[llvm-profgen] Compute and show profile density

[wlei]

Summary

AutoFDO performance is sensitive to profile density, i.e., the amount of samples in the profile relative to the program size, because profiles with insufficient samples could be inaccurate due to statistical noise and thus hurt AutoFDO performance. A previous investigation showed that AutoFDO performed better on MySQL with increased amount of samples. Therefore, we implement a profile-density computation feature to give hints about profile density to users and the compiler.

We define the density of a profile Prof as follows:

• For each function A in the profile, density(A) = total_samples(A) / sizeof(A).
• density(Prof) = min(density(A)) for all functions A that are warm (defined below).

A function is considered warm if its total-samples is within top N percent of the profile. For implementation, we reuse the ProfileSummaryBuilder::getHotCountThreshold(..) as threshold which can be set by percent(--profile-summary-cutoff-hot) or by value(--profile-summary-hot-count).

We also introduce --hot-function-density-threshold to set hot function density threshold and will give suggestion if profile density is below it which implies we should increase samples.

This also applies for CS profile with all profiles merged into base.

[wenlei]

llvm/test/tools/llvm-profgen/inline-cs-pseudoprobe.test

23–24

I suggest we use a dedicated test case instead of adding feature test into various existing test cases. That way it helps keep testing isolated separated.

llvm/tools/llvm-profgen/ProfileGenerator.cpp

143–149

Maybe these two messages should be warning?

735

Perhaps we can let calculateAndShowDensity take a merged profile map, then calculateAndShowDensity doesn't have to be a virtual function and can have full implementation in ProfileGeneratorBase. We can prepare merged profiles here and pass into calculateAndShowDensity.

llvm/tools/llvm-profgen/ProfileGenerator.h

89

Can we omit raw_fd_ostream &OS as a parameter and use outs() directly inside?

With that, can we also remove the include for raw_os_ostream.h?

llvm/tools/llvm-profgen/ProfiledBinary.h

80

nit: we've been using uint64_t for size. would be good to be consistent, even though they're the same thing here.

[wlei]

Addressing Wenlei's feedback

llvm/test/tools/llvm-profgen/inline-cs-pseudoprobe.test

23–24

Sounds good!

llvm/tools/llvm-profgen/ProfileGenerator.cpp

143–149

Good point!

735

Changed.

llvm/tools/llvm-profgen/ProfileGenerator.h

89

Sounds good!

llvm/tools/llvm-profgen/ProfiledBinary.h

80

Good to know this!

[hoy]

lgtm, thx.

[wenlei]

lgtm, thanks.

llvm/test/tools/llvm-profgen/inline-cs-pseudoprobe.test

 ; RUN: llvm-profgen --format=text --perfscript=%S/Inputs/inline-cs-pseudoprobe.perfscript --binary=%S/Inputs/inline-cs-pseudoprobe.perfbin --output=%t --skip-symbolization --profile-summary-cold-count=0 --use-offset=0
 ; RUN: FileCheck %s --input-file %t --check-prefix=CHECK-UNWINDER
-; RUN: llvm-profgen --format=text --perfscript=%S/Inputs/inline-cs-pseudoprobe.perfscript --binary=%S/Inputs/inline-cs-pseudoprobe.perfbin --output=%t --profile-summary-cold-count=0
+; RUN: llvm-profgen --format=text --perfscript=%S/Inputs/inline-cs-pseudoprobe.perfscript --binary=%S/Inputs/inline-cs-pseudoprobe.perfbin --output=%t --profile-summary-cold-count=0 --show-density &> %t1
 ; RUN: FileCheck %s --input-file %t
+; RUN: FileCheck %s --input-file %t1 --check-prefix=CHECK-DENSITY
 
 ; CHECK:     [main:2 @ foo]:74:0
 ; CHECK-NEXT: 1: 0
 ; CHECK-NEXT: 2: 15
 ; CHECK-NEXT: 3: 15
 ; CHECK-NEXT: 4: 14
 ; CHECK-NEXT: 5: 1
 ; CHECK-NEXT: 6: 15
 ; CHECK-NEXT: 7: 0
 ; CHECK-NEXT: 8: 14 bar:14
 ; CHECK-NEXT: 9: 0
 ; CHECK-NEXT: !CFGChecksum: 563088904013236
 ; CHECK:[main:2 @ foo:8 @ bar]:28:14
 ; CHECK-NEXT: 1: 14
 ; CHECK-NEXT: 4: 14
 ; CHECK-NEXT: !CFGChecksum: 72617220756
 
