lib/Analysis/InstructionPrecedenceTracking.cpp

   1 //===-- InstructionPrecedenceTracking.cpp -----------------------*- C++ -*-===//
   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 // Implements a class that is able to define some instructions as "special"
  10 // (e.g. as having implicit control flow, or writing memory, or having another
  11 // interesting property) and then efficiently answers queries of the types:
  12 // 1. Are there any special instructions in the block of interest?
  13 // 2. Return first of the special instructions in the given block;
  14 // 3. Check if the given instruction is preceeded by the first special
  15 //    instruction in the same block.
  16 // The class provides caching that allows to answer these queries quickly. The
  17 // user must make sure that the cached data is invalidated properly whenever
  18 // a content of some tracked block is changed.
  19 //===----------------------------------------------------------------------===//
  20
  21 #include "llvm/Analysis/InstructionPrecedenceTracking.h"
  22 #include "llvm/Analysis/ValueTracking.h"
  23
  24 using namespace llvm;
  25
  26 #ifndef NDEBUG
  27 static cl::opt<bool> ExpensiveAsserts(
  28     "ipt-expensive-asserts",
  29     cl::desc("Perform expensive assert validation on every query to Instruction"
  30              " Precedence Tracking"),
  31     cl::init(false), cl::Hidden);
  32 #endif
  33
  34 const Instruction *InstructionPrecedenceTracking::getFirstSpecialInstruction(
  35     const BasicBlock *BB) {
  36 #ifndef NDEBUG
  37   // If there is a bug connected to invalid cache, turn on ExpensiveAsserts to
  38   // catch this situation as early as possible.
  39   if (ExpensiveAsserts)
  40     validateAll();
  41   else
  42     validate(BB);
  43 #endif
  44
  45   if (FirstSpecialInsts.find(BB) == FirstSpecialInsts.end()) {
  46     fill(BB);
  47     assert(FirstSpecialInsts.find(BB) != FirstSpecialInsts.end() && "Must be!");
  48   }
  49   return FirstSpecialInsts[BB];
  50 }
  51
  52 bool InstructionPrecedenceTracking::hasSpecialInstructions(
  53     const BasicBlock *BB) {
  54   return getFirstSpecialInstruction(BB) != nullptr;
  55 }
  56
  57 bool InstructionPrecedenceTracking::isPreceededBySpecialInstruction(
  58     const Instruction *Insn) {
  59   const Instruction *MaybeFirstSpecial =
  60       getFirstSpecialInstruction(Insn->getParent());
  61   return MaybeFirstSpecial && OI.dominates(MaybeFirstSpecial, Insn);
  62 }
  63
  64 void InstructionPrecedenceTracking::fill(const BasicBlock *BB) {
  65   FirstSpecialInsts.erase(BB);
  66   for (auto &I : *BB)
  67     if (isSpecialInstruction(&I)) {
  68       FirstSpecialInsts[BB] = &I;
  69       return;
  70     }
  71
  72   // Mark this block as having no special instructions.
  73   FirstSpecialInsts[BB] = nullptr;
  74 }
  75
  76 #ifndef NDEBUG
  77 void InstructionPrecedenceTracking::validate(const BasicBlock *BB) const {
  78   auto It = FirstSpecialInsts.find(BB);
  79   // Bail if we don't have anything cached for this block.
  80   if (It == FirstSpecialInsts.end())
  81     return;
  82
  83   for (const Instruction &Insn : *BB)
  84     if (isSpecialInstruction(&Insn)) {
  85       assert(It->second == &Insn &&
  86              "Cached first special instruction is wrong!");
  87       return;
  88     }
  89
  90   assert(It->second == nullptr &&
  91          "Block is marked as having special instructions but in fact it  has "
  92          "none!");
  93 }
  94
  95 void InstructionPrecedenceTracking::validateAll() const {
  96   // Check that for every known block the cached value is correct.
  97   for (auto &It : FirstSpecialInsts)
  98     validate(It.first);
  99 }
 100 #endif
 101
 102 void InstructionPrecedenceTracking::invalidateBlock(const BasicBlock *BB) {
 103   OI.invalidateBlock(BB);
 104   FirstSpecialInsts.erase(BB);
 105 }
 106
 107 void InstructionPrecedenceTracking::clear() {
 108   for (auto It : FirstSpecialInsts)
 109     OI.invalidateBlock(It.first);
 110   FirstSpecialInsts.clear();
 111 #ifndef NDEBUG
 112   // The map should be valid after clearing (at least empty).
 113   validateAll();
 114 #endif
 115 }
 116
 117 bool ImplicitControlFlowTracking::isSpecialInstruction(
 118     const Instruction *Insn) const {
 119   // If a block's instruction doesn't always pass the control to its successor
 120   // instruction, mark the block as having implicit control flow. We use them
 121   // to avoid wrong assumptions of sort "if A is executed and B post-dominates
 122   // A, then B is also executed". This is not true is there is an implicit
 123   // control flow instruction (e.g. a guard) between them.
 124   //
 125   // TODO: Currently, isGuaranteedToTransferExecutionToSuccessor returns false
 126   // for volatile stores and loads because they can trap. The discussion on
 127   // whether or not it is correct is still ongoing. We might want to get rid
 128   // of this logic in the future. Anyways, trapping instructions shouldn't
 129   // introduce implicit control flow, so we explicitly allow them here. This
 130   // must be removed once isGuaranteedToTransferExecutionToSuccessor is fixed.
 131   if (isGuaranteedToTransferExecutionToSuccessor(Insn))
 132     return false;
 133   if (isa<LoadInst>(Insn)) {
 134     assert(cast<LoadInst>(Insn)->isVolatile() &&
 135            "Non-volatile load should transfer execution to successor!");
 136     return false;
 137   }
 138   if (isa<StoreInst>(Insn)) {
 139     assert(cast<StoreInst>(Insn)->isVolatile() &&
 140            "Non-volatile store should transfer execution to successor!");
 141     return false;
 142   }
 143   return true;
 144 }