llvm/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/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: Invalid cost";
 111
 112       OS << " for instruction: " << Inst << "\n";
 113     }
 114   }
 115 }