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//===- OrderedBasicBlock.cpp --------------------------------- -*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements the OrderedBasicBlock class. OrderedBasicBlock
// maintains an interface where clients can query if one instruction comes
// before another in a BasicBlock. Since BasicBlock currently lacks a reliable
// way to query relative position between instructions one can use
// OrderedBasicBlock to do such queries. OrderedBasicBlock is lazily built on a
// source BasicBlock and maintains an internal Instruction -> Position map. A
// OrderedBasicBlock instance should be discarded whenever the source
// BasicBlock changes.
// It's currently used by the CaptureTracker in order to find relative
// positions of a pair of instructions inside a BasicBlock.
#include "llvm/Analysis/OrderedBasicBlock.h"
#include "llvm/IR/Instruction.h"
using namespace llvm;
OrderedBasicBlock::OrderedBasicBlock(const BasicBlock *BasicB)
: NextInstPos(0), BB(BasicB) {
LastInstFound = BB->end();
/// Given no cached results, find if \p A comes before \p B in \p BB.
/// Cache and number out instruction while walking \p BB.
bool OrderedBasicBlock::comesBefore(const Instruction *A,
const Instruction *B) {
const Instruction *Inst = nullptr;
assert(!(LastInstFound == BB->end() && NextInstPos != 0) &&
"Instruction supposed to be in NumberedInsts");
// Start the search with the instruction found in the last lookup round.
auto II = BB->begin();
auto IE = BB->end();
if (LastInstFound != IE)
II = std::next(LastInstFound);
// Number all instructions up to the point where we find 'A' or 'B'.
for (; II != IE; ++II) {
Inst = cast<Instruction>(II);
NumberedInsts[Inst] = NextInstPos++;
if (Inst == A || Inst == B)
assert(II != IE && "Instruction not found?");
assert((Inst == A || Inst == B) && "Should find A or B");
LastInstFound = II;
return Inst != B;
/// Find out whether \p A dominates \p B, meaning whether \p A
/// comes before \p B in \p BB. This is a simplification that considers
/// cached instruction positions and ignores other basic blocks, being
/// only relevant to compare relative instructions positions inside \p BB.
bool OrderedBasicBlock::dominates(const Instruction *A, const Instruction *B) {
assert(A->getParent() == B->getParent() &&
"Instructions must be in the same basic block!");
// First we lookup the instructions. If they don't exist, lookup will give us
// back ::end(). If they both exist, we compare the numbers. Otherwise, if NA
// exists and NB doesn't, it means NA must come before NB because we would
// have numbered NB as well if it didn't. The same is true for NB. If it
// exists, but NA does not, NA must come after it. If neither exist, we need
// to number the block and cache the results (by calling comesBefore).
auto NAI = NumberedInsts.find(A);
auto NBI = NumberedInsts.find(B);
if (NAI != NumberedInsts.end() && NBI != NumberedInsts.end())
return NAI->second < NBI->second;
if (NAI != NumberedInsts.end())
return true;
if (NBI != NumberedInsts.end())
return false;
return comesBefore(A, B);