[BOLT] stale profile matching [part 1 out of 2]

[spupyrev]

Summary

BOLT often has to deal with profiles collected on binaries built from several
revisions behind release. As a result, a certain percentage of functions is
considered stale and not optimized. This diff adds an ability to match profile
to functions that are not 100% binary identical, which increases the
optimization coverage and boosts the performance of applications.

The algorithm consists of two phases: matching and inference:

• At the matching phase, we try to "guess" as many block and jump counts from the stale profile as possible. To this end, the content of each basic block is hashed and stored in the (yaml) profile. When BOLT optimizes a binary, it computes block hashes and identifies the corresponding entries in the stale profile. It yields a partial profile for every CFG in the binary.
• At the inference phase, we employ a network flow-based algorithm (profi) to reconstruct "realistic" block and jump counts from the partial profile generated at the first stage. In practice, we don't always produce proper profile data but the majority (e.g., >90%) of CFGs get the correct counts.

This is a first part of the change; the next stacked diff extends the block hashing
and provides perf evaluation numbers.

[yota9]

Wow, would definitely take a look to this patch, thank you!

[spupyrev]

extended comments

[spupyrev]

Added a test and adjusted logging

[spupyrev]

fixing a typo

[spupyrev]

rebase

[spupyrev]

Adjusting tests. Should be ready for review now

[spupyrev]

nit

[spupyrev]

@maksfb this is ready for review

[Amir]

llvm/lib/Transforms/Utils/SampleProfileInference.cpp

1325 ↗

(On Diff #519886)

There's an issue in shared build mode:

ld.lld: error: undefined symbol: llvm::applyFlowInference(llvm::ProfiParams const&, llvm::FlowFunction&)
>>> referenced by StaleProfileMatching.cpp
>>>               tools/bolt/lib/Profile/CMakeFiles/LLVMBOLTProfile.dir/StaleProfileMatching.cpp.o:(llvm::bolt::applyInference(llvm::FlowFunction&))

[spupyrev]

fixed the build and rebased

[Amir]

I'd rather move changes to llvm/lib/Transforms/Utils/SampleProfileInference.cpp to a separate diff to test with existing users and land separately as the changes don't seem to be specific to BOLT.

bolt/lib/Profile/StaleProfileMatching.cpp

139

Can we return FlowFunction object from this function?

147

Do we really need this? since we're calling updateLayoutIndices, we have BB->getLayoutIndex().

286–288

290–292

313

Can we use llvm/ADT/GraphTraits + BreadthFirstIterator to avoid reimplementing this?

384–387

Please use llvm::any_of

506

It's just removing CTCMispredCount annotation if it exists.

[spupyrev]

bolt/lib/Profile/StaleProfileMatching.cpp

147

great, thanks

313

If I understand correctly, that would require adding some boilerplate code for iterators over the nodes and their neighbors? In addition, I need to run an "inverse" BFS starting from all reachable sinks... It certainly can be done, but I'm not sure if the implementation would be any simpler. Is there a good example of how to implement something similar with built-in types?

506

which is exactly what we're trying to accomplish, no?

[spupyrev]

comments

[Amir]

bolt/lib/Profile/StaleProfileMatching.cpp

313

If I understand correctly, that would require adding some boilerplate code for iterators over the nodes and their neighbors? In addition, I need to run an "inverse" BFS starting from all reachable sinks... It certainly can be done, but I'm not sure if the implementation would be any simpler. Is there a good example of how to implement something similar with built-in types?

No, I don't have any good examples of using ADT algorithms with built-in types. LLVM classes use them throughout, with examples in llvm/unittests/ADT/. I tried using them with our BinaryBasicBlock but we implement GraphTraits interfaces. I guess it's fine to leave the BFS implementation as-is if FlowFunction doesn't already use any of ADT stuff, but in general it's a good idea to reuse LLVM's algorithms if you end up reimplementing them in several places.

506

I think it's better to be explicit and use removeAnnotation directly.

[spupyrev]

removeAnnotation

[maksfb]

Thanks for this patch, Sergey! Now that it's no longer a draft, can you update the title and summary? Do you plan to turn the option on by default in a separate patch?

bolt/lib/Profile/StaleProfileMatching.cpp

198

Some multi-entry functions will have secondary entry points reachable from the "main" entry point. In that case, their InDegree will be non-zero. Do you want to capture those here as well? The proper way to check if the block is reachable from outside the function is to check isEntryPoint().

349

nit:

352

nit:

422

Would assignProfile() describe the function more accurately?

520

Does the algorithm depend on the layout? I.e. will inferred profile be different if we supply a different order of basic blocks?

[spupyrev]

Now that it's no longer a draft, can you update the title and summary? Do you plan to turn the option on by default in a separate patch?

For context, this diff alone isn't enough for the feature; we still have https://reviews.llvm.org/D146661, which provides an actual hash computation. I'll adjust the summary to highlight that.
Once both diffs are reviewed, we plan to turn this on for a couple of services and monitor the results. If there are no issues, we can make this on by default.

bolt/lib/Profile/StaleProfileMatching.cpp

198

These dummy entry blocks are designed exactly to capture extra entry points. From my tests on the clang binary, they do _not_ have jumps from the main entry point, and thus, their InDegree is zero. Perhaps, there are some other cases that I don't see on clang?

Anyhow, for blocks with InDegree>0, we don't need to make special adjustments; they will be handled naturally.

520

In theory, the order shouldn't matter much. Here we updateLayoutIndices only to be able to index the blocks.

