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      1 //===-- IndirectCallPromotionAnalysis.cpp - Find promotion candidates ===//
      2 //
      3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
      4 // See https://llvm.org/LICENSE.txt for license information.
      5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
      6 //
      7 //===----------------------------------------------------------------------===//
      8 //
      9 // Helper methods for identifying profitable indirect call promotion
     10 // candidates for an instruction when the indirect-call value profile metadata
     11 // is available.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/Analysis/IndirectCallPromotionAnalysis.h"
     16 #include "llvm/ADT/STLExtras.h"
     17 #include "llvm/Analysis/IndirectCallVisitor.h"
     18 #include "llvm/IR/InstIterator.h"
     19 #include "llvm/IR/InstVisitor.h"
     20 #include "llvm/IR/Instructions.h"
     21 #include "llvm/IR/IntrinsicInst.h"
     22 #include "llvm/ProfileData/InstrProf.h"
     23 #include "llvm/Support/CommandLine.h"
     24 #include "llvm/Support/Debug.h"
     25 #include <memory>
     26 
     27 using namespace llvm;
     28 
     29 #define DEBUG_TYPE "pgo-icall-prom-analysis"
     30 
     31 // The percent threshold for the direct-call target (this call site vs the
     32 // remaining call count) for it to be considered as the promotion target.
     33 static cl::opt<unsigned> ICPRemainingPercentThreshold(
     34     "icp-remaining-percent-threshold", cl::init(30), cl::Hidden, cl::ZeroOrMore,
     35     cl::desc("The percentage threshold against remaining unpromoted indirect "
     36              "call count for the promotion"));
     37 
     38 // The percent threshold for the direct-call target (this call site vs the
     39 // total call count) for it to be considered as the promotion target.
     40 static cl::opt<unsigned>
     41     ICPTotalPercentThreshold("icp-total-percent-threshold", cl::init(5),
     42                              cl::Hidden, cl::ZeroOrMore,
     43                              cl::desc("The percentage threshold against total "
     44                                       "count for the promotion"));
     45 
     46 // Set the maximum number of targets to promote for a single indirect-call
     47 // callsite.
     48 static cl::opt<unsigned>
     49     MaxNumPromotions("icp-max-prom", cl::init(3), cl::Hidden, cl::ZeroOrMore,
     50                      cl::desc("Max number of promotions for a single indirect "
     51                               "call callsite"));
     52 
     53 ICallPromotionAnalysis::ICallPromotionAnalysis() {
     54   ValueDataArray = std::make_unique<InstrProfValueData[]>(MaxNumPromotions);
     55 }
     56 
     57 bool ICallPromotionAnalysis::isPromotionProfitable(uint64_t Count,
     58                                                    uint64_t TotalCount,
     59                                                    uint64_t RemainingCount) {
     60   return Count * 100 >= ICPRemainingPercentThreshold * RemainingCount &&
     61          Count * 100 >= ICPTotalPercentThreshold * TotalCount;
     62 }
     63 
     64 // Indirect-call promotion heuristic. The direct targets are sorted based on
     65 // the count. Stop at the first target that is not promoted. Returns the
     66 // number of candidates deemed profitable.
     67 uint32_t ICallPromotionAnalysis::getProfitablePromotionCandidates(
     68     const Instruction *Inst, uint32_t NumVals, uint64_t TotalCount) {
     69   ArrayRef<InstrProfValueData> ValueDataRef(ValueDataArray.get(), NumVals);
     70 
     71   LLVM_DEBUG(dbgs() << " \nWork on callsite " << *Inst
     72                     << " Num_targets: " << NumVals << "\n");
     73 
     74   uint32_t I = 0;
     75   uint64_t RemainingCount = TotalCount;
     76   for (; I < MaxNumPromotions && I < NumVals; I++) {
     77     uint64_t Count = ValueDataRef[I].Count;
     78     assert(Count <= RemainingCount);
     79     LLVM_DEBUG(dbgs() << " Candidate " << I << " Count=" << Count
     80                       << "  Target_func: " << ValueDataRef[I].Value << "\n");
     81 
     82     if (!isPromotionProfitable(Count, TotalCount, RemainingCount)) {
     83       LLVM_DEBUG(dbgs() << " Not promote: Cold target.\n");
     84       return I;
     85     }
     86     RemainingCount -= Count;
     87   }
     88   return I;
     89 }
     90 
     91 ArrayRef<InstrProfValueData>
     92 ICallPromotionAnalysis::getPromotionCandidatesForInstruction(
     93     const Instruction *I, uint32_t &NumVals, uint64_t &TotalCount,
     94     uint32_t &NumCandidates) {
     95   bool Res =
     96       getValueProfDataFromInst(*I, IPVK_IndirectCallTarget, MaxNumPromotions,
     97                                ValueDataArray.get(), NumVals, TotalCount);
     98   if (!Res) {
     99     NumCandidates = 0;
    100     return ArrayRef<InstrProfValueData>();
    101   }
    102   NumCandidates = getProfitablePromotionCandidates(I, NumVals, TotalCount);
    103   return ArrayRef<InstrProfValueData>(ValueDataArray.get(), NumVals);
    104 }
    105