PeelCount value set by the target in
+  // TTI.getUnrollingPreferences or by the flag -unroll-peel-count.
+  unsigned TargetPeelCount = UP.PeelCount;
   UP.PeelCount = 0;
   if (!canPeel(L))
     return;
@@ -240,7 +243,9 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
     SmallDenseMap<PHINode *, unsigned> IterationsToInvariance;
     // Now go through all Phis to calculate their the number of iterations they
     // need to become invariants.
-    unsigned DesiredPeelCount = 0;
+    // Start the max computation with the UP.PeelCount value set by the target
+    // in TTI.getUnrollingPreferences or by the flag -unroll-peel-count.
+    unsigned DesiredPeelCount = TargetPeelCount;
     BasicBlock *BackEdge = L->getLoopLatch();
     assert(BackEdge && "Loop is not in simplified form?");
     for (auto BI = L->getHeader()->begin(); isa<PHINode>(&*BI); ++BI) {

diff --git a/llvm/test/Transforms/LoopUnroll/Hexagon/peel-small-loop.ll b/llvm/test/Transforms/LoopUnroll/Hexagon/peel-small-loop.ll
new file mode 100644 (file)
index 0000000..45c2553
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/Hexagon/peel-small-loop.ll
@@ -0,0 +1,37 @@
+; RUN: opt -loop-unroll -mtriple=hexagon -S < %s | FileCheck %s
+; Check that the loop is peeled twice for Hexagon.
+; CHECK: while.body.peel
+; CHECK: while.body.peel2
+
+%struct.STREAM = type { %union.anon, i32, i32 }
+%union.anon = type { i32* }
+
+define void @function(%struct.STREAM* nocapture readonly %b) local_unnamed_addr {
+entry:
+  %bitPtr3 = getelementptr inbounds %struct.STREAM, %struct.STREAM* %b, i32 0, i32 2
+  %0 = load i32, i32* %bitPtr3, align 4
+  %cmp11 = icmp ult i32 %0, 32
+  br i1 %cmp11, label %while.body.preheader, label %do.end
+
+while.body.preheader:
+  %value2 = getelementptr inbounds %struct.STREAM, %struct.STREAM* %b, i32 0, i32 1
+  %1 = load i32, i32* %value2, align 4
+  %w = getelementptr inbounds %struct.STREAM, %struct.STREAM* %b, i32 0, i32 0, i32 0
+  %2 = load i32*, i32** %w, align 4
+  br label %while.body
+
+while.body:
+  %bitPtr.014 = phi i32 [ %add, %while.body ], [ %0, %while.body.preheader ]
+  %value.013 = phi i32 [ %shl, %while.body ], [ %1, %while.body.preheader ]
+  %ptr.012 = phi i32* [ %incdec.ptr, %while.body ], [ %2, %while.body.preheader ]
+  %add = add nuw i32 %bitPtr.014, 8
+  %shr = lshr i32 %value.013, 24
+  %incdec.ptr = getelementptr inbounds i32, i32* %ptr.012, i32 1
+  store i32 %shr, i32* %ptr.012, align 4
+  %shl = shl i32 %value.013, 8
+  %cmp = icmp ult i32 %add, 17
+  br i1 %cmp, label %while.body, label %do.end
+
+do.end:
+  ret void
+}