In practice however, there are "almost identical" basic blocks, especially with aggressive inlining. Such blocks have identical bodies, neighbors with identical bodies etc. There is no way to locally determine which blocks belong to which profile item. In order to break ties in such cases, I decided to simply use block addresses (that is, offsets) as a tie breaking rule. It doesn't happen too often but without the tie breaking rule the inference quality is slightly worse. (Using block indices in the layout also works here but provides a tiny regression in comparison to addresses)

[spupyrev]

review

[maksfb]

bolt/lib/Profile/StaleProfileMatching.cpp

198

Secondary entry points normally come from assembly code, perhaps from some Fortran programs as well. HHVM has memcpy written like that: https://github.com/facebook/hhvm/blob/master/hphp/util/memcpy-x64.S

263–264

nit:

520

I see. So then BlockOrder is used only for mapping blocks in BinaryFunction to FlowFunction and back?

[spupyrev]

comments

[spupyrev]

bolt/lib/Profile/StaleProfileMatching.cpp

198

You're right, i checked the HHVM binary and modified the condition here. Surprisingly, there are also instances where !BB->isEntryPoint() && InDegree[I] == 0, so we need both conditions here

520

Correct

[spupyrev]

making code compile

[maksfb]

LGTM. See comments with mostly nits. We can probably be more graceful while mapping basic blocks, but this issue can be addressed separately.

bolt/lib/Profile/StaleProfileMatching.cpp

200–201

nit: remove those variables

213–214

likewise

332

465–467

nit:

488–489

nit:

524

Will this work?

[Amir]

I'm trying to see if we can reuse SampleProfileInference code as much as possible given it's already templated and used with LLVM IR/MIR (Flow-Sensitive Sample AutoFDO and Context-Sensitive Sample PGO respectively).

bolt/lib/Profile/StaleProfileMatching.cpp

139

What's the primary motivation behind adding a BinaryFunction-specific version instead of using SampleProfileInference<BT>::initFunction?
Is it the fact that BF can have multiple entry points hence we need a dummy source node, and/or custom handling of EH control flow?

161

What outdegree is used for?

313

Does it make a difference to use BFS instead of DFS if the goal is collecting (un)reachable blocks?

There's an overlap in functionality with SampleProfileInference<BT>::apply which finds nodes reachable from source and sink using depth_first_ext and inverse_depth_first_ext.

[spupyrev]

suggested (minor) edits

bolt/include/bolt/Core/BinaryFunction.h

private:
   float ProfileMatchRatio{0.0f};
 
   /// Raw branch count for this function in the profile
   uint64_t RawBranchCount{0};
 
   /// Indicates the type of profile the function is using.
   uint16_t ProfileFlags{PF_NONE};
 
+  /// True if the function's input profile data has been inaccurate but has
+  /// been adjusted by the profile inference algorithm.
+  bool HasInferredProfile{false};
+
   /// For functions with mismatched profile we store all call profile
   /// information at a function level (as opposed to tying it to
   /// specific call sites).
   IndirectCallSiteProfile AllCallSites;
 
   /// Score of the function (estimated number of instructions executed,
   /// according to profile data). -1 if the score has not been calculated yet.
   mutable int64_t FunctionScore{-1};
if (ExecutionCount == COUNT_NO_PROFILE)
       ExecutionCount = 0;
     ProfileFlags = Flags;
     ProfileMatchRatio = 1.0f;
   }
 
   /// Return flags describing a profile for this function.
   uint16_t getProfileFlags() const { return ProfileFlags; }
 
+  /// Return true if the function's input profile data has been inaccurate but
+  /// has been corrected by the profile inference algorithm.
+  bool hasInferredProfile() const { return HasInferredProfile; }
+
+  void setHasInferredProfile(bool Inferred) { HasInferredProfile = Inferred; }
+
   void addCFIInstruction(uint64_t Offset, MCCFIInstruction &&Inst) {
     assert(!Instructions.empty());
 
     // Fix CFI instructions skipping NOPs. We need to fix this because changing
     // CFI state after a NOP, besides being wrong and inaccurate,  makes it
     // harder for us to recover this information, since we can create empty BBs
     // with NOPs and then reorder it away.
     // We fix this by moving the CFI instruction just before any NOPs.

bolt/include/bolt/Profile/YAMLProfileReader.h

private:
 
   /// Strict matching of a name in a profile to its contents.
   StringMap<yaml::bolt::BinaryFunctionProfile *> ProfileNameToProfile;
 
   /// Populate \p Function profile with the one supplied in YAML format.
   bool parseFunctionProfile(BinaryFunction &Function,
                             const yaml::bolt::BinaryFunctionProfile &YamlBF);
 
+  /// Infer function profile from stale data (collected on older binaries).
+  bool inferStaleProfile(BinaryFunction &Function,
+                         const yaml::bolt::BinaryFunctionProfile &YamlBF);
+
   /// Initialize maps for profile matching.
   void buildNameMaps(std::map<uint64_t, BinaryFunction> &Functions);
 
   /// Update matched YAML -> BinaryFunction pair.
   void matchProfileToFunction(yaml::bolt::BinaryFunctionProfile &YamlBF,
                               BinaryFunction &BF) {
     if (YamlBF.Id >= YamlProfileToFunction.size())
       YamlProfileToFunction.resize(YamlBF.Id + 1);

bolt/lib/Passes/BinaryPasses.cpp

outs() << "Worst average bias observed in " << WorstBiasFunc->getPrintName()
            << "\n";
     LLVM_DEBUG(WorstBiasFunc->dump());
   }
 }
 
 void PrintProgramStats::runOnFunctions(BinaryContext &BC) {
   uint64_t NumRegularFunctions = 0;
   uint64_t NumStaleProfileFunctions = 0;
+  uint64_t NumAllStaleFunctions = 0;
+  uint64_t NumInferredFunctions = 0;
   uint64_t NumNonSimpleProfiledFunctions = 0;
   uint64_t NumUnknownControlFlowFunctions = 0;
   uint64_t TotalSampleCount = 0;
   uint64_t StaleSampleCount = 0;
+  uint64_t InferredSampleCount = 0;
   std::vector<const BinaryFunction *> ProfiledFunctions;
   const char *StaleFuncsHeader = "BOLT-INFO: Functions with stale profile:\n";
   for (auto &BFI : BC.getBinaryFunctions()) {
     const BinaryFunction &Function = BFI.second;
 
