[clang] [llvm] [PseudoProbe] Mix and reorder block and call probe ID in lexical order (PR #75092)

[via cfe-commits]

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@@ -471,6 +471,66 @@ ProfileGenerator::getTopLevelFunctionProfile(FunctionId FuncName) {
   return ProfileMap.Create(Context);
 }
 
+// Use a heuristic to determine probe order by checking callsite insertion
+// position relative to the BB probes. Sort all the BB probes is in a list, for
+// each calliste, compute "ratio = insert position / length of the list". For
+// the old order, the probe ids for BB should be all before(smaller than) the
+// probe ids for callsite, this ratio should be equal to or close to 1.
+bool checkProbeIDIsInMixedOrder(const SampleProfileMap &Profiles) {
+  // Use flattned profile to maximize the callsites in the profile.
+  SampleProfileMap flattenProfile;
+  ProfileConverter::flattenProfile(Profiles, flattenProfile);
+
+  uint32_t PossibleOldOrderNum = 0;
+  uint32_t PossibleNewOrderNum = 0;
+
+  for (const auto &I : flattenProfile) {
+    const FunctionSamples &FS = I.second;
+    // Skip small functions whose profile order are likely random.
+    if (FS.getBodySamples().size() < 10)
+      continue;
+
+    std::set<uint32_t> PossibleBBProbeIDs;
+    std::set<uint32_t> CallsiteIDs;
+    for (const auto &I : FS.getBodySamples()) {
+      if (I.second.getCallTargets().empty())
+        PossibleBBProbeIDs.insert(I.first.LineOffset);
+      else
+        CallsiteIDs.insert(I.first.LineOffset);
+    }
+
+    if (PossibleBBProbeIDs.empty() || CallsiteIDs.empty())
+      continue;
+
+    uint32_t DistanceToBeginSum = 0;
+    for (const auto &ID : CallsiteIDs)
+      DistanceToBeginSum += std::distance(PossibleBBProbeIDs.begin(),
+                                          PossibleBBProbeIDs.upper_bound(ID));
+    uint32_t LengthSum = PossibleBBProbeIDs.size() * CallsiteIDs.size();
+
+    // Note that PossibleBBProbeIDs could contains some callsite probe id, the
+    // ratio is not exactly 1 for the old order, hence use a smaller threshold
+    // to determine.
+    if ((float)DistanceToBeginSum / LengthSum > 0.8)
+      PossibleOldOrderNum++;
+    else
+      PossibleNewOrderNum++;
+  }
+  return PossibleNewOrderNum >= PossibleOldOrderNum;
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WenleiHe wrote:

This seems quite fuzzy.. If we can't decide whether a probe is originally a call probe or a block probe, that makes the checking less reliable.. I wasn't anticipating that, but if that's case we can't even confidently issue error.

In reality, do we see cases where there are many `PossibleNewOrderNum` and `PossibleOldOrderNum` for one profile? 

https://github.com/llvm/llvm-project/pull/75092