[llvm-profgen] Filter out ambiguous cold profiles during profile generation (#81803)

For the built-in local initialization function(`__cxx_global_var_init`,
`__tls_init` prefix), there could be multiple versions of the functions
in the final binary, e.g. `__cxx_global_var_init`, which is a wrapper of
global variable ctors, the compiler could assign suffixes like
`__cxx_global_var_init.N` for different ctors.
However, in the profile generation, we call `getCanonicalFnName` to
canonicalize the names which strip the suffixes. Therefore, samples from
different functions queries the same profile(only
`__cxx_global_var_init`) and the counts are merged. As the functions are
essentially different, entries of the merged profile are ambiguous. In
sample loading, for each version of this function, the IR from one
version would be attributed towards a merged entries, which is
inaccurate, especially for fuzzy profile matching, it gets multiple
callsites(from different function) but using to match one callsite,
which mislead the matching and report a lot of false positives.
Hence, we want to filter them out from the profile map during the
profile generation time. The profiles are all cold functions, it won't
have perf impact.

2 files changed, 52 insertions, 0 deletions

diff --git a/llvm/tools/llvm-profgen/ProfileGenerator.cpp b/llvm/tools/llvm-profgen/ProfileGenerator.cpp
index c4028e6..5aa4410 100644
--- a/llvm/tools/llvm-profgen/ProfileGenerator.cpp
+++ b/llvm/tools/llvm-profgen/ProfileGenerator.cpp
@@ -196,6 +196,48 @@ void ProfileGeneratorBase::showDensitySuggestion(double Density) {
            << "% total samples: " << format("%.1f", Density) << "\n";
 }
 
+bool ProfileGeneratorBase::filterAmbiguousProfile(FunctionSamples &FS) {
+  for (const auto &Prefix : FuncPrefixsToFilter) {
+    if (FS.getFuncName().starts_with(Prefix))
+      return true;
+  }
+
+  // Filter the function profiles for the inlinees. It's useful for fuzzy
+  // profile matching which flattens the profile and inlinees' samples are
+  // merged into top-level function.
+  for (auto &Callees :
+       const_cast<CallsiteSampleMap &>(FS.getCallsiteSamples())) {
+    auto &CalleesMap = Callees.second;
+    for (auto I = CalleesMap.begin(); I != CalleesMap.end();) {
+      auto FS = I++;
+      if (filterAmbiguousProfile(FS->second))
+        CalleesMap.erase(FS);
+    }
+  }
+  return false;
+}
+
+// For built-in local initialization function such as __cxx_global_var_init,
+// __tls_init prefix function, there could be multiple versions of the functions
+// in the final binary. However, in the profile generation, we call
+// getCanonicalFnName to canonicalize the names which strips the suffixes.
+// Therefore, samples from different functions queries the same profile and the
+// samples are merged. As the functions are essentially different, entries of
+// the merged profile are ambiguous. In sample loader, the IR from one version
+// would be attributed towards a merged entries, which is inaccurate. Especially
+// for fuzzy profile matching, it gets multiple callsites(from different
+// function) but used to match one callsite, which misleads the matching and
+// causes a lot of false positives report. Hence, we want to filter them out
+// from the profile map during the profile generation time. The profiles are all
+// cold functions, it won't have perf impact.
+void ProfileGeneratorBase::filterAmbiguousProfile(SampleProfileMap &Profiles) {
+  for (auto I = ProfileMap.begin(); I != ProfileMap.end();) {
+    auto FS = I++;
+    if (filterAmbiguousProfile(FS->second))
+      ProfileMap.erase(FS);
+  }
+}
+
 double ProfileGeneratorBase::calculateDensity(const SampleProfileMap &Profiles,
                                               uint64_t HotCntThreshold) {
   double Density = DBL_MAX;
@@ -491,6 +533,7 @@ void ProfileGenerator::generateProfile() {
 void ProfileGenerator::postProcessProfiles() {
   computeSummaryAndThreshold(ProfileMap);
   trimColdProfiles(ProfileMap, ColdCountThreshold);
+  filterAmbiguousProfile(ProfileMap);
   calculateAndShowDensity(ProfileMap);
 }
 
@@ -1024,6 +1067,7 @@ void CSProfileGenerator::postProcessProfiles() {
     CSConverter.convertCSProfiles();
     FunctionSamples::ProfileIsCS = false;
   }
+  filterAmbiguousProfile(ProfileMap);
 }
 
 void ProfileGeneratorBase::computeSummaryAndThreshold(
diff --git a/llvm/tools/llvm-profgen/ProfileGenerator.h b/llvm/tools/llvm-profgen/ProfileGenerator.h
index 88cf2dc..d258fb7 100644
--- a/llvm/tools/llvm-profgen/ProfileGenerator.h
+++ b/llvm/tools/llvm-profgen/ProfileGenerator.h
@@ -108,6 +108,10 @@ protected:
 
   void updateCallsiteSamples();
 
+  void filterAmbiguousProfile(SampleProfileMap &Profiles);
+
+  bool filterAmbiguousProfile(FunctionSamples &FS);
+
   StringRef getCalleeNameForAddress(uint64_t TargetAddress);
 
   void computeSummaryAndThreshold(SampleProfileMap &ProfileMap);
@@ -128,6 +132,10 @@ protected:
   virtual bool collectFunctionsFromLLVMProfile(
       std::unordered_set<const BinaryFunction *> &ProfiledFunctions) = 0;
 
+  // List of function prefix to filter out.
+  static constexpr const char *FuncPrefixsToFilter[] = {"__cxx_global_var_init",
+                                                        "__tls_init"};
+
   // Thresholds from profile summary to answer isHotCount/isColdCount queries.
   uint64_t HotCountThreshold;