+; CHECK-DENSITY: AutoFDO is estimated to optimize better with 1675.7x more samples. Please consider increasing sampling rate or profiling for longer duration to get more samples.
+; CHECK-DENSITY: Minimum profile density for hot functions with top 99.00% total samples: 0.6
+
 ; CHECK-UNWINDER:      3
 ; CHECK-UNWINDER-NEXT: 201800-201858:1
 ; CHECK-UNWINDER-NEXT: 20180e-20182b:1
 ; CHECK-UNWINDER-NEXT: 20180e-201858:13
 ; CHECK-UNWINDER-NEXT: 2
 ; CHECK-UNWINDER-NEXT: 20182b->201800:1
 ; CHECK-UNWINDER-NEXT: 201858->20180e:15
 

llvm/test/tools/llvm-profgen/inline-noprobe2.test

 ; RUN: llvm-profgen --format=text --perfscript=%S/Inputs/artificial-branch.perfscript --binary=%S/Inputs/inline-noprobe2.perfbin --output=%t  --skip-symbolization --use-offset=0
 ; RUN: FileCheck %s --input-file %t --check-prefix=CHECK-ARTIFICIAL-BRANCH
 ; RUN: llvm-profgen --format=text --perfscript=%S/Inputs/inline-noprobe2.perfscript --binary=%S/Inputs/inline-noprobe2.perfbin --output=%t  --skip-symbolization --use-offset=0
 ; RUN: FileCheck %s --input-file %t --check-prefix=CHECK-RAW-PROFILE
-; RUN: llvm-profgen --format=text --unsymbolized-profile=%t --binary=%S/Inputs/inline-noprobe2.perfbin --output=%t1 --use-offset=0
+; RUN: llvm-profgen --format=text --unsymbolized-profile=%t --binary=%S/Inputs/inline-noprobe2.perfbin --output=%t1 --use-offset=0 --show-density -hot-function-density-threshold=1 &> %t2
 ; RUN: FileCheck %s --input-file %t1 --check-prefix=CHECK
+; RUN: FileCheck %s --input-file %t2 --check-prefix=CHECK-DENSITY
 
 ; RUN: llvm-profgen --format=extbinary --perfscript=%S/Inputs/inline-noprobe2.perfscript --binary=%S/Inputs/inline-noprobe2.perfbin --output=%t --populate-profile-symbol-list=1
 ; RUN: llvm-profdata show -show-prof-sym-list -sample %t | FileCheck %s --check-prefix=CHECK-SYM-LIST
 
 ; CHECK-ARTIFICIAL-BRANCH: 3
 ; CHECK-ARTIFICIAL-BRANCH: 400540-400540:1
 ; CHECK-ARTIFICIAL-BRANCH: 400870-400870:2
 ; CHECK-ARTIFICIAL-BRANCH: 400875-4008bf:1

 ;CHECK-NEXT:  6.1: 12
 ;CHECK-NEXT:  6.3: 10
 ;CHECK-NEXT:  7: 0
 ;CHECK-NEXT:  8: 0 quick_sort:1
 ;CHECK-NEXT:  9: 0
 ;CHECK-NEXT:  11: 0
 ;CHECK-NEXT:  14: 0
 
+;CHECK-DENSITY: AutoFDO is estimated to optimize better with 4.9x more samples. Please consider increasing sampling rate or profiling for longer duration to get more samples.
+;CHECK-DENSITY: Minimum profile density for hot functions with top 99.00% total samples: 0.2
+
 ; original code:
 ; clang -O3 -g -fno-optimize-sibling-calls -fdebug-info-for-profiling qsort.c -o a.out
 #include <stdio.h>
 #include <stdlib.h>
 
 void swap(int *a, int *b) {
   int t = *a;
   *a = *b;

llvm/tools/llvm-profgen/ProfileGenerator.h

 //===-- ProfileGenerator.h - Profile Generator -----------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TOOLS_LLVM_PROGEN_PROFILEGENERATOR_H
 #define LLVM_TOOLS_LLVM_PROGEN_PROFILEGENERATOR_H
 #include "CSPreInliner.h"
 #include "ErrorHandling.h"
 #include "PerfReader.h"
 #include "ProfiledBinary.h"
 #include "llvm/ProfileData/SampleProfWriter.h"
+#include "llvm/Support/raw_os_ostream.h"
 #include <memory>
 #include <unordered_set>
 
 using namespace llvm;
 using namespace sampleprof;
 
