[clang] [compiler-rt] [llvm] [TypeProf][InstrFDO]Implement more efficient comparison sequence for indirect-call-promotion with vtable profiles. (PR #81442)

[David Li via cfe-commits]

================
@@ -321,14 +790,114 @@ bool IndirectCallPromoter::processFunction(ProfileSummaryInfo *PSI) {
     if (!NumCandidates ||
         (PSI && PSI->hasProfileSummary() && !PSI->isHotCount(TotalCount)))
       continue;
+
     auto PromotionCandidates = getPromotionCandidatesForCallSite(
         *CB, ICallProfDataRef, TotalCount, NumCandidates);
-    Changed |= tryToPromoteWithFuncCmp(*CB, PromotionCandidates, TotalCount,
-                                       ICallProfDataRef, NumCandidates);
+
+    VTableGUIDCountsMap VTableGUIDCounts;
+    Instruction *VPtr =
+        computeVTableInfos(CB, VTableGUIDCounts, PromotionCandidates);
+
+    if (isProfitableToCompareVTables(PromotionCandidates, TotalCount))
+      Changed |= tryToPromoteWithVTableCmp(*CB, VPtr, PromotionCandidates,
+                                           TotalCount, NumCandidates,
+                                           ICallProfDataRef, VTableGUIDCounts);
+    else
+      Changed |= tryToPromoteWithFuncCmp(*CB, VPtr, PromotionCandidates,
+                                         TotalCount, ICallProfDataRef,
+                                         NumCandidates, VTableGUIDCounts);
   }
   return Changed;
 }
 
+// TODO: Returns false if the function addressing and vtable load instructions
+// cannot sink to indirect fallback.
+bool IndirectCallPromoter::isProfitableToCompareVTables(
+    const std::vector<PromotionCandidate> &Candidates, uint64_t TotalCount) {
+  if (!EnableVTableProfileUse || Candidates.empty())
+    return false;
+  uint64_t RemainingVTableCount = TotalCount;
+  for (size_t I = 0; I < Candidates.size(); I++) {
+    auto &Candidate = Candidates[I];
+    uint64_t VTableSumCount = 0;
+    for (auto &[GUID, Count] : Candidate.VTableGUIDAndCounts)
+      VTableSumCount += Count;
+
+    if (VTableSumCount < Candidate.Count * ICPVTablePercentageThreshold)
+      return false;
+
+    RemainingVTableCount -= Candidate.Count;
+
+    int MaxNumVTable = 1;
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david-xl wrote:

Makes sense, but the comment may need to be updated. For non-last candidates, it conservatively limits the NumVtable to be 1 while for the last candidate it is relaxed to be a value controlled by the parameter.

https://github.com/llvm/llvm-project/pull/81442