1 //===-- InstructionPrecedenceTracking.cpp -----------------------*- C++ -*-===//
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 // Implements a class that is able to define some instructions as "special"
9 // (e.g. as having implicit control flow, or writing memory, or having another
10 // interesting property) and then efficiently answers queries of the types:
11 // 1. Are there any special instructions in the block of interest?
12 // 2. Return first of the special instructions in the given block;
13 // 3. Check if the given instruction is preceeded by the first special
14 // instruction in the same block.
15 // The class provides caching that allows to answer these queries quickly. The
16 // user must make sure that the cached data is invalidated properly whenever
17 // a content of some tracked block is changed.
18 //===----------------------------------------------------------------------===//
19
20 #include "llvm/Analysis/InstructionPrecedenceTracking.h"
21 #include "llvm/Analysis/ValueTracking.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/IR/PatternMatch.h"
24 #include "llvm/Support/CommandLine.h"
25
26 using namespace llvm;
27
28 #define DEBUG_TYPE "ipt"
29 STATISTIC(NumInstScanned, "Number of insts scanned while updating ibt");
30
31 #ifndef NDEBUG
32 static cl::opt<bool> ExpensiveAsserts(
33 "ipt-expensive-asserts",
34 cl::desc("Perform expensive assert validation on every query to Instruction"
35 " Precedence Tracking"),
36 cl::init(false), cl::Hidden);
37 #endif
38
getFirstSpecialInstruction(const BasicBlock * BB)39 const Instruction *InstructionPrecedenceTracking::getFirstSpecialInstruction(
40 const BasicBlock *BB) {
41 #ifndef NDEBUG
42 // If there is a bug connected to invalid cache, turn on ExpensiveAsserts to
43 // catch this situation as early as possible.
44 if (ExpensiveAsserts)
45 validateAll();
46 else
47 validate(BB);
48 #endif
49
50 if (!FirstSpecialInsts.contains(BB)) {
51 fill(BB);
52 assert(FirstSpecialInsts.contains(BB) && "Must be!");
53 }
54 return FirstSpecialInsts[BB];
55 }
56
hasSpecialInstructions(const BasicBlock * BB)57 bool InstructionPrecedenceTracking::hasSpecialInstructions(
58 const BasicBlock *BB) {
59 return getFirstSpecialInstruction(BB) != nullptr;
60 }
61
isPreceededBySpecialInstruction(const Instruction * Insn)62 bool InstructionPrecedenceTracking::isPreceededBySpecialInstruction(
63 const Instruction *Insn) {
64 const Instruction *MaybeFirstSpecial =
65 getFirstSpecialInstruction(Insn->getParent());
66 return MaybeFirstSpecial && MaybeFirstSpecial->comesBefore(Insn);
67 }
68
fill(const BasicBlock * BB)69 void InstructionPrecedenceTracking::fill(const BasicBlock *BB) {
70 FirstSpecialInsts.erase(BB);
71 for (const auto &I : *BB) {
72 NumInstScanned++;
73 if (isSpecialInstruction(&I)) {
74 FirstSpecialInsts[BB] = &I;
75 return;
76 }
77 }
78
79 // Mark this block as having no special instructions.
80 FirstSpecialInsts[BB] = nullptr;
81 }
82
83 #ifndef NDEBUG
validate(const BasicBlock * BB) const84 void InstructionPrecedenceTracking::validate(const BasicBlock *BB) const {
85 auto It = FirstSpecialInsts.find(BB);
86 // Bail if we don't have anything cached for this block.
87 if (It == FirstSpecialInsts.end())
88 return;
89
90 for (const Instruction &Insn : *BB)
91 if (isSpecialInstruction(&Insn)) {
92 assert(It->second == &Insn &&
93 "Cached first special instruction is wrong!");
94 return;
95 }
96
97 assert(It->second == nullptr &&
98 "Block is marked as having special instructions but in fact it has "
99 "none!");
100 }
101
validateAll() const102 void InstructionPrecedenceTracking::validateAll() const {
103 // Check that for every known block the cached value is correct.
104 for (const auto &It : FirstSpecialInsts)
105 validate(It.first);
106 }
107 #endif
108
insertInstructionTo(const Instruction * Inst,const BasicBlock * BB)109 void InstructionPrecedenceTracking::insertInstructionTo(const Instruction *Inst,
110 const BasicBlock *BB) {
111 if (isSpecialInstruction(Inst))
112 FirstSpecialInsts.erase(BB);
113 }
114
removeInstruction(const Instruction * Inst)115 void InstructionPrecedenceTracking::removeInstruction(const Instruction *Inst) {
116 auto *BB = Inst->getParent();
117 assert(BB && "must be called before instruction is actually removed");
118 if (FirstSpecialInsts.count(BB) && FirstSpecialInsts[BB] == Inst)
119 FirstSpecialInsts.erase(BB);
120 }
121
removeUsersOf(const Instruction * Inst)122 void InstructionPrecedenceTracking::removeUsersOf(const Instruction *Inst) {
123 for (const auto *U : Inst->users()) {
124 if (const auto *UI = dyn_cast<Instruction>(U))
125 removeInstruction(UI);
126 }
127 }
128
clear()129 void InstructionPrecedenceTracking::clear() {
130 FirstSpecialInsts.clear();
131 #ifndef NDEBUG
132 // The map should be valid after clearing (at least empty).
133 validateAll();
134 #endif
135 }
136
isSpecialInstruction(const Instruction * Insn) const137 bool ImplicitControlFlowTracking::isSpecialInstruction(
138 const Instruction *Insn) const {
139 // If a block's instruction doesn't always pass the control to its successor
140 // instruction, mark the block as having implicit control flow. We use them
141 // to avoid wrong assumptions of sort "if A is executed and B post-dominates
142 // A, then B is also executed". This is not true is there is an implicit
143 // control flow instruction (e.g. a guard) between them.
144 return !isGuaranteedToTransferExecutionToSuccessor(Insn);
145 }
146
isSpecialInstruction(const Instruction * Insn) const147 bool MemoryWriteTracking::isSpecialInstruction(
148 const Instruction *Insn) const {
149 using namespace PatternMatch;
150 if (match(Insn, m_Intrinsic<Intrinsic::experimental_widenable_condition>()))
151 return false;
152 return Insn->mayWriteToMemory();
153 }
154