1 //===- OrderedBasicBlock.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 //
10 // This file implements the OrderedBasicBlock class. OrderedBasicBlock
11 // maintains an interface where clients can query if one instruction comes
12 // before another in a BasicBlock. Since BasicBlock currently lacks a reliable
13 // way to query relative position between instructions one can use
14 // OrderedBasicBlock to do such queries. OrderedBasicBlock is lazily built on a
15 // source BasicBlock and maintains an internal Instruction -> Position map. A
16 // OrderedBasicBlock instance should be discarded whenever the source
17 // BasicBlock changes.
18 //
19 // It's currently used by the CaptureTracker in order to find relative
20 // positions of a pair of instructions inside a BasicBlock.
21 //
22 //===----------------------------------------------------------------------===//
23
24 #include "llvm/Analysis/OrderedBasicBlock.h"
25 #include "llvm/IR/Instruction.h"
26 using namespace llvm;
27
OrderedBasicBlock(const BasicBlock * BasicB)28 OrderedBasicBlock::OrderedBasicBlock(const BasicBlock *BasicB)
29 : NextInstPos(0), BB(BasicB) {
30 LastInstFound = BB->end();
31 }
32
33 /// Given no cached results, find if \p A comes before \p B in \p BB.
34 /// Cache and number out instruction while walking \p BB.
comesBefore(const Instruction * A,const Instruction * B)35 bool OrderedBasicBlock::comesBefore(const Instruction *A,
36 const Instruction *B) {
37 const Instruction *Inst = nullptr;
38 assert(!(LastInstFound == BB->end() && NextInstPos != 0) &&
39 "Instruction supposed to be in NumberedInsts");
40 assert(A->getParent() == BB && "Instruction supposed to be in the block!");
41 assert(B->getParent() == BB && "Instruction supposed to be in the block!");
42
43 // Start the search with the instruction found in the last lookup round.
44 auto II = BB->begin();
45 auto IE = BB->end();
46 if (LastInstFound != IE)
47 II = std::next(LastInstFound);
48
49 // Number all instructions up to the point where we find 'A' or 'B'.
50 for (; II != IE; ++II) {
51 Inst = cast<Instruction>(II);
52 NumberedInsts[Inst] = NextInstPos++;
53 if (Inst == A || Inst == B)
54 break;
55 }
56
57 assert(II != IE && "Instruction not found?");
58 assert((Inst == A || Inst == B) && "Should find A or B");
59 LastInstFound = II;
60 return Inst != B;
61 }
62
63 /// Find out whether \p A dominates \p B, meaning whether \p A
64 /// comes before \p B in \p BB. This is a simplification that considers
65 /// cached instruction positions and ignores other basic blocks, being
66 /// only relevant to compare relative instructions positions inside \p BB.
dominates(const Instruction * A,const Instruction * B)67 bool OrderedBasicBlock::dominates(const Instruction *A, const Instruction *B) {
68 assert(A->getParent() == B->getParent() &&
69 "Instructions must be in the same basic block!");
70 assert(A->getParent() == BB && "Instructions must be in the tracked block!");
71
72 // First we lookup the instructions. If they don't exist, lookup will give us
73 // back ::end(). If they both exist, we compare the numbers. Otherwise, if NA
74 // exists and NB doesn't, it means NA must come before NB because we would
75 // have numbered NB as well if it didn't. The same is true for NB. If it
76 // exists, but NA does not, NA must come after it. If neither exist, we need
77 // to number the block and cache the results (by calling comesBefore).
78 auto NAI = NumberedInsts.find(A);
79 auto NBI = NumberedInsts.find(B);
80 if (NAI != NumberedInsts.end() && NBI != NumberedInsts.end())
81 return NAI->second < NBI->second;
82 if (NAI != NumberedInsts.end())
83 return true;
84 if (NBI != NumberedInsts.end())
85 return false;
86
87 return comesBefore(A, B);
88 }
89