CostModel.cpp revision 360784 
1138573Ssam//===- CostModel.cpp ------ Cost Model Analysis ---------------------------===//
2138573Ssam//
3138573Ssam// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4138573Ssam// See https://llvm.org/LICENSE.txt for license information.
5138573Ssam// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6138573Ssam//
7178354Ssam//===----------------------------------------------------------------------===//
8138573Ssam//
9178354Ssam// This file defines the cost model analysis. It provides a very basic cost
10178354Ssam// estimation for LLVM-IR. This analysis uses the services of the codegen
11182668Simp// to approximate the cost of any IR instruction when lowered to machine
12178354Ssam// instructions. The cost results are unit-less and the cost number represents
13178354Ssam// the throughput of the machine assuming that all loads hit the cache, all
14138573Ssam// 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"
28using namespace llvm;
29
30static 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
40#define CM_NAME "cost-model"
41#define DEBUG_TYPE CM_NAME
42
43namespace {
44  class CostModelAnalysis : public FunctionPass {
45
46  public:
47    static char ID; // Class identification, replacement for typeinfo
48    CostModelAnalysis() : FunctionPass(ID), F(nullptr), TTI(nullptr) {
49      initializeCostModelAnalysisPass(
50        *PassRegistry::getPassRegistry());
51    }
52
53    /// Returns the expected cost of the instruction.
54    /// Returns -1 if the cost is unknown.
55    /// Note, this method does not cache the cost calculation and it
56    /// can be expensive in some cases.
57    unsigned getInstructionCost(const Instruction *I) const {
58      return TTI->getInstructionCost(I, TargetTransformInfo::TCK_RecipThroughput);
59    }
60
61  private:
62    void getAnalysisUsage(AnalysisUsage &AU) const override;
63    bool runOnFunction(Function &F) override;
64    void print(raw_ostream &OS, const Module*) const override;
65
66    /// The function that we analyze.
67    Function *F;
68    /// Target information.
69    const TargetTransformInfo *TTI;
70  };
71}  // End of anonymous namespace
72
73// Register this pass.
74char CostModelAnalysis::ID = 0;
75static const char cm_name[] = "Cost Model Analysis";
76INITIALIZE_PASS_BEGIN(CostModelAnalysis, CM_NAME, cm_name, false, true)
77INITIALIZE_PASS_END  (CostModelAnalysis, CM_NAME, cm_name, false, true)
78
79FunctionPass *llvm::createCostModelAnalysisPass() {
80  return new CostModelAnalysis();
81}
82
83void
84CostModelAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
85  AU.setPreservesAll();
86}
87
88bool
89CostModelAnalysis::runOnFunction(Function &F) {
90 this->F = &F;
91 auto *TTIWP = getAnalysisIfAvailable<TargetTransformInfoWrapperPass>();
92 TTI = TTIWP ? &TTIWP->getTTI(F) : nullptr;
93
94 return false;
95}
96
97void CostModelAnalysis::print(raw_ostream &OS, const Module*) const {
98  if (!F)
99    return;
100
101  for (BasicBlock &B : *F) {
102    for (Instruction &Inst : B) {
103      unsigned Cost = TTI->getInstructionCost(&Inst, CostKind);
104      if (Cost != (unsigned)-1)
105        OS << "Cost Model: Found an estimated cost of " << Cost;
106      else
107        OS << "Cost Model: Unknown cost";
108
109      OS << " for instruction: " << Inst << "\n";
110    }
111  }
112}
113