1 files changed, 256 insertions, 0 deletions
diff --git a/contrib/llvm-project/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/contrib/llvm-project/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
new file mode 100644
index 000000000000..613c52900331
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
@@ -0,0 +1,256 @@
+//===-- MachineFunctionSplitter.cpp - Split machine functions //-----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// \file
+// Uses profile information to split out cold blocks.
+//
+// This pass splits out cold machine basic blocks from the parent function. This
+// implementation leverages the basic block section framework. Blocks marked
+// cold by this pass are grouped together in a separate section prefixed with
+// ".text.unlikely.*". The linker can then group these together as a cold
+// section. The split part of the function is a contiguous region identified by
+// the symbol "foo.cold". Grouping all cold blocks across functions together
+// decreases fragmentation and improves icache and itlb utilization. Note that
+// the overall changes to the binary size are negligible; only a small number of
+// additional jump instructions may be introduced.
+//
+// For the original RFC of this pass please see
+// https://groups.google.com/d/msg/llvm-dev/RUegaMg-iqc/wFAVxa6fCgAJ
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Analysis/ProfileSummaryInfo.h"
+#include "llvm/CodeGen/BasicBlockSectionUtils.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Support/CommandLine.h"
+#include <optional>
+
+using namespace llvm;
+
+// FIXME: This cutoff value is CPU dependent and should be moved to
+// TargetTransformInfo once we consider enabling this on other platforms.
+// The value is expressed as a ProfileSummaryInfo integer percentile cutoff.
+// Defaults to 999950, i.e. all blocks colder than 99.995 percentile are split.
+// The default was empirically determined to be optimal when considering cutoff
+// values between 99%-ile to 100%-ile with respect to iTLB and icache metrics on
+// Intel CPUs.
+static cl::opt<unsigned>
+    PercentileCutoff("mfs-psi-cutoff",
+                     cl::desc("Percentile profile summary cutoff used to "
+                              "determine cold blocks. Unused if set to zero."),
+                     cl::init(999950), cl::Hidden);
+
+static cl::opt<unsigned> ColdCountThreshold(
+    "mfs-count-threshold",
+    cl::desc(
+        "Minimum number of times a block must be executed to be retained."),
+    cl::init(1), cl::Hidden);
+
+static cl::opt<bool> SplitAllEHCode(
+    "mfs-split-ehcode",
+    cl::desc("Splits all EH code and it's descendants by default."),
+    cl::init(false), cl::Hidden);
+
+namespace {
+
+class MachineFunctionSplitter : public MachineFunctionPass {
+public:
+  static char ID;
+  MachineFunctionSplitter() : MachineFunctionPass(ID) {
+    initializeMachineFunctionSplitterPass(*PassRegistry::getPassRegistry());
+  }
+
+  StringRef getPassName() const override {
+    return "Machine Function Splitter Transformation";
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override;
+
+  bool runOnMachineFunction(MachineFunction &F) override;
+};
+} // end anonymous namespace
+
+/// setDescendantEHBlocksCold - This splits all EH pads and blocks reachable
+/// only by EH pad as cold. This will help mark EH pads statically cold instead
+/// of relying on profile data.
+static void
+setDescendantEHBlocksCold(SmallVectorImpl<MachineBasicBlock *> &EHBlocks,
+                          MachineFunction &MF) {
+  MachineBasicBlock *StartBlock = &MF.front();
+  // A block can be unknown if its not reachable from anywhere
+  // EH if its only reachable from start blocks via some path through EH pads
+  // NonEH if it's reachable from Non EH blocks as well.
+  enum Status { Unknown = 0, EH = 1, NonEH = 2 };
+  DenseSet<MachineBasicBlock *> WorkList;
+  DenseMap<MachineBasicBlock *, Status> Statuses;
+
+  auto getStatus = [&](MachineBasicBlock *MBB) {
+    if (Statuses.find(MBB) != Statuses.end())
+      return Statuses[MBB];
+    else
+      return Unknown;
+  };
+
+  auto checkPredecessors = [&](MachineBasicBlock *MBB, Status Stat) {
+    for (auto *PredMBB : MBB->predecessors()) {
+      Status PredStatus = getStatus(PredMBB);
+      // If status of predecessor block has gone above current block
+      // we update current blocks status.
+      if (PredStatus > Stat)
+        Stat = PredStatus;
+    }
+    return Stat;
+  };
+
+  auto addSuccesors = [&](MachineBasicBlock *MBB) {
+    for (auto *SuccMBB : MBB->successors()) {
+      if (!SuccMBB->isEHPad())
+        WorkList.insert(SuccMBB);
+    }
+  };
+
+  // Insert the successors of start block
+  // and landing pads successor.
+  Statuses[StartBlock] = NonEH;
+  addSuccesors(StartBlock);
+  for (auto *LP : EHBlocks) {
+    addSuccesors(LP);
+    Statuses[LP] = EH;
+  }
+
+  // Worklist iterative algorithm.