     // Ignore PLT functions for stats.
     if (Function.isPLTFunction())
       continue;
for (auto &BFI : BC.getBinaryFunctions()) {
     if (!Function.hasProfile())
       continue;
 
     uint64_t SampleCount = Function.getRawBranchCount();
     TotalSampleCount += SampleCount;
 
     if (Function.hasValidProfile()) {
       ProfiledFunctions.push_back(&Function);
+      if (Function.hasInferredProfile()) {
+        ++NumInferredFunctions;
+        InferredSampleCount += SampleCount;
+        ++NumAllStaleFunctions;
+      }
     } else {
       if (opts::ReportStaleFuncs) {
         outs() << StaleFuncsHeader;
         StaleFuncsHeader = "";
         outs() << "  " << Function << '\n';
       }
       ++NumStaleProfileFunctions;
       StaleSampleCount += SampleCount;
+      ++NumAllStaleFunctions;
     }
   }
   BC.NumProfiledFuncs = ProfiledFunctions.size();
   BC.NumStaleProfileFuncs = NumStaleProfileFunctions;
 
   const size_t NumAllProfiledFunctions =
       ProfiledFunctions.size() + NumStaleProfileFunctions;
   outs() << "BOLT-INFO: " << NumAllProfiledFunctions << " out of "
if (TotalSampleCount > 0) {
                 " (possibly stale) profile.\n";
     }
     if (PctStale > opts::StaleThreshold) {
       errs() << "BOLT-ERROR: stale functions exceed specified threshold of "
              << opts::StaleThreshold << "%. Exiting.\n";
       exit(1);
     }
   }
+  if (NumInferredFunctions) {
+    outs() << format("BOLT-INFO: inferred profile for %d (%.2f%% of profiled, "
+                     "%.2f%% of stale) functions responsible for %.2f%% samples"
+                     " (%zu out of %zu)\n",
+                     NumInferredFunctions,
+                     100.0 * NumInferredFunctions / NumAllProfiledFunctions,
+                     100.0 * NumInferredFunctions / NumAllStaleFunctions,
+                     100.0 * InferredSampleCount / TotalSampleCount,
+                     InferredSampleCount, TotalSampleCount);
+  }
 
   if (const uint64_t NumUnusedObjects = BC.getNumUnusedProfiledObjects()) {
     outs() << "BOLT-INFO: profile for " << NumUnusedObjects
            << " objects was ignored\n";
   }
 
   if (ProfiledFunctions.size() > 10) {
     if (opts::Verbosity >= 1) {

bolt/lib/Profile/CMakeLists.txt

 add_llvm_library(LLVMBOLTProfile
   BoltAddressTranslation.cpp
   DataAggregator.cpp
   DataReader.cpp
   Heatmap.cpp
   ProfileReaderBase.cpp
+  StaleProfileMatching.cpp
   YAMLProfileReader.cpp
   YAMLProfileWriter.cpp
 
   DISABLE_LLVM_LINK_LLVM_DYLIB
 
   LINK_COMPONENTS
   Support
+  TransformUtils
   )
 
 target_link_libraries(LLVMBOLTProfile
   PRIVATE
   LLVMBOLTCore
   LLVMBOLTPasses
   LLVMBOLTUtils
   )

