[BOLT] Add profile density computation

bolt/include/bolt/Core/BinaryFunction.h

@@ -386,6 +386,9 @@ class BinaryFunction {
   /// Raw branch count for this function in the profile.
   uint64_t RawBranchCount{0};
 
+  /// Dynamically executed function bytes, used for density computation.
+  uint64_t SampleCountInBytes{0};
+
   /// Indicates the type of profile the function is using.
   uint16_t ProfileFlags{PF_NONE};
 
@@ -1844,6 +1847,9 @@ class BinaryFunction {
   /// to this function.
   void setRawBranchCount(uint64_t Count) { RawBranchCount = Count; }
 
+  /// Return the number of dynamically executed bytes, from raw perf data.
+  uint64_t getSampleCountInBytes() const { return SampleCountInBytes; }
+
   /// Return the execution count for functions with known profile.
   /// Return 0 if the function has no profile.
   uint64_t getKnownExecutionCount() const {

bolt/include/bolt/Utils/CommandLineOpts.h

@@ -55,6 +55,7 @@ extern llvm::cl::opt<bool> PrintSections;
 enum ProfileFormatKind { PF_Fdata, PF_YAML };
 
 extern llvm::cl::opt<ProfileFormatKind> ProfileFormat;
+extern llvm::cl::opt<bool> ShowDensity;
 extern llvm::cl::opt<bool> SplitEH;
 extern llvm::cl::opt<bool> StrictMode;
 extern llvm::cl::opt<bool> TimeOpts;

bolt/lib/Passes/BinaryPasses.cpp

@@ -15,6 +15,7 @@
 #include "bolt/Core/ParallelUtilities.h"
 #include "bolt/Passes/ReorderAlgorithm.h"
 #include "bolt/Passes/ReorderFunctions.h"
+#include "bolt/Utils/CommandLineOpts.h"
 #include "llvm/Support/CommandLine.h"
 #include <atomic>
 #include <mutex>
@@ -223,6 +224,18 @@ static cl::opt<unsigned> TopCalledLimit(
              "functions section"),
     cl::init(100), cl::Hidden, cl::cat(BoltCategory));
 
+// Profile density options, synced with llvm-profgen/ProfileGenerator.cpp
+static cl::opt<int> ProfileDensityCutOffHot(
+    "profile-density-cutoff-hot", cl::init(990000),
+    cl::desc("Total samples cutoff for functions used to calculate "
+             "profile density."));
+
+static cl::opt<double> ProfileDensityThreshold(
+    "profile-density-threshold", cl::init(60),
+    cl::desc("If the profile density is below the given threshold, it "
+             "will be suggested to increase the sampling rate."),
+    cl::Optional);
+
 } // namespace opts
 
 namespace llvm {
@@ -1383,6 +1396,7 @@ Error PrintProgramStats::runOnFunctions(BinaryContext &BC) {
   uint64_t StaleSampleCount = 0;
   uint64_t InferredSampleCount = 0;
   std::vector<const BinaryFunction *> ProfiledFunctions;
+  std::vector<std::pair<double, uint64_t>> FuncDensityList;
   const char *StaleFuncsHeader = "BOLT-INFO: Functions with stale profile:\n";
   for (auto &BFI : BC.getBinaryFunctions()) {
     const BinaryFunction &Function = BFI.second;
@@ -1441,6 +1455,22 @@ Error PrintProgramStats::runOnFunctions(BinaryContext &BC) {
       StaleSampleCount += SampleCount;
       ++NumAllStaleFunctions;
     }
+
+    if (opts::ShowDensity) {
+      uint64_t Size = Function.getSize();
+      // In case of BOLT split functions registered in BAT, executed traces are
+      // automatically attributed to the main fragment. Add up function sizes
+      // for all fragments.
+      if (IsHotParentOfBOLTSplitFunction)
+        for (const BinaryFunction *Fragment : Function.getFragments())
+          Size += Fragment->getSize();
+      double Density = (double)1.0 * Function.getSampleCountInBytes() / Size;
+      FuncDensityList.emplace_back(Density, SampleCount);
+      LLVM_DEBUG(BC.outs() << Function << ": executed bytes "
+                           << Function.getSampleCountInBytes() << ", size (b) "
+                           << Size << ", density " << Density
+                           << ", sample count " << SampleCount << '\n');
+    }
   }
   BC.NumProfiledFuncs = ProfiledFunctions.size();
   BC.NumStaleProfileFuncs = NumStaleProfileFunctions;
@@ -1684,6 +1714,50 @@ Error PrintProgramStats::runOnFunctions(BinaryContext &BC) {
       BC.outs() << ". Use -print-unknown to see the list.";
     BC.outs() << '\n';
   }
+
+  if (opts::ShowDensity) {
+    double Density = 0.0;
+    // Sorted by the density in descending order.
+    llvm::stable_sort(FuncDensityList,
+                      [&](const std::pair<double, uint64_t> &A,
+                          const std::pair<double, uint64_t> &B) {
+                        if (A.first != B.first)
+                          return A.first > B.first;
+                        return A.second < B.second;
+                      });
+
+    uint64_t AccumulatedSamples = 0;
+    uint32_t I = 0;
+    assert(opts::ProfileDensityCutOffHot <= 1000000 &&
+           "The cutoff value is greater than 1000000(100%)");
+    while (AccumulatedSamples <
+               TotalSampleCount *
+                   static_cast<float>(opts::ProfileDensityCutOffHot) /
+                   1000000 &&
+           I < FuncDensityList.size()) {
+      AccumulatedSamples += FuncDensityList[I].second;
+      Density = FuncDensityList[I].first;
+      I++;
+    }
+    if (Density == 0.0) {
+      BC.errs() << "BOLT-WARNING: the output profile is empty or the "
+                   "--profile-density-cutoff-hot option is "
+                   "set too low. Please check your command.\n";
+    } else if (Density < opts::ProfileDensityThreshold) {
+      BC.errs()
+          << "BOLT-WARNING: BOLT is estimated to optimize better with "
+          << format("%.1f", opts::ProfileDensityThreshold / Density)
+          << "x more samples. Please consider increasing sampling rate or "
+             "profiling for longer duration to get more samples.\n";
+    }
+
+    BC.outs() << "BOLT-INFO: Functions with density >= "
+              << format("%.1f", Density) << " account for "
+              << format("%.2f",
+                        static_cast<double>(opts::ProfileDensityCutOffHot) /
+                            10000)
+              << "% total sample counts.\n";
+  }
   return Error::success();
 }
 

