lib/Analysis/CostModel.cpp

   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/Pass.h"
  24 #include "llvm/Support/CommandLine.h"
  25 #include "llvm/Support/Debug.h"
  26 #include "llvm/Support/raw_ostream.h"
  27 using namespace llvm;
  28
  29 static cl::opt<TargetTransformInfo::TargetCostKind> CostKind(
  30     "cost-kind", cl::desc("Target cost kind"),
  31     cl::init(TargetTransformInfo::TCK_RecipThroughput),
  32     cl::values(clEnumValN(TargetTransformInfo::TCK_RecipThroughput,
  33                           "throughput", "Reciprocal throughput"),
  34                clEnumValN(TargetTransformInfo::TCK_Latency,
  35                           "latency", "Instruction latency"),
  36                clEnumValN(TargetTransformInfo::TCK_CodeSize,
  37                           "code-size", "Code size")));
  38
  39 #define CM_NAME "cost-model"
  40 #define DEBUG_TYPE CM_NAME
  41
  42 namespace {
  43   class CostModelAnalysis : public FunctionPass {
  44
  45   public:
  46     static char ID; // Class identification, replacement for typeinfo
  47     CostModelAnalysis() : FunctionPass(ID), F(nullptr), TTI(nullptr) {
  48       initializeCostModelAnalysisPass(
  49         *PassRegistry::getPassRegistry());
  50     }
  51
  52     /// Returns the expected cost of the instruction.
  53     /// Returns -1 if the cost is unknown.
  54     /// Note, this method does not cache the cost calculation and it
  55     /// can be expensive in some cases.
  56     unsigned getInstructionCost(const Instruction *I) const {
  57       return TTI->getInstructionCost(I, TargetTransformInfo::TCK_RecipThroughput);
  58     }
  59
  60   private:
  61     void getAnalysisUsage(AnalysisUsage &AU) const override;
  62     bool runOnFunction(Function &F) override;
  63     void print(raw_ostream &OS, const Module*) const override;
  64
  65     /// The function that we analyze.
  66     Function *F;
  67     /// Target information.
  68     const TargetTransformInfo *TTI;
  69   };
  70 }  // End of anonymous namespace
  71
  72 // Register this pass.
  73 char CostModelAnalysis::ID = 0;
  74 static const char cm_name[] = "Cost Model Analysis";
  75 INITIALIZE_PASS_BEGIN(CostModelAnalysis, CM_NAME, cm_name, false, true)
  76 INITIALIZE_PASS_END  (CostModelAnalysis, CM_NAME, cm_name, false, true)
  77
  78 FunctionPass *llvm::createCostModelAnalysisPass() {
  79   return new CostModelAnalysis();
  80 }
  81
  82 void
  83 CostModelAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  84   AU.setPreservesAll();
  85 }
  86
  87 bool
  88 CostModelAnalysis::runOnFunction(Function &F) {
  89  this->F = &F;
  90  auto *TTIWP = getAnalysisIfAvailable<TargetTransformInfoWrapperPass>();
  91  TTI = TTIWP ? &TTIWP->getTTI(F) : nullptr;
  92
  93  return false;
  94 }
  95
  96 void CostModelAnalysis::print(raw_ostream &OS, const Module*) const {
  97   if (!F)
  98     return;
  99
 100   for (BasicBlock &B : *F) {
 101     for (Instruction &Inst : B) {
 102       unsigned Cost = TTI->getInstructionCost(&Inst, CostKind);
 103       if (Cost != (unsigned)-1)
 104         OS << "Cost Model: Found an estimated cost of " << Cost;
 105       else
 106         OS << "Cost Model: Unknown cost";
 107
 108       OS << " for instruction: " << Inst << "\n";
 109     }
 110   }
 111 }