Add Loop Sink pass to reverse the LICM based of basic block frequency.

Summary: LICM may hoist instructions to preheader speculatively. Before code generation, we need to sink down the hoisted instructions inside to loop if it's beneficial. This pass is a reverse of LICM: looking at instructions in preheader and sinks the instruction to basic blocks inside the loop body if basic block frequency is smaller than the preheader frequency.

Reviewers: hfinkel, davidxl, chandlerc

Subscribers: anna, modocache, mgorny, beanz, reames, dberlin, chandlerc, mcrosier, junbuml, sanjoy, mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D22778

llvm-svn: 285308

4 files changed, 337 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/Scalar/CMakeLists.txt b/llvm/lib/Transforms/Scalar/CMakeLists.txt
index 9f04344b8b0..d6cef5fd351 100644
--- a/llvm/lib/Transforms/Scalar/CMakeLists.txt
+++ b/llvm/lib/Transforms/Scalar/CMakeLists.txt
@@ -17,6 +17,7 @@ add_llvm_library(LLVMScalarOpts
   IndVarSimplify.cpp
   JumpThreading.cpp
   LICM.cpp
+  LoopSink.cpp
   LoadCombine.cpp
   LoopDeletion.cpp
   LoopDataPrefetch.cpp
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 6659c5ad1c2..172459beaea 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -100,10 +100,6 @@ static Instruction *
 CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
                             const LoopInfo *LI,
                             const LoopSafetyInfo *SafetyInfo);
-static bool canSinkOrHoistInst(Instruction &I, AliasAnalysis *AA,
-                               DominatorTree *DT,
-                               Loop *CurLoop, AliasSetTracker *CurAST,
-                               LoopSafetyInfo *SafetyInfo);
 
 namespace {
 struct LoopInvariantCodeMotion {
@@ -436,16 +432,9 @@ void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
         SafetyInfo->BlockColors = colorEHFunclets(*Fn);
 }
 