bolt/lib/Profile/DataAggregator.cpp

@@ -849,6 +849,12 @@ bool DataAggregator::doTrace(const LBREntry &First, const LBREntry &Second,
     return false;
   }
 
+  // Set ParentFunc to BAT parent function or FromFunc itself.
+  BinaryFunction *ParentFunc = getBATParentFunction(*FromFunc);
+  if (!ParentFunc)
+    ParentFunc = FromFunc;
+  ParentFunc->SampleCountInBytes += Count * (Second.From - First.To);
+
   std::optional<BoltAddressTranslation::FallthroughListTy> FTs =
       BAT ? BAT->getFallthroughsInTrace(FromFunc->getAddress(), First.To,
                                         Second.From)
@@ -868,13 +874,12 @@ bool DataAggregator::doTrace(const LBREntry &First, const LBREntry &Second,
                     << FromFunc->getPrintName() << ":"
                     << Twine::utohexstr(First.To) << " to "
                     << Twine::utohexstr(Second.From) << ".\n");
-  BinaryFunction *ParentFunc = getBATParentFunction(*FromFunc);
   for (auto [From, To] : *FTs) {
     if (BAT) {
       From = BAT->translate(FromFunc->getAddress(), From, /*IsBranchSrc=*/true);
       To = BAT->translate(FromFunc->getAddress(), To, /*IsBranchSrc=*/false);
     }
-    doIntraBranch(ParentFunc ? *ParentFunc : *FromFunc, From, To, Count, false);
+    doIntraBranch(*ParentFunc, From, To, Count, false);
   }
 
   return true;

bolt/lib/Utils/CommandLineOpts.cpp

@@ -175,6 +175,10 @@ cl::opt<std::string> SaveProfile("w",
                                  cl::desc("save recorded profile to a file"),
                                  cl::cat(BoltOutputCategory));
 
+cl::opt<bool> ShowDensity("show-density",
+                          cl::desc("show profile density details"),
+                          cl::Optional, cl::cat(AggregatorCategory));
+
 cl::opt<bool> SplitEH("split-eh", cl::desc("split C++ exception handling code"),
                       cl::Hidden, cl::cat(BoltOptCategory));
 

bolt/test/X86/pre-aggregated-perf.test

@@ -11,7 +11,19 @@ REQUIRES: system-linux
 
 RUN: yaml2obj %p/Inputs/blarge.yaml &> %t.exe
 RUN: perf2bolt %t.exe -o %t --pa -p %p/Inputs/pre-aggregated.txt -w %t.new \
-RUN:   --profile-use-dfs | FileCheck %s
+RUN:   --profile-density-threshold=9 --profile-density-cutoff-hot=970000 \
+RUN:   --profile-use-dfs | FileCheck %s --check-prefix=CHECK-P2B
+
+CHECK-P2B: BOLT-INFO: 4 out of 7 functions in the binary (57.1%) have non-empty execution profile
+CHECK-P2B: BOLT-INFO: Functions with density >= 21.7 account for 97.00% total sample counts.
+
+RUN: perf2bolt %t.exe -o %t --pa -p %p/Inputs/pre-aggregated.txt -w %t.new \
+RUN:   --profile-density-cutoff-hot=970000 \
+RUN:   --profile-use-dfs 2>&1 | FileCheck %s --check-prefix=CHECK-WARNING
+
+CHECK-WARNING: BOLT-INFO: 4 out of 7 functions in the binary (57.1%) have non-empty execution profile
+CHECK-WARNING: BOLT-WARNING: BOLT is estimated to optimize better with 2.8x more samples.
+CHECK-WARNING: BOLT-INFO: Functions with density >= 21.7 account for 97.00% total sample counts.
 
 RUN: llvm-bolt %t.exe -data %t -o %t.null | FileCheck %s
 RUN: llvm-bolt %t.exe -data %t.new -o %t.null | FileCheck %s

bolt/tools/driver/llvm-bolt.cpp

@@ -129,6 +129,7 @@ void perf2boltMode(int argc, char **argv) {
     exit(1);
   }
   opts::AggregateOnly = true;
+  opts::ShowDensity = true;
 }
 
 void boltDiffMode(int argc, char **argv) {