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//===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This family of functions performs analyses on basic blocks, and instructions
// contained within basic blocks.
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;
// The max number of basic blocks explored during reachability analysis between
// two basic blocks. This is kept reasonably small to limit compile time when
// repeatedly used by clients of this analysis (such as captureTracking).
static cl::opt<unsigned> DefaultMaxBBsToExplore(
"dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
cl::desc("Max number of BBs to explore for reachability analysis"),
/// FindFunctionBackedges - Analyze the specified function to find all of the
/// loop backedges in the function and return them. This is a relatively cheap
/// (compared to computing dominators and loop info) analysis.
/// The output is added to Result, as pairs of <from,to> edge info.
void llvm::FindFunctionBackedges(const Function &F,
SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
const BasicBlock *BB = &F.getEntryBlock();
if (succ_empty(BB))
SmallPtrSet<const BasicBlock*, 8> Visited;
SmallVector<std::pair<const BasicBlock *, const_succ_iterator>, 8> VisitStack;
SmallPtrSet<const BasicBlock*, 8> InStack;
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
do {
std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
const BasicBlock *ParentBB = Top.first;
const_succ_iterator &I = Top.second;
bool FoundNew = false;
while (I != succ_end(ParentBB)) {
BB = *I++;
if (Visited.insert(BB).second) {
FoundNew = true;
// Successor is in VisitStack, it's a back edge.
if (InStack.count(BB))
Result.push_back(std::make_pair(ParentBB, BB));
if (FoundNew) {
// Go down one level if there is a unvisited successor.
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
} else {
// Go up one level.
} while (!VisitStack.empty());
/// GetSuccessorNumber - Search for the specified successor of basic block BB
/// and return its position in the terminator instruction's list of
/// successors. It is an error to call this with a block that is not a
/// successor.
unsigned llvm::GetSuccessorNumber(const BasicBlock *BB,
const BasicBlock *Succ) {
const Instruction *Term = BB->getTerminator();
#ifndef NDEBUG
unsigned e = Term->getNumSuccessors();
for (unsigned i = 0; ; ++i) {
assert(i != e && "Didn't find edge?");
if (Term->getSuccessor(i) == Succ)
return i;
/// isCriticalEdge - Return true if the specified edge is a critical edge.
/// Critical edges are edges from a block with multiple successors to a block
/// with multiple predecessors.
bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
bool AllowIdenticalEdges) {
assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
bool AllowIdenticalEdges) {
assert(TI->isTerminator() && "Must be a terminator to have successors!");
if (TI->getNumSuccessors() == 1) return false;
assert(is_contained(predecessors(Dest), TI->getParent()) &&
"No edge between TI's block and Dest.");
const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
// If there is more than one predecessor, this is a critical edge...
assert(I != E && "No preds, but we have an edge to the block?");
const BasicBlock *FirstPred = *I;
++I; // Skip one edge due to the incoming arc from TI.
if (!AllowIdenticalEdges)
return I != E;
// If AllowIdenticalEdges is true, then we allow this edge to be considered
// non-critical iff all preds come from TI's block.
for (; I != E; ++I)
if (*I != FirstPred)
return true;
return false;
// LoopInfo contains a mapping from basic block to the innermost loop. Find
// the outermost loop in the loop nest that contains BB.
static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
const Loop *L = LI->getLoopFor(BB);
if (L) {
while (const Loop *Parent = L->getParentLoop())
L = Parent;
return L;
bool llvm::isPotentiallyReachableFromMany(
SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
const LoopInfo *LI) {
// When the stop block is unreachable, it's dominated from everywhere,
// regardless of whether there's a path between the two blocks.
if (DT && !DT->isReachableFromEntry(StopBB))
DT = nullptr;
// We can't skip directly from a block that dominates the stop block if the
// exclusion block is potentially in between.
if (ExclusionSet && !ExclusionSet->empty())
DT = nullptr;
// Normally any block in a loop is reachable from any other block in a loop,
// however excluded blocks might partition the body of a loop to make that
// untrue.
SmallPtrSet<const Loop *, 8> LoopsWithHoles;
if (LI && ExclusionSet) {
for (auto BB : *ExclusionSet) {
if (const Loop *L = getOutermostLoop(LI, BB))
const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
unsigned Limit = DefaultMaxBBsToExplore;
SmallPtrSet<const BasicBlock*, 32> Visited;
do {
BasicBlock *BB = Worklist.pop_back_val();
if (!Visited.insert(BB).second)
if (BB == StopBB)
return true;
if (ExclusionSet && ExclusionSet->count(BB))
if (DT && DT->dominates(BB, StopBB))
return true;
const Loop *Outer = nullptr;
if (LI) {
Outer = getOutermostLoop(LI, BB);
// If we're in a loop with a hole, not all blocks in the loop are
// reachable from all other blocks. That implies we can't simply jump to
// the loop's exit blocks, as that exit might need to pass through an
// excluded block. Clear Outer so we process BB's successors.
if (LoopsWithHoles.count(Outer))
Outer = nullptr;
if (StopLoop && Outer == StopLoop)
return true;
if (!--Limit) {
// We haven't been able to prove it one way or the other. Conservatively
// answer true -- that there is potentially a path.
return true;
if (Outer) {
// All blocks in a single loop are reachable from all other blocks. From
// any of these blocks, we can skip directly to the exits of the loop,
// ignoring any other blocks inside the loop body.
} else {
Worklist.append(succ_begin(BB), succ_end(BB));
} while (!Worklist.empty());
// We have exhausted all possible paths and are certain that 'To' can not be
// reached from 'From'.
return false;
bool llvm::isPotentiallyReachable(
const BasicBlock *A, const BasicBlock *B,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
const LoopInfo *LI) {
assert(A->getParent() == B->getParent() &&
"This analysis is function-local!");
if (DT) {
if (DT->isReachableFromEntry(A) && !DT->isReachableFromEntry(B))
return false;
if (!ExclusionSet || ExclusionSet->empty()) {
if (A->isEntryBlock() && DT->isReachableFromEntry(B))
return true;
if (B->isEntryBlock() && DT->isReachableFromEntry(A))
return false;
SmallVector<BasicBlock*, 32> Worklist;
return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
ExclusionSet, DT, LI);
bool llvm::isPotentiallyReachable(
const Instruction *A, const Instruction *B,
const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
const LoopInfo *LI) {
assert(A->getParent()->getParent() == B->getParent()->getParent() &&
"This analysis is function-local!");
if (A->getParent() == B->getParent()) {
// The same block case is special because it's the only time we're looking
// within a single block to see which instruction comes first. Once we
// start looking at multiple blocks, the first instruction of the block is
// reachable, so we only need to determine reachability between whole
// blocks.
BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
// If the block is in a loop then we can reach any instruction in the block
// from any other instruction in the block by going around a backedge.
if (LI && LI->getLoopFor(BB) != nullptr)
return true;
// If A comes before B, then B is definitively reachable from A.
if (A == B || A->comesBefore(B))
return true;
// Can't be in a loop if it's the entry block -- the entry block may not
// have predecessors.
if (BB->isEntryBlock())
return false;
// Otherwise, continue doing the normal per-BB CFG walk.
SmallVector<BasicBlock*, 32> Worklist;
Worklist.append(succ_begin(BB), succ_end(BB));
if (Worklist.empty()) {
// We've proven that there's no path!
return false;
return isPotentiallyReachableFromMany(
Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet,
DT, LI);
return isPotentiallyReachable(
A->getParent(), B->getParent(), ExclusionSet, DT, LI);