-/// Returns true if the hoister and sinker can handle this instruction.
-/// If SafetyInfo is nullptr, we are checking for sinking instructions from
-/// preheader to loop body (no speculation).
-/// If SafetyInfo is not nullptr, we are checking for hoisting/sinking
-/// instructions from loop body to preheader/exit. Check if the instruction
-/// can execute specultatively.
-///
-bool canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
-                        Loop *CurLoop, AliasSetTracker *CurAST,
-                        LoopSafetyInfo *SafetyInfo) {
+bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
+                              Loop *CurLoop, AliasSetTracker *CurAST,
+                              LoopSafetyInfo *SafetyInfo) {
   // Loads have extra constraints we have to verify before we can hoist them.
   if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
     if (!LI->isUnordered())
diff --git a/llvm/lib/Transforms/Scalar/LoopSink.cpp b/llvm/lib/Transforms/Scalar/LoopSink.cpp
new file mode 100644
index 00000000000..85249b83033
--- /dev/null
+++ b/llvm/lib/Transforms/Scalar/LoopSink.cpp
@@ -0,0 +1,328 @@
+//===-- LoopSink.cpp - Loop Sink Pass ------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass does the inverse transformation of what LICM does.
+// It traverses all of the instructions in the loop's preheader and sinks
+// them to the loop body where frequency is lower than the loop's preheader.
+// This pass is a reverse-transformation of LICM. It differs from the Sink
+// pass in the following ways:
+//
+// * It only handles sinking of instructions from the loop's preheader to the
+//   loop's body
+// * It uses alias set tracker to get more accurate alias info
+// * It uses block frequency info to find the optimal sinking locations
+//
+// Overall algorithm:
+//
+// For I in Preheader:
+//   InsertBBs = BBs that uses I
+//   For BB in sorted(LoopBBs):
+//     DomBBs = BBs in InsertBBs that are dominated by BB
+//     if freq(DomBBs) > freq(BB)
+//       InsertBBs = UseBBs - DomBBs + BB
+//   For BB in InsertBBs:
+//     Insert I at BB's beginning
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/AliasSetTracker.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/LoopPassManager.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "loopsink"
+
+STATISTIC(NumLoopSunk, "Number of instructions sunk into loop");
+STATISTIC(NumLoopSunkCloned, "Number of cloned instructions sunk into loop");
+
+static cl::opt<unsigned> SinkFrequencyPercentThreshold(
+    "sink-freq-percent-threshold", cl::Hidden, cl::init(90),
+    cl::desc("Do not sink instructions that require cloning unless they "
+             "execute less than this percent of the time."));
+
+static cl::opt<unsigned> MaxNumberOfUseBBsForSinking(
+    "max-uses-for-sinking", cl::Hidden, cl::init(30),
+    cl::desc("Do not sink instructions that have too many uses."));
+
+/// Return adjusted total frequency of \p BBs.
+///
+/// * If there is only one BB, sinking instruction will not introduce code
+///   size increase. Thus there is no need to adjust the frequency.
+/// * If there are more than one BB, sinking would lead to code size increase.
+///   In this case, we add some "tax" to the total frequency to make it harder
+///   to sink. E.g.
+///     Freq(Preheader) = 100
+///     Freq(BBs) = sum(50, 49) = 99
+///   Even if Freq(BBs) < Freq(Preheader), we will not sink from Preheade to
+///   BBs as the difference is too small to justify the code size increase.
+///   To model this, The adjusted Freq(BBs) will be:
+///     AdjustedFreq(BBs) = 99 / SinkFrequencyPercentThreshold%
+static BlockFrequency adjustedSumFreq(SmallPtrSetImpl<BasicBlock *> &BBs,
+                                      BlockFrequencyInfo &BFI) {
+  BlockFrequency T = 0;
+  for (BasicBlock *B : BBs)
+    T += BFI.getBlockFreq(B);
+  if (BBs.size() > 1)
+    T /= BranchProbability(SinkFrequencyPercentThreshold, 100);
+  return T;
+}
+
+/// Return a set of basic blocks to insert sinked instructions.
+///
+/// The returned set of basic blocks (BBsToSinkInto) should satisfy:
+///
+/// * Inside the loop \p L
+/// * For each UseBB in \p UseBBs, there is at least one BB in BBsToSinkInto
+///   that domintates the UseBB
+/// * Has minimum total frequency that is no greater than preheader frequency
+///
+/// The purpose of the function is to find the optimal sinking points to
+/// minimize execution cost, which is defined as "sum of frequency of
+/// BBsToSinkInto".
+/// As a result, the returned BBsToSinkInto needs to have minimum total
+/// frequency.
+/// Additionally, if the total frequency of BBsToSinkInto exceeds preheader
+/// frequency, the optimal solution is not sinking (return empty set).
+///
+/// \p ColdLoopBBs is used to help find the optimal sinking locations.