bolt/lib/Profile/StaleProfileMatching.cpp

+//===- bolt/Profile/StaleProfileMatching.cpp - Profile data matching   ----===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// BOLT often has to deal with profiles collected on binaries built from several
+// revisions behind release. As a result, a certain percentage of functions is
+// considered stale and not optimized. This file implements an ability to match
+// profile to functions that are not 100% binary identical, and thus, increasing
+// the optimization coverage and boost the performance of applications.
+//
+// The algorithm consists of two phases: matching and inference:
+// - At the matching phase, we try to "guess" as many block and jump counts from
+//   the stale profile as possible. To this end, the content of each basic block
+//   is hashed and stored in the (yaml) profile. When BOLT optimizes a binary,
+//   it computes block hashes and identifies the corresponding entries in the
+//   stale profile. It yields a partial profile for every CFG in the binary.
+// - At the inference phase, we employ a network flow-based algorithm (profi) to
+//   reconstruct "realistic" block and jump counts from the partial profile
+//   generated at the first stage. In practice, we don't always produce proper
+//   profile data but the majority (e.g., >90%) of CFGs get the correct counts.
+//
+//===----------------------------------------------------------------------===//
+
+#include "bolt/Core/HashUtilities.h"
+#include "bolt/Profile/YAMLProfileReader.h"
+#include "llvm/ADT/Hashing.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Transforms/Utils/SampleProfileInference.h"
+
+#include <queue>
+
+using namespace llvm;
+
+namespace opts {
+
+extern cl::OptionCategory BoltOptCategory;
+
+cl::opt<bool>
+    InferStaleProfile("infer-stale-profile",
+                      cl::desc("Infer counts from stale profile data."),
+                      cl::init(false), cl::Hidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingMaxFuncSize(
+    "stale-matching-max-func-size",
+    cl::desc("The maximum size of a function to consider for inference."),
+    cl::init(10000), cl::Hidden, cl::cat(BoltOptCategory));
+
+// Parameters of the profile inference algorithm. The default values are tuned
+// on several benchmarks.
+cl::opt<bool> StaleMatchingEvenFlowDistribution(
+    "stale-matching-even-flow-distribution",
+    cl::desc("Try to evenly distribute flow when there are multiple equally "
+             "likely options."),
+    cl::init(true), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<bool> StaleMatchingRebalanceUnknown(
+    "stale-matching-rebalance-unknown",
+    cl::desc("Evenly re-distribute flow among unknown subgraphs."),
+    cl::init(false), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<bool> StaleMatchingJoinIslands(
+    "stale-matching-join-islands",
+    cl::desc("Join isolated components having positive flow."), cl::init(true),
+    cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockInc(
+    "stale-matching-cost-block-inc",
+    cl::desc("The cost of increasing a block's count by one."), cl::init(110),
+    cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockDec(
+    "stale-matching-cost-block-dec",
+    cl::desc("The cost of decreasing a block's count by one."), cl::init(100),
+    cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockEntryInc(
+    "stale-matching-cost-block-entry-inc",
+    cl::desc("The cost of increasing the entry block's count by one."),
+    cl::init(110), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockEntryDec(
+    "stale-matching-cost-block-entry-dec",
+    cl::desc("The cost of decreasing the entry block's count by one."),
+    cl::init(100), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockZeroInc(
+    "stale-matching-cost-block-zero-inc",
+    cl::desc("The cost of increasing a count of zero-weight block by one."),
+    cl::init(10), cl::Hidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostBlockUnknownInc(
+    "stale-matching-cost-block-unknown-inc",
+    cl::desc("The cost of increasing an unknown block's count by one."),
+    cl::init(10), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpInc(
+    "stale-matching-cost-jump-inc",
+    cl::desc("The cost of increasing a jump's count by one."), cl::init(100),
+    cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpFTInc(
+    "stale-matching-cost-jump-ft-inc",
+    cl::desc("The cost of increasing a fall-through jump's count by one."),
+    cl::init(100), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpDec(
+    "stale-matching-cost-jump-dec",
+    cl::desc("The cost of decreasing a jump's count by one."), cl::init(110),
+    cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpFTDec(
+    "stale-matching-cost-jump-ft-dec",
+    cl::desc("The cost of decreasing a fall-through jump's count by one."),
+    cl::init(110), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpUnknownInc(
+    "stale-matching-cost-jump-unknown-inc",
+    cl::desc("The cost of increasing an unknown jump's count by one."),
+    cl::init(50), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+cl::opt<unsigned> StaleMatchingCostJumpUnknownFTInc(
+    "stale-matching-cost-jump-unknown-ft-inc",
+    cl::desc(
+        "The cost of increasing an unknown fall-through jump's count by one."),
+    cl::init(5), cl::ReallyHidden, cl::cat(BoltOptCategory));
