Hot cold splitting pass

Find cold blocks based on profile information (or optionally with static analysis).
Forward propagate profile information to all cold-blocks.
Outline a cold region.
Set calling conv and prof hint for the callsite of the outlined function.

Worked in collaboration with: Sebastian Pop <s.pop@samsung.com>
Differential Revision: https://reviews.llvm.org/D50658

llvm-svn: 341669

include/llvm/InitializePasses.h

@@ -161,6 +161,7 @@ void initializeGlobalOptLegacyPassPass(PassRegistry&);
 void initializeGlobalSplitPass(PassRegistry&);
 void initializeGlobalsAAWrapperPassPass(PassRegistry&);
 void initializeGuardWideningLegacyPassPass(PassRegistry&);
+void initializeHotColdSplittingLegacyPassPass(PassRegistry&);
 void initializeHWAddressSanitizerPass(PassRegistry&);
 void initializeIPCPPass(PassRegistry&);
 void initializeIPSCCPLegacyPassPass(PassRegistry&);

include/llvm/Transforms/IPO.h

@@ -201,6 +201,11 @@ Pass *createReversePostOrderFunctionAttrsPass();
 ///
 ModulePass *createMergeFunctionsPass();
 
+//===----------------------------------------------------------------------===//
+/// createHotColdSplittingPass - This pass outlines cold blocks into a separate
+/// function(s).
+ModulePass *createHotColdSplittingPass();
+
 //===----------------------------------------------------------------------===//
 /// createPartialInliningPass - This pass inlines parts of functions.
 ///

lib/Transforms/IPO/CMakeLists.txt

@@ -15,6 +15,7 @@ add_llvm_library(LLVMipo
   GlobalDCE.cpp
   GlobalOpt.cpp
   GlobalSplit.cpp
+  HotColdSplitting.cpp
   IPConstantPropagation.cpp
   IPO.cpp
   InferFunctionAttrs.cpp