+/// It stores a list of BBs that is:
+///
+/// * Inside the loop \p L
+/// * Has a frequency no larger than the loop's preheader
+/// * Sorted by BB frequency
+///
+/// The complexity of the function is O(UseBBs.size() * ColdLoopBBs.size()).
+/// To avoid expensive computation, we cap the maximum UseBBs.size() in its
+/// caller.
+static SmallPtrSet<BasicBlock *, 2>
+findBBsToSinkInto(const Loop &L, const SmallPtrSetImpl<BasicBlock *> &UseBBs,
+                  const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
+                  DominatorTree &DT, BlockFrequencyInfo &BFI) {
+  SmallPtrSet<BasicBlock *, 2> BBsToSinkInto;
+  if (UseBBs.size() == 0)
+    return BBsToSinkInto;
+
+  BBsToSinkInto.insert(UseBBs.begin(), UseBBs.end());
+  SmallPtrSet<BasicBlock *, 2> BBsDominatedByColdestBB;
+
+  // For every iteration:
+  //   * Pick the ColdestBB from ColdLoopBBs
+  //   * Find the set BBsDominatedByColdestBB that satisfy:
+  //     - BBsDominatedByColdestBB is a subset of BBsToSinkInto
+  //     - Every BB in BBsDominatedByColdestBB is dominated by ColdestBB
+  //   * If Freq(ColdestBB) < Freq(BBsDominatedByColdestBB), remove
+  //     BBsDominatedByColdestBB from BBsToSinkInto, add ColdestBB to
+  //     BBsToSinkInto
+  for (BasicBlock *ColdestBB : ColdLoopBBs) {
+    BBsDominatedByColdestBB.clear();
+    for (BasicBlock *SinkedBB : BBsToSinkInto)
+      if (DT.dominates(ColdestBB, SinkedBB))
+        BBsDominatedByColdestBB.insert(SinkedBB);
+    if (BBsDominatedByColdestBB.size() == 0)
+      continue;
+    if (adjustedSumFreq(BBsDominatedByColdestBB, BFI) >
+        BFI.getBlockFreq(ColdestBB)) {
+      for (BasicBlock *DominatedBB : BBsDominatedByColdestBB) {
+        BBsToSinkInto.erase(DominatedBB);
+      }
+      BBsToSinkInto.insert(ColdestBB);
+    }
+  }
+
+  // If the total frequency of BBsToSinkInto is larger than preheader frequency,
+  // do not sink.
+  if (adjustedSumFreq(BBsToSinkInto, BFI) >
+      BFI.getBlockFreq(L.getLoopPreheader()))
+    BBsToSinkInto.clear();
+  return BBsToSinkInto;
+}
+
+// Sinks \p I from the loop \p L's preheader to its uses. Returns true if
+// sinking is successful.
+// \p LoopBlockNumber is used to sort the insertion blocks to ensure
+// determinism.
+static bool sinkInstruction(Loop &L, Instruction &I,
+                            const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
+                            const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber,
+                            LoopInfo &LI, DominatorTree &DT,
+                            BlockFrequencyInfo &BFI) {
+  // Compute the set of blocks in loop L which contain a use of I.
+  SmallPtrSet<BasicBlock *, 2> BBs;
+  for (auto &U : I.uses()) {
+    Instruction *UI = cast<Instruction>(U.getUser());
+    // We cannot sink I to PHI-uses.
+    if (dyn_cast<PHINode>(UI))
+      return false;
+    // We cannot sink I if it has uses outside of the loop.
+    if (!L.contains(LI.getLoopFor(UI->getParent())))
+      return false;
+    BBs.insert(UI->getParent());
+  }
+
+  // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
+  // BBs.size() to avoid expensive computation.
+  // FIXME: Handle code size growth for min_size and opt_size.
+  if (BBs.size() > MaxNumberOfUseBBsForSinking)
+    return false;
+
+  // Find the set of BBs that we should insert a copy of I.
+  SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
+      findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI);
+  if (BBsToSinkInto.empty())
+    return false;
+
+  // Copy the final BBs into a vector and sort them using the total ordering
+  // of the loop block numbers as iterating the set doesn't give a useful
+  // order. No need to stable sort as the block numbers are a total ordering.
+  SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
+  SortedBBsToSinkInto.insert(SortedBBsToSinkInto.begin(), BBsToSinkInto.begin(),
+                             BBsToSinkInto.end());
+  std::sort(SortedBBsToSinkInto.begin(), SortedBBsToSinkInto.end(),
+            [&](BasicBlock *A, BasicBlock *B) {
+              return *LoopBlockNumber.find(A) < *LoopBlockNumber.find(B);
+            });
+
+  BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
+  // FIXME: Optimize the efficiency for cloned value replacement. The current
+  //        implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
+  for (BasicBlock *N : SortedBBsToSinkInto) {
+    if (N == MoveBB)
+      continue;
+    // Clone I and replace its uses.
+    Instruction *IC = I.clone();
+    IC->setName(I.getName());
+    IC->insertBefore(&*N->getFirstInsertionPt());
+    // Replaces uses of I with IC in N
+    for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
+      Use &U = *UI++;
+      auto *I = cast<Instruction>(U.getUser());
