lib/Analysis/CostModel.cpp

   1 //===- CostModel.cpp ------ Cost Model Analysis ---------------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file defines the cost model analysis. It provides a very basic cost
  11 // estimation for LLVM-IR. This analysis uses the services of the codegen
  12 // to approximate the cost of any IR instruction when lowered to machine
  13 // instructions. The cost results are unit-less and the cost number represents
  14 // the throughput of the machine assuming that all loads hit the cache, all
  15 // branches are predicted, etc. The cost numbers can be added in order to
  16 // compare two or more transformation alternatives.
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #include "llvm/ADT/STLExtras.h"
  21 #include "llvm/Analysis/Passes.h"
  22 #include "llvm/Analysis/TargetTransformInfo.h"
  23 #include "llvm/IR/Function.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
  40 #define CM_NAME "cost-model"
  41 #define DEBUG_TYPE CM_NAME
  42
  43 namespace {
  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.
  74 char CostModelAnalysis::ID = 0;
  75 static const char cm_name[] = "Cost Model Analysis";
  76 INITIALIZE_PASS_BEGIN(CostModelAnalysis, CM_NAME, cm_name, false, true)
  77 INITIALIZE_PASS_END  (CostModelAnalysis, CM_NAME, cm_name, false, true)
  78
  79 FunctionPass *llvm::createCostModelAnalysisPass() {
  80   return new CostModelAnalysis();
  81 }
  82
  83 void
  84 CostModelAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  85   AU.setPreservesAll();
  86 }
  87
  88 bool
  89 CostModelAnalysis::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
  97 void 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 }