+
+} // namespace opts
+
+namespace llvm {
+namespace bolt {
+
+/// Create a wrapper flow function to use with the profile inference algorithm,
+/// and initialize its jumps and metadata.
+FlowFunction
+createFlowFunction(const BinaryFunction::BasicBlockOrderType &BlockOrder) {
+  FlowFunction Func;
+
+  // Add a special "dummy" source so that there is always a unique entry point.
+  // Because of the extra source, for all other blocks in FlowFunction it holds
+  // that Block.Index == BB->getLayoutIndex() + 1
+  FlowBlock EntryBlock;
+  EntryBlock.Index = 0;
+  Func.Blocks.push_back(EntryBlock);
+
+  // Create FlowBlock for every basic block in the binary function
+  for (const BinaryBasicBlock *BB : BlockOrder) {
+    Func.Blocks.emplace_back();
+    FlowBlock &Block = Func.Blocks.back();
+    Block.Index = Func.Blocks.size() - 1;
+    (void)BB;
+    assert(Block.Index == BB->getLayoutIndex() + 1 &&
+           "incorrectly assigned basic block index");
+  }
+
+  // Create FlowJump for each jump between basic blocks in the binary function
+  std::vector<uint64_t> InDegree(Func.Blocks.size(), 0);
+  std::vector<uint64_t> OutDegree(Func.Blocks.size(), 0);
+  for (const BinaryBasicBlock *SrcBB : BlockOrder) {
+    std::unordered_set<const BinaryBasicBlock *> UniqueSuccs;
+    // Collect regular jumps
+    for (const BinaryBasicBlock *DstBB : SrcBB->successors()) {
+      // Ignoring parallel edges
+      if (UniqueSuccs.find(DstBB) != UniqueSuccs.end())
+        continue;
+
+      Func.Jumps.emplace_back();
+      FlowJump &Jump = Func.Jumps.back();
+      Jump.Source = SrcBB->getLayoutIndex() + 1;
+      Jump.Target = DstBB->getLayoutIndex() + 1;
+      InDegree[Jump.Target]++;
+      OutDegree[Jump.Source]++;
+      UniqueSuccs.insert(DstBB);
+    }
+    // Collect jumps to landing pads
+    for (const BinaryBasicBlock *DstBB : SrcBB->landing_pads()) {
+      // Ignoring parallel edges
+      if (UniqueSuccs.find(DstBB) != UniqueSuccs.end())
+        continue;
+
+      Func.Jumps.emplace_back();
+      FlowJump &Jump = Func.Jumps.back();
+      Jump.Source = SrcBB->getLayoutIndex() + 1;
+      Jump.Target = DstBB->getLayoutIndex() + 1;
+      InDegree[Jump.Target]++;
+      OutDegree[Jump.Source]++;
+      UniqueSuccs.insert(DstBB);
+    }
+  }
+
+  // Add dummy edges to the extra sources. If there are multiple entry blocks,
+  // add an unlikely edge from 0 to the subsequent ones
+  assert(InDegree[0] == 0 && "dummy entry blocks shouldn't have predecessors");
+  for (uint64_t I = 1; I < Func.Blocks.size(); I++) {
+    const BinaryBasicBlock *BB = BlockOrder[I - 1];
+    if (BB->isEntryPoint() || InDegree[I] == 0) {
+      uint64_t Src = 0;
+      uint64_t Dst = I;
+      Func.Jumps.emplace_back();
+      FlowJump &Jump = Func.Jumps.back();
+      Jump.Source = Src;
+      Jump.Target = Dst;
+      if (!BB->isEntryPoint())
+        Jump.IsUnlikely = true;
+    }
+  }
+
+  // Create necessary metadata for the flow function
+  for (FlowJump &Jump : Func.Jumps) {
+    uint64_t Src = Jump.Source;
+    uint64_t Dst = Jump.Target;
+    Func.Blocks.at(Src).SuccJumps.push_back(&Jump);
+    Func.Blocks.at(Dst).PredJumps.push_back(&Jump);
+  }
+  return Func;
+}
+
+/// Assign initial block/jump weights based on the stale profile data. The goal
+/// is to extract as much information from the stale profile as possible. Here
+/// we assume that each basic block is specified via a hash value computed from
+/// its content and the hashes of the unchanged basic blocks stay the same
+/// across different revisions of the binary.
+/// Whenever there is a count in the profile with the hash corresponding to one
+/// of the basic blocks in the binary, the count is "matched" to the block.
+/// Similarly, if both the source and the target of a count in the profile are
+/// matched to a jump in the binary, the count is recorded in CFG.
+void matchWeightsByHashes(const BinaryFunction::BasicBlockOrderType &BlockOrder,
+                          const yaml::bolt::BinaryFunctionProfile &YamlBF,
+                          FlowFunction &Func) {
+  assert(Func.Blocks.size() == BlockOrder.size() + 1);
+  // Initialize stale matcher
+  DenseMap<uint64_t, std::vector<FlowBlock *>> HashToBlocks;
+  for (uint64_t I = 0; I < BlockOrder.size(); I++) {
+    const BinaryBasicBlock *BB = BlockOrder[I];
+    assert(BB->getHash() != 0 && "empty hash of BinaryBasicBlock");
+    HashToBlocks[BB->getHash()].push_back(&Func.Blocks[I + 1]);
+  }
+
+  // Index in yaml profile => corresponding (matched) block
+  DenseMap<uint64_t, const FlowBlock *> MatchedBlocks;
+  // Match blocks from the profile to the blocks in CFG
+  for (const yaml::bolt::BinaryBasicBlockProfile &YamlBB : YamlBF.Blocks) {
+    assert(YamlBB.Hash != 0 && "empty hash of BinaryBasicBlockProfile");
+    auto It = HashToBlocks.find(YamlBB.Hash);
+    if (It != HashToBlocks.end()) {
+      const FlowBlock *MatchedBlock = It->second.front();
+      MatchedBlocks[YamlBB.Index] = MatchedBlock;
+    }
+  }
+
+  // Match jumps from the profile to the jumps from CFG
+  std::vector<uint64_t> OutWeight(Func.Blocks.size(), 0);
+  std::vector<uint64_t> InWeight(Func.Blocks.size(), 0);
+  for (const yaml::bolt::BinaryBasicBlockProfile &YamlBB : YamlBF.Blocks) {
+    for (const yaml::bolt::SuccessorInfo &YamlSI : YamlBB.Successors) {
+      if (YamlSI.Count == 0)
+        continue;
+