 namespace llvm {
 namespace sampleprof {
protected:
   void findDisjointRanges(RangeSample &DisjointRanges,
                           const RangeSample &Ranges);
   // Helper function for updating body sample for a leaf location in
   // FunctionProfile
   void updateBodySamplesforFunctionProfile(FunctionSamples &FunctionProfile,
                                            const SampleContextFrame &LeafLoc,
                                            uint64_t Count);
   void updateTotalSamples();
+
   StringRef getCalleeNameForOffset(uint64_t TargetOffset);
+
+  void computeSummaryAndThreshold();
+
+  virtual void calculateAndShowDensity(const SampleProfileMap &Profiles) = 0;
+
+  double calculateDensity(const SampleProfileMap &Profiles,
+                          uint64_t HotCntThreshold);
+
+  void showDensitySuggestion(double Density, raw_fd_ostream &OS);
+
+  // Thresholds from profile summary to answer isHotCount/isColdCount queries.
+  uint64_t HotCountThreshold;
+
+  uint64_t ColdCountThreshold;
+
   // Used by SampleProfileWriter
   SampleProfileMap ProfileMap;
 
   ProfiledBinary *Binary = nullptr;
 
   const ContextSampleCounterMap &SampleCounters;
 };
 
private:
   // Helper function to get the leaf frame's FunctionProfile by traversing the
   // inline stack and meanwhile it adds the total samples for each frame's
   // function profile.
   FunctionSamples &
   getLeafFrameProfile(const SampleContextFrameVector &FrameVec);
   void populateBodySamplesForAllFunctions(const RangeSample &RangeCounter);
   void
   populateBoundarySamplesForAllFunctions(const BranchSample &BranchCounters);
+  void postProcessProfiles();
+  void calculateAndShowDensity(const SampleProfileMap &Profiles) override;
 };
 
 using ProbeCounterMap =
     std::unordered_map<const MCDecodedPseudoProbe *, uint64_t>;
 
 class CSProfileGenerator : public ProfileGeneratorBase {
 public:
   CSProfileGenerator(ProfiledBinary *Binary,
getFunctionProfileForContext(const SampleContextFrameVector &Context,
                                bool WasLeafInlined = false);
   // For profiled only functions, on-demand compute their inline context
   // function byte size which is used by the pre-inliner.
   void computeSizeForProfiledFunctions();
   // Post processing for profiles before writing out, such as mermining
   // and trimming cold profiles, running preinliner on profiles.
   void postProcessProfiles();
 
-  void computeSummaryAndThreshold();
-
   void populateBodySamplesForFunction(FunctionSamples &FunctionProfile,
                                       const RangeSample &RangeCounters);
   void populateBoundarySamplesForFunction(SampleContextFrames ContextId,
                                           FunctionSamples &FunctionProfile,
                                           const BranchSample &BranchCounters);
   void populateInferredFunctionSamples();
 
   void generateProbeBasedProfile();
   // Go through each address from range to extract the top frame probe by
   // looking up in the Address2ProbeMap
   void extractProbesFromRange(const RangeSample &RangeCounter,
                               ProbeCounterMap &ProbeCounter);
   // Fill in function body samples from probes
   void populateBodySamplesWithProbes(const RangeSample &RangeCounter,
                                      SampleContextFrames ContextStack);
   // Fill in boundary samples for a call probe
   void populateBoundarySamplesWithProbes(const BranchSample &BranchCounter,
                                          SampleContextFrames ContextStack);
   // Helper function to get FunctionSamples for the leaf probe
   FunctionSamples &
   getFunctionProfileForLeafProbe(SampleContextFrames ContextStack,
                                  const MCDecodedPseudoProbe *LeafProbe);
-  // Thresholds from profile summary to answer isHotCount/isColdCount queries.
-  uint64_t HotCountThreshold;
-  uint64_t ColdCountThreshold;
+
+  void calculateAndShowDensity(const SampleProfileMap &Profiles) override;
 
   // Underlying context table serves for sample profile writer.
   std::unordered_set<SampleContextFrameVector, SampleContextFrameHash> Contexts;
 
 public:
   // Deduplicate adjacent repeated context sequences up to a given sequence
   // length. -1 means no size limit.
   static int32_t MaxCompressionSize;
   static int MaxContextDepth;
 };
 
 } // end namespace sampleprof
 } // end namespace llvm
 
 #endif

llvm/tools/llvm-profgen/ProfileGenerator.cpp

 //===-- ProfileGenerator.cpp - Profile Generator  ---------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "ProfileGenerator.h"
 #include "ProfiledBinary.h"
 #include "llvm/ProfileData/ProfileCommon.h"
+#include <float.h>
 #include <unordered_set>
 
 cl::opt<std::string> OutputFilename("output", cl::value_desc("output"),
                                     cl::Required,
                                     cl::desc("Output profile file"));
 static cl::alias OutputA("o", cl::desc("Alias for --output"),
                          cl::aliasopt(OutputFilename));
 
cl::desc("Keep the last K contexts while merging cold profile. 1 means the "
              "context-less base profile"));
 
 static cl::opt<int, true> CSProfMaxContextDepth(
     "csprof-max-context-depth", cl::ZeroOrMore,
     cl::desc("Keep the last K contexts while merging profile. -1 means no "
              "depth limit."),
     cl::location(llvm::sampleprof::CSProfileGenerator::MaxContextDepth));
 