lib/Transforms/IPO/HotColdSplitting.cpp

@@ -0,0 +1,370 @@
+//===- HotColdSplitting.cpp -- Outline Cold Regions -------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Outline cold regions to a separate function.
+// TODO: Update BFI and BPI
+// TODO: Add all the outlined functions to a separate section.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/BranchProbabilityInfo.h"
+#include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/OptimizationRemarkEmitter.h"
+#include "llvm/Analysis/PostDominators.h"
+#include "llvm/Analysis/ProfileSummaryInfo.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/IR/Type.h"
+#include "llvm/IR/Use.h"
+#include "llvm/IR/User.h"
+#include "llvm/IR/Value.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/BlockFrequency.h"
+#include "llvm/Support/BranchProbability.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/CodeExtractor.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/SSAUpdater.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+#include <algorithm>
+#include <cassert>
+
+#define DEBUG_TYPE "hotcoldsplit"
+
+STATISTIC(NumColdSESEFound,
+          "Number of cold single entry single exit (SESE) regions found.");
+STATISTIC(NumColdSESEOutlined,
+          "Number of cold single entry single exit (SESE) regions outlined.");
+
+using namespace llvm;
+
+static cl::opt<bool> EnableStaticAnalyis("hot-cold-static-analysis",
+                              cl::init(true), cl::Hidden);
+
+
+namespace {
+
+struct PostDomTree : PostDomTreeBase<BasicBlock> {
+  PostDomTree(Function &F) { recalculate(F); }
+};
+
+typedef DenseSet<const BasicBlock *> DenseSetBB;
+
+// From: https://reviews.llvm.org/D22558
+// Exit is not part of the region.
+static bool isSingleEntrySingleExit(BasicBlock *Entry, const BasicBlock *Exit,
+                                    DominatorTree *DT, PostDomTree *PDT,
+                                    SmallVectorImpl<BasicBlock *> &Region) {
+  if (!DT->dominates(Entry, Exit))
+    return false;
+
+  if (!PDT->dominates(Exit, Entry))
+    return false;
+
+  Region.push_back(Entry);
+  for (auto I = df_begin(Entry), E = df_end(Entry); I != E;) {
+    if (*I == Exit) {
+      I.skipChildren();
+      continue;
+    }
+    if (!DT->dominates(Entry, *I))
+      return false;
+    Region.push_back(*I);
+    ++I;
+  }
+  return true;
+}
+
+bool blockEndsInUnreachable(const BasicBlock &BB) {
+  if (BB.empty())
+    return true;
+  const TerminatorInst *I = BB.getTerminator();
+  if (isa<ReturnInst>(I) || isa<IndirectBrInst>(I))
+    return true;
+  // Unreachable blocks do not have any successor.
+  return succ_empty(&BB);
+}
+
+static
+bool unlikelyExecuted(const BasicBlock &BB) {
+  if (blockEndsInUnreachable(BB))
+    return true;
+  // Exception handling blocks are unlikely executed.
+  if (BB.isEHPad())
+    return true;
+  for (const Instruction &I : BB)
+    if (const CallInst *CI = dyn_cast<CallInst>(&I)) {
+      // The block is cold if it calls functions tagged as cold or noreturn.
+      if (CI->hasFnAttr(Attribute::Cold) ||
+          CI->hasFnAttr(Attribute::NoReturn))
+        return true;
+
+      // Assume that inline assembly is hot code.
+      if (isa<InlineAsm>(CI->getCalledValue()))
+        return false;
+    }
+  return false;
+}
+
+static DenseSetBB getHotBlocks(Function &F) {
+  // First mark all function basic blocks as hot or cold.
+  DenseSet<const BasicBlock *> ColdBlocks;
+  for (BasicBlock &BB : F)
+    if (unlikelyExecuted(BB))
+      ColdBlocks.insert(&BB);
+  // Forward propagation.
+  DenseSetBB AllColdBlocks;
+  SmallVector<const BasicBlock *, 8> WL;
+  DenseSetBB Visited; // Track hot blocks.
+
+  const BasicBlock *It = &F.front();
+  const TerminatorInst *TI = It->getTerminator();
+  if (!ColdBlocks.count(It)) {
+    Visited.insert(It);
+    // Breadth First Search to mark edges not reachable from cold.
+    WL.push_back(It);
+    while (WL.size() > 0) {
+      It = WL.pop_back_val();
+      for (const BasicBlock *Succ : successors(TI)) {
+        // Do not visit blocks that are cold.
+        if (!ColdBlocks.count(Succ) && !Visited.count(Succ)) {
+          Visited.insert(Succ);
+          WL.push_back(Succ);
+        }
+      }
+    }
+  }
+
+  return Visited;
+}
+
+class HotColdSplitting {
+public:
+  HotColdSplitting(ProfileSummaryInfo *ProfSI,
+                   function_ref<BlockFrequencyInfo *(Function &)> GBFI,
+                   std::function<OptimizationRemarkEmitter &(Function &)> *GORE)
+      : PSI(ProfSI), GetBFI(GBFI), GetORE(GORE) {}
+  bool run(Module &M);
+
+private:
+  bool shouldOutlineFrom(const Function &F) const;
+  Function *outlineColdBlocks(Function &F,
+                              const DenseSet<const BasicBlock *> &ColdBlock,
+                              DominatorTree *DT, PostDomTree *PDT);
+  Function *extractColdRegion(const SmallVectorImpl<BasicBlock *> &Region,
+                              DominatorTree *DT, BlockFrequencyInfo *BFI,
+                              OptimizationRemarkEmitter &ORE);
+  bool isOutlineCandidate(const SmallVectorImpl<BasicBlock *> &Region,
+                          const BasicBlock *Exit) const {
+    if (!Exit)
+      return false;
+    // TODO: Find a better metric to compute the size of region being outlined.
+    if (Region.size() == 1)
+      return false;
+    // Regions with landing pads etc.
+    for (const BasicBlock *BB : Region) {
+      if (BB->isEHPad() || BB->hasAddressTaken())
+        return false;
+    }
+    return true;
+  }
+  SmallPtrSet<const Function *, 2> OutlinedFunctions;
+  ProfileSummaryInfo *PSI;
+  function_ref<BlockFrequencyInfo *(Function &)> GetBFI;
+  std::function<OptimizationRemarkEmitter &(Function &)> *GetORE;
+};
+
+class HotColdSplittingLegacyPass : public ModulePass {
+public:
+  static char ID;