+      // Try to find the jump for a given (src, dst) pair from the profile and
+      // assign the jump weight based on the profile count
+      const uint64_t SrcIndex = YamlBB.Index;
+      const uint64_t DstIndex = YamlSI.Index;
+
+      const FlowBlock *MatchedSrcBlock =
+          MatchedBlocks.find(SrcIndex) != MatchedBlocks.end()
+              ? MatchedBlocks[SrcIndex]
+              : nullptr;
+      const FlowBlock *MatchedDstBlock =
+          MatchedBlocks.find(DstIndex) != MatchedBlocks.end()
+              ? MatchedBlocks[DstIndex]
+              : nullptr;
+
+      if (MatchedSrcBlock != nullptr && MatchedDstBlock != nullptr) {
+        // Find a jump between the two blocks
+        FlowJump *Jump = nullptr;
+        for (FlowJump *SuccJump : MatchedSrcBlock->SuccJumps) {
+          if (SuccJump->Target == MatchedDstBlock->Index) {
+            Jump = SuccJump;
+            break;
+          }
+        }
+        // Assign the weight, if the corresponding jump is found
+        if (Jump != nullptr) {
+          Jump->Weight = YamlSI.Count;
+          Jump->HasUnknownWeight = false;
+        }
+      }
+      // Assign the weight for the src block, if it is found
+      if (MatchedSrcBlock != nullptr)
+        OutWeight[MatchedSrcBlock->Index] += YamlSI.Count;
+      // Assign the weight for the dst block, if it is found
+      if (MatchedDstBlock != nullptr)
+        InWeight[MatchedDstBlock->Index] += YamlSI.Count;
+    }
+  }
+
+  // Assign block counts based on in-/out- jumps
+  for (FlowBlock &Block : Func.Blocks) {
+    if (OutWeight[Block.Index] == 0 && InWeight[Block.Index] == 0) {
+      assert(Block.HasUnknownWeight && "unmatched block with positive count");
+      continue;
+    }
+    Block.HasUnknownWeight = false;
+    Block.Weight = std::max(OutWeight[Block.Index], InWeight[Block.Index]);
+  }
+}
+
+/// The function finds all blocks that are (i) reachable from the Entry block
+/// and (ii) do not have a path to an exit, and marks all such blocks 'cold'
+/// so that profi does not send any flow to such blocks.
+void preprocessUnreachableBlocks(FlowFunction &Func) {
+  const uint64_t NumBlocks = Func.Blocks.size();
+
+  // Start bfs from the source
+  std::queue<uint64_t> Queue;
+  std::vector<bool> VisitedEntry(NumBlocks, false);
+  for (uint64_t I = 0; I < NumBlocks; I++) {
+    FlowBlock &Block = Func.Blocks[I];
+    if (Block.isEntry()) {
+      Queue.push(I);
+      VisitedEntry[I] = true;
+      break;
+    }
+  }
+  while (!Queue.empty()) {
+    const uint64_t Src = Queue.front();
+    Queue.pop();
+    for (FlowJump *Jump : Func.Blocks[Src].SuccJumps) {
+      uint64_t Dst = Jump->Target;
+      if (!VisitedEntry[Dst]) {
+        Queue.push(Dst);
+        VisitedEntry[Dst] = true;
+      }
+    }
+  }
+
+  // Start bfs from all sinks
+  std::vector<bool> VisitedExit(NumBlocks, false);
+  for (uint64_t I = 0; I < NumBlocks; I++) {
+    FlowBlock &Block = Func.Blocks[I];
+    if (Block.isExit() && VisitedEntry[I]) {
+      Queue.push(I);
+      VisitedExit[I] = true;
+    }
+  }
+  while (!Queue.empty()) {
+    const uint64_t Src = Queue.front();
+    Queue.pop();
+    for (FlowJump *Jump : Func.Blocks[Src].PredJumps) {
+      uint64_t Dst = Jump->Source;
+      if (!VisitedExit[Dst]) {
+        Queue.push(Dst);
+        VisitedExit[Dst] = true;
+      }
+    }
+  }
+
+  // Make all blocks of zero weight so that flow is not sent
+  for (uint64_t I = 0; I < NumBlocks; I++) {
+    FlowBlock &Block = Func.Blocks[I];
+    if (Block.Weight == 0)
+      continue;
+    if (!VisitedEntry[I] || !VisitedExit[I]) {
+      Block.Weight = 0;
+      Block.HasUnknownWeight = true;
+      Block.IsUnlikely = true;
+      for (FlowJump *Jump : Block.SuccJumps) {
+        if (Jump->Source == Block.Index && Jump->Target == Block.Index) {
+          Jump->Weight = 0;
+          Jump->HasUnknownWeight = true;
+          Jump->IsUnlikely = true;
+        }
+      }
+    }
+  }
+}
+
+/// Decide if stale profile matching can be applied for a given function.
+/// Currently we skip inference for (very) large instances and for instances
+/// having "unexpected" control flow (e.g., having no sink basic blocks).
+bool canApplyInference(const FlowFunction &Func) {
+  if (Func.Blocks.size() > opts::StaleMatchingMaxFuncSize)
+    return false;
+
+  bool HasExitBlocks = llvm::any_of(
+      Func.Blocks, [&](const FlowBlock &Block) { return Block.isExit(); });
+  if (!HasExitBlocks)
+    return false;
+
+  return true;
+}
+
+/// Apply the profile inference algorithm for a given flow function.
+void applyInference(FlowFunction &Func) {
+  ProfiParams Params;
+  // Set the params from the command-line flags.
+  Params.EvenFlowDistribution = opts::StaleMatchingEvenFlowDistribution;
+  Params.RebalanceUnknown = opts::StaleMatchingRebalanceUnknown;
+  Params.JoinIslands = opts::StaleMatchingJoinIslands;
+
+  Params.CostBlockInc = opts::StaleMatchingCostBlockInc;
+  Params.CostBlockDec = opts::StaleMatchingCostBlockDec;
+  Params.CostBlockEntryInc = opts::StaleMatchingCostBlockEntryInc;
+  Params.CostBlockEntryDec = opts::StaleMatchingCostBlockEntryDec;
+  Params.CostBlockZeroInc = opts::StaleMatchingCostBlockZeroInc;
+  Params.CostBlockUnknownInc = opts::StaleMatchingCostBlockUnknownInc;
+
+  Params.CostJumpInc = opts::StaleMatchingCostJumpInc;
+  Params.CostJumpFTInc = opts::StaleMatchingCostJumpFTInc;
+  Params.CostJumpDec = opts::StaleMatchingCostJumpDec;