-extern cl::opt<int> ProfileSummaryCutoffCold;
+static cl::opt<double> HotFunctionDensityThreshold(
+    "hot-function-density-threshold", llvm::cl::init(1000),
+    llvm::cl::desc(
+        "specify density threshold for hot functions (default: 1000)"),
+    llvm::cl::Optional);
+static cl::opt<bool> ShowDensity("show-density", llvm::cl::init(false),
+                                 llvm::cl::desc("show profile density details"),
+                                 llvm::cl::Optional);
+
+extern cl::opt<int> ProfileSummaryCutoffHot;
 
 using namespace llvm;
 using namespace sampleprof;
 
 namespace llvm {
 namespace sampleprof {
 
 // Initialize the MaxCompressionSize to -1 which means no size limit
if (OutputFormat != SPF_Ext_Binary)
                               "--format=extbinary to enable it\n";
     else
       WriterOrErr.get()->setUseMD5();
   }
 
   write(std::move(WriterOrErr.get()), ProfileMap);
 }
 
+void ProfileGeneratorBase::showDensitySuggestion(double Density,
+                                                 raw_fd_ostream &OS) {
+  if (Density == 0.0)
+    OS << "The --profile-summary-cutoff-hot option may be set too low. Please "
+          "check your command.\n";
+  else if (Density < HotFunctionDensityThreshold)
+    OS << "AutoFDO is estimated to optimize better with "
+       << format("%.1f", HotFunctionDensityThreshold / Density)
+       << "x more samples. Please consider increasing sampling rate or "
+          "profiling for longer duration to get more samples.\n";
+
+  if (ShowDensity)
+    OS << "Minimum profile density for hot functions with top "
+       << format("%.2f",
+                 static_cast<double>(ProfileSummaryCutoffHot.getValue()) /
+                     10000)
+       << "% total samples: " << format("%.1f", Density) << "\n";
+}
+
+double ProfileGeneratorBase::calculateDensity(const SampleProfileMap &Profiles,
+                                              uint64_t HotCntThreshold) {
+  double Density = DBL_MAX;
+  std::vector<const FunctionSamples *> HotFuncs;
+  for (auto &I : Profiles) {
+    auto &FuncSamples = I.second;
+    if (FuncSamples.getTotalSamples() < HotCntThreshold)
+      break;
+    HotFuncs.emplace_back(&FuncSamples);
+  }
+
+  for (auto *FuncSamples : HotFuncs) {
+    auto *Func = Binary->getBinaryFunction(FuncSamples->getName());
+    if (!Func)
+      continue;
+    size_t FuncSize = Func->getFuncSize();
+    if (FuncSize == 0)
+      continue;
+    Density =
+        std::min(Density, static_cast<double>(FuncSamples->getTotalSamples()) /
+                              FuncSize);
+  }
+
+  return Density == DBL_MAX ? 0.0 : Density;
+}
+
 void ProfileGeneratorBase::findDisjointRanges(RangeSample &DisjointRanges,
                                               const RangeSample &Ranges) {
 
   /*
   Regions may overlap with each other. Using the boundary info, find all
   disjoint ranges and their sample count. BoundaryPoint contains the count
   multiple samples begin/end at this points.
 

 }
 
 void ProfileGenerator::generateProfile() {
   if (Binary->usePseudoProbes()) {
     // TODO: Support probe based profile generation
   } else {
     generateLineNumBasedProfile();
   }
+  postProcessProfiles();
+}
+
+void ProfileGenerator::postProcessProfiles() {
+  computeSummaryAndThreshold();
+  calculateAndShowDensity(ProfileMap);
 }
 
 void ProfileGenerator::generateLineNumBasedProfile() {
   assert(SampleCounters.size() == 1 &&
          "Must have one entry for profile generation.");
   const SampleCounter &SC = SampleCounters.begin()->second;
   // Fill in function body samples
   populateBodySamplesForAllFunctions(SC.RangeCounter);
if (!FrameVec.empty()) {
           CalleeName, Count);
     }
     // Add head samples for callee.
     FunctionSamples &CalleeProfile = getTopLevelFunctionProfile(CalleeName);
     CalleeProfile.addHeadSamples(Count);
   }
 }
 