+  while (!WorkList.empty()) {
+    auto *MBB = *WorkList.begin();
+    WorkList.erase(MBB);
+
+    Status OldStatus = getStatus(MBB);
+
+    // Check on predecessors and check for
+    // Status update.
+    Status NewStatus = checkPredecessors(MBB, OldStatus);
+
+    // Did the block status change?
+    bool changed = OldStatus != NewStatus;
+    if (changed) {
+      addSuccesors(MBB);
+      Statuses[MBB] = NewStatus;
+    }
+  }
+
+  for (auto Entry : Statuses) {
+    if (Entry.second == EH)
+      Entry.first->setSectionID(MBBSectionID::ColdSectionID);
+  }
+}
+
+static bool isColdBlock(const MachineBasicBlock &MBB,
+                        const MachineBlockFrequencyInfo *MBFI,
+                        ProfileSummaryInfo *PSI) {
+  std::optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB);
+  if (!Count)
+    return true;
+
+  if (PercentileCutoff > 0) {
+    return PSI->isColdCountNthPercentile(PercentileCutoff, *Count);
+  }
+  return (*Count < ColdCountThreshold);
+}
+
+bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
+  // We target functions with profile data. Static information in the form
+  // of exception handling code may be split to cold if user passes the
+  // mfs-split-ehcode flag.
+  bool UseProfileData = MF.getFunction().hasProfileData();
+  if (!UseProfileData && !SplitAllEHCode)
+    return false;
+
+  // TODO: We don't split functions where a section attribute has been set
+  // since the split part may not be placed in a contiguous region. It may also
+  // be more beneficial to augment the linker to ensure contiguous layout of
+  // split functions within the same section as specified by the attribute.
+  if (MF.getFunction().hasSection() ||
+      MF.getFunction().hasFnAttribute("implicit-section-name"))
+    return false;
+
+  // We don't want to proceed further for cold functions
+  // or functions of unknown hotness. Lukewarm functions have no prefix.
+  std::optional<StringRef> SectionPrefix = MF.getFunction().getSectionPrefix();
+  if (SectionPrefix &&
+      (*SectionPrefix == "unlikely" || *SectionPrefix == "unknown")) {
+    return false;
+  }
+
+  // Renumbering blocks here preserves the order of the blocks as
+  // sortBasicBlocksAndUpdateBranches uses the numeric identifier to sort
+  // blocks. Preserving the order of blocks is essential to retaining decisions
+  // made by prior passes such as MachineBlockPlacement.
+  MF.RenumberBlocks();
+  MF.setBBSectionsType(BasicBlockSection::Preset);
+
+  MachineBlockFrequencyInfo *MBFI = nullptr;
+  ProfileSummaryInfo *PSI = nullptr;
+  if (UseProfileData) {
+    MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
+    PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
+  }
+
+  SmallVector<MachineBasicBlock *, 2> LandingPads;
+  for (auto &MBB : MF) {
+    if (MBB.isEntryBlock())
+      continue;
+
+    if (MBB.isEHPad())
+      LandingPads.push_back(&MBB);
+    else if (UseProfileData && isColdBlock(MBB, MBFI, PSI) && !SplitAllEHCode)
+      MBB.setSectionID(MBBSectionID::ColdSectionID);
+  }
+
+  // Split all EH code and it's descendant statically by default.
+  if (SplitAllEHCode)
+    setDescendantEHBlocksCold(LandingPads, MF);
+  // We only split out eh pads if all of them are cold.
+  else {
+    bool HasHotLandingPads = false;
+    for (const MachineBasicBlock *LP : LandingPads) {
+      if (!isColdBlock(*LP, MBFI, PSI))
+        HasHotLandingPads = true;
+    }
+    if (!HasHotLandingPads) {
+      for (MachineBasicBlock *LP : LandingPads)
+        LP->setSectionID(MBBSectionID::ColdSectionID);
+    }
+  }
+  auto Comparator = [](const MachineBasicBlock &X, const MachineBasicBlock &Y) {
+    return X.getSectionID().Type < Y.getSectionID().Type;
+  };
+  llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator);
+  llvm::avoidZeroOffsetLandingPad(MF);
+  return true;
+}
+
+void MachineFunctionSplitter::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<MachineModuleInfoWrapperPass>();
+  AU.addRequired<MachineBlockFrequencyInfo>();
+  AU.addRequired<ProfileSummaryInfoWrapperPass>();
+}
+
+char MachineFunctionSplitter::ID = 0;
+INITIALIZE_PASS(MachineFunctionSplitter, "machine-function-splitter",
+                "Split machine functions using profile information", false,
+                false)
+
+MachineFunctionPass *llvm::createMachineFunctionSplitterPass() {
+  return new MachineFunctionSplitter();
+}