+  Params.CostJumpFTDec = opts::StaleMatchingCostJumpFTDec;
+  Params.CostJumpUnknownInc = opts::StaleMatchingCostJumpUnknownInc;
+  Params.CostJumpUnknownFTInc = opts::StaleMatchingCostJumpUnknownFTInc;
+
+  applyFlowInference(Params, Func);
+}
+
+/// Collect inferred counts from the flow function and update annotations in
+/// the binary function.
+void assignProfile(BinaryFunction &BF,
+                   const BinaryFunction::BasicBlockOrderType &BlockOrder,
+                   FlowFunction &Func) {
+  BinaryContext &BC = BF.getBinaryContext();
+
+  assert(Func.Blocks.size() == BlockOrder.size() + 1);
+  for (uint64_t I = 0; I < BlockOrder.size(); I++) {
+    FlowBlock &Block = Func.Blocks[I + 1];
+    BinaryBasicBlock *BB = BlockOrder[I];
+
+    // Update block's count
+    BB->setExecutionCount(Block.Flow);
+
+    // Update jump counts: (i) clean existing counts and then (ii) set new ones
+    auto BI = BB->branch_info_begin();
+    for (const BinaryBasicBlock *DstBB : BB->successors()) {
+      (void)DstBB;
+      BI->Count = 0;
+      BI->MispredictedCount = 0;
+      ++BI;
+    }
+    for (FlowJump *Jump : Block.SuccJumps) {
+      if (Jump->IsUnlikely)
+        continue;
+      if (Jump->Flow == 0)
+        continue;
+
+      BinaryBasicBlock &SuccBB = *BlockOrder[Jump->Target - 1];
+      // Check if the edge corresponds to a regular jump or a landing pad
+      if (BB->getSuccessor(SuccBB.getLabel())) {
+        BinaryBasicBlock::BinaryBranchInfo &BI = BB->getBranchInfo(SuccBB);
+        BI.Count += Jump->Flow;
+      } else {
+        BinaryBasicBlock *LP = BB->getLandingPad(SuccBB.getLabel());
+        if (LP && LP->getKnownExecutionCount() < Jump->Flow)
+          LP->setExecutionCount(Jump->Flow);
+      }
+    }
+
+    // Update call-site annotations
+    auto setOrUpdateAnnotation = [&](MCInst &Instr, StringRef Name,
+                                     uint64_t Count) {
+      if (BC.MIB->hasAnnotation(Instr, Name)) {
+        BC.MIB->removeAnnotation(Instr, Name);
+      }
+      // Do not add zero-count annotations
+      if (Count == 0)
+        return;
+      BC.MIB->addAnnotation(Instr, Name, Count);
+    };
+
+    for (MCInst &Instr : *BB) {
+      // Ignore pseudo instructions
+      if (BC.MIB->isPseudo(Instr))
+        continue;
+      // Ignore jump tables
+      const MCInst *LastInstr = BB->getLastNonPseudoInstr();
+      if (BC.MIB->getJumpTable(*LastInstr) && LastInstr == &Instr)
+        continue;
+
+      if (BC.MIB->isIndirectCall(Instr) || BC.MIB->isIndirectBranch(Instr)) {
+        auto &ICSP = BC.MIB->getOrCreateAnnotationAs<IndirectCallSiteProfile>(
+            Instr, "CallProfile");
+        if (!ICSP.empty()) {
+          // Try to evenly distribute the counts among the call sites
+          uint64_t TotalCount = Block.Flow;
+          uint64_t NumSites = ICSP.size();
+          for (uint64_t Idx = 0; Idx < ICSP.size(); Idx++) {
+            IndirectCallProfile &CSP = ICSP[Idx];
+            uint64_t CountPerSite = TotalCount / NumSites;
+            // When counts cannot be exactly distributed, increase by 1 the
+            // counts of the first (TotalCount % NumSites) call sites
+            if (Idx < TotalCount % NumSites)
+              CountPerSite++;
+            CSP.Count = CountPerSite;
+          }
+        } else {
+          ICSP.emplace_back(nullptr, Block.Flow, 0);
+        }
+      } else if (BC.MIB->getConditionalTailCall(Instr)) {
+        // We don't know exactly the number of times the conditional tail call
+        // is executed; conservatively, setting it to the count of the block
+        setOrUpdateAnnotation(Instr, "CTCTakenCount", Block.Flow);
+        BC.MIB->removeAnnotation(Instr, "CTCMispredCount");
+      } else if (BC.MIB->isCall(Instr)) {
+        setOrUpdateAnnotation(Instr, "Count", Block.Flow);
+      }
+    }
+  }
+
+  // Update function's execution count and mark the function inferred.
+  BF.setExecutionCount(Func.Blocks[0].Flow);
+  BF.setHasInferredProfile(true);
+}
+
+bool YAMLProfileReader::inferStaleProfile(
+    BinaryFunction &BF, const yaml::bolt::BinaryFunctionProfile &YamlBF) {
+  // Make sure that block indices and hashes are up to date
+  BF.getLayout().updateLayoutIndices();
+  BF.computeBlockHashes();
+
+  BinaryFunction::BasicBlockOrderType BlockOrder(BF.getLayout().block_begin(),
+                                                 BF.getLayout().block_end());
+
+  // Create a wrapper flow function to use with the profile inference algorithm
+  FlowFunction Func = createFlowFunction(BlockOrder);
+
+  // Match as many block/jump counts from the stale profile as possible
+  matchWeightsByHashes(BlockOrder, YamlBF, Func);
+
+  // Adjust the flow function by marking unreachable blocks Unlikely so that
+  // they don't get any counts assigned
+  preprocessUnreachableBlocks(Func);
+
+  // Check if profile inference can be applied for the instance
+  if (!canApplyInference(Func))
+    return false;
+
+  // Apply the profile inference algorithm
+  applyInference(Func);
+
+  // Collect inferred counts and update function annotations
+  assignProfile(BF, BlockOrder, Func);
+
+  // As of now, we always mark the binary function having "correct" profile.
+  // In the future, we may discard the results for instances with poor inference
+  // metrics and keep such functions un-optimized.
+  return true;
+}
+
+} // end namespace bolt
+} // end namespace llvm