+      if (I->getParent() == N)
+        U.set(IC);
+    }
+    // Replaces uses of I with IC in blocks dominated by N
+    replaceDominatedUsesWith(&I, IC, DT, N);
+    DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
+                 << '\n');
+    NumLoopSunkCloned++;
+  }
+  DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
+  NumLoopSunk++;
+  I.moveBefore(&*MoveBB->getFirstInsertionPt());
+
+  return true;
+}
+
+/// Sinks instructions from loop's preheader to the loop body if the
+/// sum frequency of inserted copy is smaller than preheader's frequency.
+static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
+                                          DominatorTree &DT,
+                                          BlockFrequencyInfo &BFI,
+                                          ScalarEvolution *SE) {
+  BasicBlock *Preheader = L.getLoopPreheader();
+  if (!Preheader)
+    return false;
+
+  const BlockFrequency PreheaderFreq = BFI.getBlockFreq(Preheader);
+  // If there are no basic blocks with lower frequency than the preheader then
+  // we can avoid the detailed analysis as we will never find profitable sinking
+  // opportunities.
+  if (all_of(L.blocks(), [&](const BasicBlock *BB) {
+        return BFI.getBlockFreq(BB) > PreheaderFreq;
+      }))
+    return false;
+
+  bool Changed = false;
+  AliasSetTracker CurAST(AA);
+
+  // Compute alias set.
+  for (BasicBlock *BB : L.blocks())
+    CurAST.add(*BB);
+
+  // Sort loop's basic blocks by frequency
+  SmallVector<BasicBlock *, 10> ColdLoopBBs;
+  SmallDenseMap<BasicBlock *, int, 16> LoopBlockNumber;
+  int i = 0;
+  for (BasicBlock *B : L.blocks())
+    if (BFI.getBlockFreq(B) < BFI.getBlockFreq(L.getLoopPreheader())) {
+      ColdLoopBBs.push_back(B);
+      LoopBlockNumber[B] = ++i;
+    }
+  std::stable_sort(ColdLoopBBs.begin(), ColdLoopBBs.end(),
+                   [&](BasicBlock *A, BasicBlock *B) {
+                     return BFI.getBlockFreq(A) < BFI.getBlockFreq(B);
+                   });
+
+  // Traverse preheader's instructions in reverse order becaue if A depends
+  // on B (A appears after B), A needs to be sinked first before B can be
+  // sinked.
+  for (auto II = Preheader->rbegin(), E = Preheader->rend(); II != E;) {
+    Instruction *I = &*II++;
+    if (!L.hasLoopInvariantOperands(I) ||
+        !canSinkOrHoistInst(*I, &AA, &DT, &L, &CurAST, nullptr))
+      continue;
+    if (sinkInstruction(L, *I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI))
+      Changed = true;
+  }
+
+  if (Changed && SE)
+    SE->forgetLoopDispositions(&L);
+  return Changed;
+}
+
+namespace {
+struct LegacyLoopSinkPass : public LoopPass {
+  static char ID;
+  LegacyLoopSinkPass() : LoopPass(ID) {
+    initializeLegacyLoopSinkPassPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnLoop(Loop *L, LPPassManager &LPM) override {
+    if (skipLoop(L))
+      return false;
+
+    auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
+    return sinkLoopInvariantInstructions(
+        *L, getAnalysis<AAResultsWrapperPass>().getAAResults(),
+        getAnalysis<LoopInfoWrapperPass>().getLoopInfo(),
+        getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
+        getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI(),
+        SE ? &SE->getSE() : nullptr);
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    AU.addRequired<BlockFrequencyInfoWrapperPass>();
+    getLoopAnalysisUsage(AU);
+  }
+};
+}
+
+char LegacyLoopSinkPass::ID = 0;
+INITIALIZE_PASS_BEGIN(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false,
+                      false)
+INITIALIZE_PASS_DEPENDENCY(LoopPass)
+INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
+INITIALIZE_PASS_END(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false, false)
+
+Pass *llvm::createLoopSinkPass() { return new LegacyLoopSinkPass(); }
diff --git a/llvm/lib/Transforms/Scalar/Scalar.cpp b/llvm/lib/Transforms/Scalar/Scalar.cpp
index 4e6bc6337cb..e6dba082695 100644
--- a/llvm/lib/Transforms/Scalar/Scalar.cpp
+++ b/llvm/lib/Transforms/Scalar/Scalar.cpp
@@ -51,6 +51,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeIndVarSimplifyLegacyPassPass(Registry);
   initializeJumpThreadingPass(Registry);
   initializeLegacyLICMPassPass(Registry);
+  initializeLegacyLoopSinkPassPass(Registry);
   initializeLoopDataPrefetchLegacyPassPass(Registry);
   initializeLoopDeletionLegacyPassPass(Registry);
   initializeLoopAccessLegacyAnalysisPass(Registry);
@@ -141,6 +142,10 @@ void LLVMAddJumpThreadingPass(LLVMPassManagerRef PM) {
   unwrap(PM)->add(createJumpThreadingPass());
 }
 
+void LLVMAddLoopSinkPass(LLVMPassManagerRef PM) {
+  unwrap(PM)->add(createLoopSinkPass());
+}
+
 void LLVMAddLICMPass(LLVMPassManagerRef PM) {
   unwrap(PM)->add(createLICMPass());
 }