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/IR/Instruction.h"
17 #include "llvm/ProfileData/InstrProf.h"
18 #include "llvm/Support/CommandLine.h"
19 #include "llvm/Support/Debug.h"
20 #include <memory>
21 
22 using namespace llvm;
23 
24 #define DEBUG_TYPE "pgo-icall-prom-analysis"
25 
26 // The percent threshold for the direct-call target (this call site vs the
27 // remaining call count) for it to be considered as the promotion target.
28 static cl::opt<unsigned> ICPRemainingPercentThreshold(
29     "icp-remaining-percent-threshold", cl::init(30), cl::Hidden,
30     cl::desc("The percentage threshold against remaining unpromoted indirect "
31              "call count for the promotion"));
32 
33 // The percent threshold for the direct-call target (this call site vs the
34 // total call count) for it to be considered as the promotion target.
35 static cl::opt<unsigned>
36     ICPTotalPercentThreshold("icp-total-percent-threshold", cl::init(5),
37                              cl::Hidden,
38                              cl::desc("The percentage threshold against total "
39                                       "count for the promotion"));
40 
41 // Set the maximum number of targets to promote for a single indirect-call
42 // callsite.
43 static cl::opt<unsigned>
44     MaxNumPromotions("icp-max-prom", cl::init(3), cl::Hidden,
45                      cl::desc("Max number of promotions for a single indirect "
46                               "call callsite"));
47 
48 ICallPromotionAnalysis::ICallPromotionAnalysis() {
49   ValueDataArray = std::make_unique<InstrProfValueData[]>(MaxNumPromotions);
50 }
51 
52 bool ICallPromotionAnalysis::isPromotionProfitable(uint64_t Count,
53                                                    uint64_t TotalCount,
54                                                    uint64_t RemainingCount) {
55   return Count * 100 >= ICPRemainingPercentThreshold * RemainingCount &&
56          Count * 100 >= ICPTotalPercentThreshold * TotalCount;
57 }
58 
59 // Indirect-call promotion heuristic. The direct targets are sorted based on
60 // the count. Stop at the first target that is not promoted. Returns the
61 // number of candidates deemed profitable.
62 uint32_t ICallPromotionAnalysis::getProfitablePromotionCandidates(
63     const Instruction *Inst, uint32_t NumVals, uint64_t TotalCount) {
64   ArrayRef<InstrProfValueData> ValueDataRef(ValueDataArray.get(), NumVals);
65 
66   LLVM_DEBUG(dbgs() << " \nWork on callsite " << *Inst
67                     << " Num_targets: " << NumVals << "\n");
68 
69   uint32_t I = 0;
70   uint64_t RemainingCount = TotalCount;
71   for (; I < MaxNumPromotions && I < NumVals; I++) {
72     uint64_t Count = ValueDataRef[I].Count;
73     assert(Count <= RemainingCount);
74     LLVM_DEBUG(dbgs() << " Candidate " << I << " Count=" << Count
75                       << "  Target_func: " << ValueDataRef[I].Value << "\n");
76 
77     if (!isPromotionProfitable(Count, TotalCount, RemainingCount)) {
78       LLVM_DEBUG(dbgs() << " Not promote: Cold target.\n");
79       return I;
80     }
81     RemainingCount -= Count;
82   }
83   return I;
84 }
85 
86 ArrayRef<InstrProfValueData>
87 ICallPromotionAnalysis::getPromotionCandidatesForInstruction(
88     const Instruction *I, uint32_t &NumVals, uint64_t &TotalCount,
89     uint32_t &NumCandidates) {
90   bool Res =
91       getValueProfDataFromInst(*I, IPVK_IndirectCallTarget, MaxNumPromotions,
92                                ValueDataArray.get(), NumVals, TotalCount);
93   if (!Res) {
94     NumCandidates = 0;
95     return ArrayRef<InstrProfValueData>();
96   }
97   NumCandidates = getProfitablePromotionCandidates(I, NumVals, TotalCount);
98   return ArrayRef<InstrProfValueData>(ValueDataArray.get(), NumVals);
99 }
100