+  HotColdSplittingLegacyPass() : ModulePass(ID) {
+    initializeHotColdSplittingLegacyPassPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<AssumptionCacheTracker>();
+    AU.addRequired<BlockFrequencyInfoWrapperPass>();
+    AU.addRequired<ProfileSummaryInfoWrapperPass>();
+  }
+
+  bool runOnModule(Module &M) override;
+};
+
+} // end anonymous namespace
+
+// Returns false if the function should not be considered for hot-cold split
+// optimization. Already outlined functions have coldcc so no need to check
+// for them here.
+bool HotColdSplitting::shouldOutlineFrom(const Function &F) const {
+  if (F.size() <= 2)
+    return false;
+
+  if (F.hasAddressTaken())
+    return false;
+
+  if (F.hasFnAttribute(Attribute::AlwaysInline))
+    return false;
+
+  if (F.hasFnAttribute(Attribute::NoInline))
+    return false;
+
+  if (F.getCallingConv() == CallingConv::Cold)
+    return false;
+
+  if (PSI->isFunctionEntryCold(&F))
+    return false;
+  return true;
+}
+
+Function *
+HotColdSplitting::extractColdRegion(const SmallVectorImpl<BasicBlock *> &Region,
+                                    DominatorTree *DT, BlockFrequencyInfo *BFI,
+                                    OptimizationRemarkEmitter &ORE) {
+  LLVM_DEBUG(for (auto *BB : Region)
+          llvm::dbgs() << "\nExtracting: " << *BB;);
+  // TODO: Pass BFI and BPI to update profile information.
+  CodeExtractor CE(Region, DT, /*AggregateArgs*/ false, nullptr, nullptr,
+                   /* AllowVarargs */ false);
+
+  SetVector<Value *> Inputs, Outputs, Sinks;
+  CE.findInputsOutputs(Inputs, Outputs, Sinks);
+
+  // Do not extract regions that have live exit variables.
+  if (Outputs.size() > 0)
+    return nullptr;
+
+  if (Function *OutF = CE.extractCodeRegion()) {
+    User *U = *OutF->user_begin();
+    CallInst *CI = cast<CallInst>(U);
+    CallSite CS(CI);
+    NumColdSESEOutlined++;
+    OutF->setCallingConv(CallingConv::Cold);
+    CS.setCallingConv(CallingConv::Cold);
+    CI->setIsNoInline();
+    LLVM_DEBUG(llvm::dbgs() << "Outlined Region at block: " << Region.front());
+    return OutF;
+  }
+
+  ORE.emit([&]() {
+    return OptimizationRemarkMissed(DEBUG_TYPE, "ExtractFailed",
+                                    &*Region[0]->begin())
+           << "Failed to extract region at block "
+           << ore::NV("Block", Region.front());
+  });
+  return nullptr;
+}
+
+// Return the function created after outlining, nullptr otherwise.
+Function *HotColdSplitting::outlineColdBlocks(Function &F,
+                                              const  DenseSetBB &HotBlock,
+                                              DominatorTree *DT,
+                                              PostDomTree *PDT) {
+  auto BFI = GetBFI(F);
+  auto &ORE = (*GetORE)(F);
+  // Walking the dominator tree allows us to find the largest
+  // cold region.
+  BasicBlock *Begin = DT->getRootNode()->getBlock();
+  for (auto I = df_begin(Begin), E = df_end(Begin); I != E; ++I) {
+    BasicBlock *BB = *I;
+    if (PSI->isColdBB(BB, BFI) || !HotBlock.count(BB)) {
+      SmallVector<BasicBlock *, 4> ValidColdRegion, Region;
+      auto *BBNode = (*PDT)[BB];
+      auto Exit = BBNode->getIDom()->getBlock();
+      // We might need a virtual exit which post-dominates all basic blocks.
+      if (!Exit)
+        continue;
+      BasicBlock *ExitColdRegion = nullptr;
+      // Estimated cold region between a BB and its dom-frontier.
+      while (isSingleEntrySingleExit(BB, Exit, DT, PDT, Region) &&
+             isOutlineCandidate(Region, Exit)) {
+        ExitColdRegion = Exit;
+        ValidColdRegion = Region;
+        Region.clear();
+        // Update Exit recursively to its dom-frontier.
+        Exit = (*PDT)[Exit]->getIDom()->getBlock();
+      }
+      if (ExitColdRegion) {
+        ++NumColdSESEFound;
+        // Candidate for outlining. FIXME: Continue outlining.
+        // FIXME: Shouldn't need uniquing, debug isSingleEntrySingleExit
+        //std::sort(ValidColdRegion.begin(), ValidColdRegion.end());
+        auto last = std::unique(ValidColdRegion.begin(), ValidColdRegion.end());
+        ValidColdRegion.erase(last, ValidColdRegion.end());
+        return extractColdRegion(ValidColdRegion, DT, BFI, ORE);
+      }
+    }
+  }
+  return nullptr;
+}
+
+bool HotColdSplitting::run(Module &M) {
+  for (auto &F : M) {
+    if (!shouldOutlineFrom(F))
+      continue;
+    DominatorTree DT(F);
+    PostDomTree PDT(F);
+    PDT.recalculate(F);
+    DenseSetBB HotBlocks;
+    if (EnableStaticAnalyis) // Static analysis of cold blocks.
+      HotBlocks = getHotBlocks(F);
+
+    auto Outlined = outlineColdBlocks(F, HotBlocks, &DT, &PDT);
+    if (Outlined)
+      OutlinedFunctions.insert(Outlined);
+  }
+  return true;
+}
+
+bool HotColdSplittingLegacyPass::runOnModule(Module &M) {
+  if (skipModule(M))
+    return false;
+  ProfileSummaryInfo *PSI =
+      getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
+  auto GBFI = [this](Function &F) {
+    return &this->getAnalysis<BlockFrequencyInfoWrapperPass>(F).getBFI();
+  };
+  std::unique_ptr<OptimizationRemarkEmitter> ORE;
+  std::function<OptimizationRemarkEmitter &(Function &)> GetORE =
+      [&ORE](Function &F) -> OptimizationRemarkEmitter & {
+    ORE.reset(new OptimizationRemarkEmitter(&F));
+    return *ORE.get();
+  };
+
+  return HotColdSplitting(PSI, GBFI, &GetORE).run(M);
+}
+
+char HotColdSplittingLegacyPass::ID = 0;
+INITIALIZE_PASS_BEGIN(HotColdSplittingLegacyPass, "hotcoldsplit",
+                      "Hot Cold Splitting", false, false)
+INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
+INITIALIZE_PASS_END(HotColdSplittingLegacyPass, "hotcoldsplit",
+                    "Hot Cold Splitting", false, false)
+
+ModulePass *llvm::createHotColdSplittingPass() {
+  return new HotColdSplittingLegacyPass();
+}