+void ProfileGenerator::calculateAndShowDensity(
+    const SampleProfileMap &Profiles) {
+  double Density = calculateDensity(Profiles, HotCountThreshold);
+  showDensitySuggestion(Density, outs());
+}
+
 FunctionSamples &CSProfileGenerator::getFunctionProfileForContext(
     const SampleContextFrameVector &Context, bool WasLeafInlined) {
   auto I = ProfileMap.find(SampleContext(Context));
   if (I == ProfileMap.end()) {
     // Save the new context for future references.
     SampleContextFrames NewContext = *Contexts.insert(Context).first;
     SampleContext FContext(NewContext, RawContext);
     auto Ret = ProfileMap.emplace(FContext, FunctionSamples());
void CSProfileGenerator::postProcessProfiles() {
 
   // Trim and merge cold context profile using cold threshold above.
   if (CSProfTrimColdContext || CSProfMergeColdContext) {
     SampleContextTrimmer(ProfileMap)
         .trimAndMergeColdContextProfiles(
             HotCountThreshold, CSProfTrimColdContext, CSProfMergeColdContext,
             CSProfMaxColdContextDepth, EnableCSPreInliner);
   }
+
+  calculateAndShowDensity(ProfileMap);
 }
 
-void CSProfileGenerator::computeSummaryAndThreshold() {
+void ProfileGeneratorBase::computeSummaryAndThreshold() {
   SampleProfileSummaryBuilder Builder(ProfileSummaryBuilder::DefaultCutoffs);
   auto Summary = Builder.computeSummaryForProfiles(ProfileMap);
   HotCountThreshold = ProfileSummaryBuilder::getHotCountThreshold(
       (Summary->getDetailedSummary()));
   ColdCountThreshold = ProfileSummaryBuilder::getColdCountThreshold(
       (Summary->getDetailedSummary()));
 }
 
FunctionSamples &CSProfileGenerator::getFunctionProfileForLeafProbe(
   const auto *FuncDesc = Binary->getFuncDescForGUID(LeafProbe->getGuid());
   bool WasLeafInlined = LeafProbe->getInlineTreeNode()->hasInlineSite();
   FunctionSamples &FunctionProile =
       getFunctionProfileForContext(NewContextStack, WasLeafInlined);
   FunctionProile.setFunctionHash(FuncDesc->FuncHash);
   return FunctionProile;
 }
 
+void CSProfileGenerator::calculateAndShowDensity(
+    const SampleProfileMap &Profiles) {
+  sampleprof::SampleProfileMap ContextLessProfiles;
+  // Merge function samples for CS profile.
+  for (const auto &I : Profiles) {
+    ContextLessProfiles[I.second.getName()].merge(I.second);
+  }
+
+  double Density = calculateDensity(ContextLessProfiles, HotCountThreshold);
+  showDensitySuggestion(Density, outs());
+}
+
 } // end namespace sampleprof
 } // end namespace llvm

llvm/tools/llvm-profgen/ProfiledBinary.h

 };
 
 using RangesTy = std::vector<std::pair<uint64_t, uint64_t>>;
 
 struct BinaryFunction {
   StringRef FuncName;
   // End of range is an exclusive bound.
   RangesTy Ranges;
+
+  size_t getFuncSize() {
+    size_t Sum = 0;
+    for (auto &R : Ranges) {
+      Sum += R.second - R.first;
+    }
+    return Sum;
+  }
 };
 
 // Info about function range. A function can be split into multiple
 // non-continuous ranges, each range corresponds to one FuncRange.
 struct FuncRange {
   uint64_t StartOffset;
   // EndOffset is an exclusive bound.
   uint64_t EndOffset;
RangesTy getRangesForOffset(uint64_t Offset) {
     return FRange->Func->Ranges;
   }
 
   const std::unordered_map<std::string, BinaryFunction> &
   getAllBinaryFunctions() {
     return BinaryFunctions;
   }
 
+  BinaryFunction *getBinaryFunction(StringRef FName) {
+    auto I = BinaryFunctions.find(FName.str());
+    if (I == BinaryFunctions.end())
+      return nullptr;
+    return &I->second;
+  }
+
   uint32_t getFuncSizeForContext(SampleContext &Context) {
     return FuncSizeTracker.getFuncSizeForContext(Context);
   }
 
   // Load the symbols from debug table and populate into symbol list.
   void populateSymbolListFromDWARF(ProfileSymbolList &SymbolList);
 
   const SampleContextFrameVector &