bolt/lib/Profile/YAMLProfileReader.cpp

 #include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 
 namespace opts {
 
 extern cl::opt<unsigned> Verbosity;
 extern cl::OptionCategory BoltOptCategory;
+extern cl::opt<bool> InferStaleProfile;
 
 static llvm::cl::opt<bool>
     IgnoreHash("profile-ignore-hash",
                cl::desc("ignore hash while reading function profile"),
                cl::Hidden, cl::cat(BoltOptCategory));
 }
 
 namespace llvm {
bool YAMLProfileReader::parseFunctionProfile(
   if (ProfileMatched)
     BF.markProfiled(YamlBP.Header.Flags);
 
   if (!ProfileMatched && opts::Verbosity >= 1)
     errs() << "BOLT-WARNING: " << MismatchedBlocks << " blocks, "
            << MismatchedCalls << " calls, and " << MismatchedEdges
            << " edges in profile did not match function " << BF << '\n';
 
+  if (!ProfileMatched && opts::InferStaleProfile) {
+    if (opts::Verbosity >= 1)
+      outs() << "BOLT-INFO: applying profile inference for "
+             << "\"" << BF.getPrintName() << "\"\n";
+    if (inferStaleProfile(BF, YamlBF)) {
+      ProfileMatched = true;
+      BF.markProfiled(YamlBP.Header.Flags);
+    }
+  }
+
   return ProfileMatched;
 }
 
 Error YAMLProfileReader::preprocessProfile(BinaryContext &BC) {
   ErrorOr<std::unique_ptr<MemoryBuffer>> MB =
       MemoryBuffer::getFileOrSTDIN(Filename);
   if (std::error_code EC = MB.getError()) {
     errs() << "ERROR: cannot open " << Filename << ": " << EC.message() << "\n";

bolt/test/X86/Inputs/blarge_profile_stale.yaml

+---
+header:
+  profile-version: 1
+  binary-name:     'reader-yaml.test.tmp.exe'
+  binary-build-id: '<unknown>'
+  profile-flags:   [ lbr ]
+  profile-origin:  branch profile reader
+  profile-events:  ''
+functions:
+  - name:            SolveCubic
+    fid:             6
+    hash:            0xC6E9098E973BBE19
+    exec:            151
+    nblocks:         18
+    blocks:
+      - bid:             0
+        insns:           43
+        hash:            0xD2411AC186118199
+        exec:            151
+        succ:            [ { bid: 1, cnt: 4, mis: 2 }, { bid: 11, cnt: 0 } ]
+      - bid:             1
+        insns:           7
+        hash:            0xDF0C9CC1FEAA70C3
+        succ:            [ { bid: 10, cnt: 0 }, { bid: 2, cnt: 0 } ]
+      - bid:             13
+        insns:           26
+        hash:            0xF05DC5524E99E56F
+        succ:            [ { bid: 15, cnt: 89 }, { bid: 14, cnt: 0 } ]
+      - bid:             15
+        insns:           9
+        hash:            0xB2E8338276A9834E
+  - name:            usqrt
+    fid:             7
+    hash:            0x8B62B1F9AD81EA35
+    exec:            20
+    nblocks:         6
+    blocks:
+      - bid:             0
+        insns:           4
+        hash:            0xE3FEB842A6548CCF
+        exec:            20
+        succ:            [ { bid: 1, cnt: 0 } ]
+      - bid:             1
+        insns:           9
+        hash:            0x85948FF2924613B7
+        succ:            [ { bid: 3, cnt: 320, mis: 171 }, { bid: 2, cnt: 0 } ]
+      - bid:             3
+        insns:           2
+        hash:            0x41D8DB2D2B01F411
+        succ:            [ { bid: 1, cnt: 300, mis: 33 }, { bid: 4, cnt: 20 } ]
+...

bolt/test/X86/reader-stale-yaml.test

+# This script checks that YamlProfileReader in llvm-bolt is reading data
+# correctly and stale data is corrected.
+
+RUN: yaml2obj %p/Inputs/blarge.yaml &> %t.exe
+RUN: llvm-bolt %t.exe -o /dev/null --b %p/Inputs/blarge_profile_stale.yaml --print-cfg --print-only=usqrt --infer-stale-profile=1 --profile-ignore-hash=1 \
+RUN:   2>&1 | FileCheck %s -check-prefix=CHECK
+
+# Verify that yaml reader works as expected.
+CHECK:  pre-processing profile using YAML profile reader
+
+# Verify the inferred counts of "usqrt" that has stale profile:
+#   - the function has nblocks=6 in the profile, which makes it stale
+#   - block with bid=0 has an incorrect (missing) count, which is inferred
+CHECK:    Binary Function "usqrt" after building cfg {
+CHECK:      State       : CFG constructed
+CHECK:      Address     : 0x401170
+CHECK:      Size        : 0x43
+CHECK:      Section     : .text
+CHECK:      IsSimple    : 1
+CHECK:      BB Count    : 5
+CHECK:      Exec Count  : 20
+CHECK:      Branch Count: 640
+CHECK:    }
+
+# Verify block counts.
+CHECK:    .LBB01 (4 instructions, align : 1)
+CHECK:      Successors: .Ltmp[[#BB13:]] (mispreds: 0, count: 20)
+CHECK:    .Ltmp[[#BB13:]] (9 instructions, align : 1)
+CHECK:      Successors: .Ltmp[[#BB12:]] (mispreds: 0, count: 320), .LFT[[#BB0:]] (mispreds: 0, count: 0)
+CHECK:    .LFT[[#BB0:]] (2 instructions, align : 1)
+CHECK:      Successors: .Ltmp[[#BB12:]] (mispreds: 0, count: 0)
+CHECK:    .Ltmp[[#BB12:]] (2 instructions, align : 1)
+CHECK:      Successors: .Ltmp[[#BB13:]] (mispreds: 0, count: 300), .LFT[[#BB1:]] (mispreds: 0, count: 20)
+CHECK:    .LFT[[#BB1:]] (2 instructions, align : 1)
+
+# Check the overal inference stats.
+CHECK:  2 out of 7 functions in the binary (28.6%) have non-empty execution profile
+CHECK:  inferred profile for 1 (50.00% of profiled, 100.00% of stale) functions responsible for 87.31% samples (640 out of 733)