lib/Transforms/IPO/IPO.cpp

@@ -34,6 +34,7 @@ void llvm::initializeIPO(PassRegistry &Registry) {
   initializeGlobalDCELegacyPassPass(Registry);
   initializeGlobalOptLegacyPassPass(Registry);
   initializeGlobalSplitPass(Registry);
+  initializeHotColdSplittingLegacyPassPass(Registry);
   initializeIPCPPass(Registry);
   initializeAlwaysInlinerLegacyPassPass(Registry);
   initializeSimpleInlinerPass(Registry);

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -104,6 +104,10 @@ static cl::opt<bool>
     EnablePrepareForThinLTO("prepare-for-thinlto", cl::init(false), cl::Hidden,
                             cl::desc("Enable preparation for ThinLTO."));
 
+cl::opt<bool> EnableHotColdSplit("hot-cold-split", cl::init(false), cl::Hidden,
+    cl::desc("Enable hot-cold splitting pass"));
+
+
 static cl::opt<bool> RunPGOInstrGen(
     "profile-generate", cl::init(false), cl::Hidden,
     cl::desc("Enable PGO instrumentation."));
@@ -497,6 +501,9 @@ void PassManagerBuilder::populateModulePassManager(
   // Infer attributes about declarations if possible.
   MPM.add(createInferFunctionAttrsLegacyPass());
 
+  if (EnableHotColdSplit)
+    MPM.add(createHotColdSplittingPass());
+
   addExtensionsToPM(EP_ModuleOptimizerEarly, MPM);
 
   if (OptLevel > 2)

test/Transforms/HotColdSplit/split-cold-1.ll

@@ -0,0 +1,34 @@
+; RUN: opt -hotcoldsplit -S < %s | FileCheck %s
+source_filename = "bugpoint-output-054409e.bc"
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.13.0"
+
+declare i32 @__gxx_personality_v0(...)
+
+; Outlined function is called from a basic block named codeRepl
+; CHECK: codeRepl:
+; CHECK-NEXT: call coldcc void @foo
+; Check that no recursive outlining is done.
+; CHECK-NOT: codeRepl:
+; Function Attrs: ssp uwtable
+define hidden void @foo() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+entry:
+  br i1 undef, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  unreachable
+
+if.end:                                           ; preds = %entry
+  br label %if.then12
+
+if.then12:                                        ; preds = %if.end
+  br label %cleanup40
+
+cleanup40:                                        ; preds = %if.then12
+  br label %return
+
+return:                                           ; preds = %cleanup40
+  ret void
+}
+
+