1 //===- CostModel.cpp ------ Cost Model Analysis ---------------------------===//
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 // This file defines the cost model analysis. It provides a very basic cost
10 // estimation for LLVM-IR. This analysis uses the services of the codegen
11 // to approximate the cost of any IR instruction when lowered to machine
12 // instructions. The cost results are unit-less and the cost number represents
13 // the throughput of the machine assuming that all loads hit the cache, all
14 // branches are predicted, etc. The cost numbers can be added in order to
15 // compare two or more transformation alternatives.
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #include "llvm/ADT/STLExtras.h"
20 #include "llvm/Analysis/Passes.h"
21 #include "llvm/Analysis/TargetTransformInfo.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/InitializePasses.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Support/CommandLine.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 using namespace llvm;
29 
30 static cl::opt<TargetTransformInfo::TargetCostKind> CostKind(
31     "cost-kind", cl::desc("Target cost kind"),
32     cl::init(TargetTransformInfo::TCK_RecipThroughput),
33     cl::values(clEnumValN(TargetTransformInfo::TCK_RecipThroughput,
34                           "throughput", "Reciprocal throughput"),
35                clEnumValN(TargetTransformInfo::TCK_Latency,
36                           "latency", "Instruction latency"),
37                clEnumValN(TargetTransformInfo::TCK_CodeSize,
38                           "code-size", "Code size"),
39                clEnumValN(TargetTransformInfo::TCK_SizeAndLatency,
40                           "size-latency", "Code size and latency")));
41 
42 
43 #define CM_NAME "cost-model"
44 #define DEBUG_TYPE CM_NAME
45 
46 namespace {
47   class CostModelAnalysis : public FunctionPass {
48 
49   public:
50     static char ID; // Class identification, replacement for typeinfo
51     CostModelAnalysis() : FunctionPass(ID), F(nullptr), TTI(nullptr) {
52       initializeCostModelAnalysisPass(
53         *PassRegistry::getPassRegistry());
54     }
55 
56     /// Returns the expected cost of the instruction.
57     /// Returns -1 if the cost is unknown.
58     /// Note, this method does not cache the cost calculation and it
59     /// can be expensive in some cases.
60     InstructionCost getInstructionCost(const Instruction *I) const {
61       return TTI->getInstructionCost(I, TargetTransformInfo::TCK_RecipThroughput);
62     }
63 
64   private:
65     void getAnalysisUsage(AnalysisUsage &AU) const override;
66     bool runOnFunction(Function &F) override;
67     void print(raw_ostream &OS, const Module*) const override;
68 
69     /// The function that we analyze.
70     Function *F;
71     /// Target information.
72     const TargetTransformInfo *TTI;
73   };
74 }  // End of anonymous namespace
75 
76 // Register this pass.
77 char CostModelAnalysis::ID = 0;
78 static const char cm_name[] = "Cost Model Analysis";
79 INITIALIZE_PASS_BEGIN(CostModelAnalysis, CM_NAME, cm_name, false, true)
80 INITIALIZE_PASS_END  (CostModelAnalysis, CM_NAME, cm_name, false, true)
81 
82 FunctionPass *llvm::createCostModelAnalysisPass() {
83   return new CostModelAnalysis();
84 }
85 
86 void
87 CostModelAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
88   AU.setPreservesAll();
89 }
90 
91 bool
92 CostModelAnalysis::runOnFunction(Function &F) {
93  this->F = &F;
94  auto *TTIWP = getAnalysisIfAvailable<TargetTransformInfoWrapperPass>();
95  TTI = TTIWP ? &TTIWP->getTTI(F) : nullptr;
96 
97  return false;
98 }
99 
100 void CostModelAnalysis::print(raw_ostream &OS, const Module*) const {
101   if (!F)
102     return;
103 
104   for (BasicBlock &B : *F) {
105     for (Instruction &Inst : B) {
106       InstructionCost Cost = TTI->getInstructionCost(&Inst, CostKind);
107       if (auto CostVal = Cost.getValue())
108         OS << "Cost Model: Found an estimated cost of " << *CostVal;
109       else
110         OS << "Cost Model: Unknown cost";
111 
112       OS << " for instruction: " << Inst << "\n";
113     }
114   }
115 }
116