LLVM  9.0.0svn
CFG.cpp
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1 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
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 //
9 // This family of functions performs analyses on basic blocks, and instructions
10 // contained within basic blocks.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Analysis/CFG.h"
15 #include "llvm/ADT/SmallPtrSet.h"
16 #include "llvm/ADT/SmallSet.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/IR/Dominators.h"
19 
20 using namespace llvm;
21 
22 /// FindFunctionBackedges - Analyze the specified function to find all of the
23 /// loop backedges in the function and return them. This is a relatively cheap
24 /// (compared to computing dominators and loop info) analysis.
25 ///
26 /// The output is added to Result, as pairs of <from,to> edge info.
28  SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
29  const BasicBlock *BB = &F.getEntryBlock();
30  if (succ_empty(BB))
31  return;
32 
36 
37  Visited.insert(BB);
38  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
39  InStack.insert(BB);
40  do {
41  std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
42  const BasicBlock *ParentBB = Top.first;
43  succ_const_iterator &I = Top.second;
44 
45  bool FoundNew = false;
46  while (I != succ_end(ParentBB)) {
47  BB = *I++;
48  if (Visited.insert(BB).second) {
49  FoundNew = true;
50  break;
51  }
52  // Successor is in VisitStack, it's a back edge.
53  if (InStack.count(BB))
54  Result.push_back(std::make_pair(ParentBB, BB));
55  }
56 
57  if (FoundNew) {
58  // Go down one level if there is a unvisited successor.
59  InStack.insert(BB);
60  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
61  } else {
62  // Go up one level.
63  InStack.erase(VisitStack.pop_back_val().first);
64  }
65  } while (!VisitStack.empty());
66 }
67 
68 /// GetSuccessorNumber - Search for the specified successor of basic block BB
69 /// and return its position in the terminator instruction's list of
70 /// successors. It is an error to call this with a block that is not a
71 /// successor.
73  const BasicBlock *Succ) {
74  const Instruction *Term = BB->getTerminator();
75 #ifndef NDEBUG
76  unsigned e = Term->getNumSuccessors();
77 #endif
78  for (unsigned i = 0; ; ++i) {
79  assert(i != e && "Didn't find edge?");
80  if (Term->getSuccessor(i) == Succ)
81  return i;
82  }
83 }
84 
85 /// isCriticalEdge - Return true if the specified edge is a critical edge.
86 /// Critical edges are edges from a block with multiple successors to a block
87 /// with multiple predecessors.
88 bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
89  bool AllowIdenticalEdges) {
90  assert(TI->isTerminator() && "Must be a terminator to have successors!");
91  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
92  if (TI->getNumSuccessors() == 1) return false;
93 
94  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
95  const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
96 
97  // If there is more than one predecessor, this is a critical edge...
98  assert(I != E && "No preds, but we have an edge to the block?");
99  const BasicBlock *FirstPred = *I;
100  ++I; // Skip one edge due to the incoming arc from TI.
101  if (!AllowIdenticalEdges)
102  return I != E;
103 
104  // If AllowIdenticalEdges is true, then we allow this edge to be considered
105  // non-critical iff all preds come from TI's block.
106  for (; I != E; ++I)
107  if (*I != FirstPred)
108  return true;
109  return false;
110 }
111 
112 // LoopInfo contains a mapping from basic block to the innermost loop. Find
113 // the outermost loop in the loop nest that contains BB.
114 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
115  const Loop *L = LI->getLoopFor(BB);
116  if (L) {
117  while (const Loop *Parent = L->getParentLoop())
118  L = Parent;
119  }
120  return L;
121 }
122 
124  SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
125  const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
126  const LoopInfo *LI) {
127  // When the stop block is unreachable, it's dominated from everywhere,
128  // regardless of whether there's a path between the two blocks.
129  if (DT && !DT->isReachableFromEntry(StopBB))
130  DT = nullptr;
131 
132  // We can't skip directly from a block that dominates the stop block if the
133  // exclusion block is potentially in between.
134  if (ExclusionSet && !ExclusionSet->empty())
135  DT = nullptr;
136 
137  // Normally any block in a loop is reachable from any other block in a loop,
138  // however excluded blocks might partition the body of a loop to make that
139  // untrue.
140  SmallPtrSet<const Loop *, 8> LoopsWithHoles;
141  if (LI && ExclusionSet) {
142  for (auto BB : *ExclusionSet) {
143  if (const Loop *L = getOutermostLoop(LI, BB))
144  LoopsWithHoles.insert(L);
145  }
146  }
147 
148  const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
149 
150  // Limit the number of blocks we visit. The goal is to avoid run-away compile
151  // times on large CFGs without hampering sensible code. Arbitrarily chosen.
152  unsigned Limit = 32;
154  do {
155  BasicBlock *BB = Worklist.pop_back_val();
156  if (!Visited.insert(BB).second)
157  continue;
158  if (BB == StopBB)
159  return true;
160  if (ExclusionSet && ExclusionSet->count(BB))
161  continue;
162  if (DT && DT->dominates(BB, StopBB))
163  return true;
164 
165  const Loop *Outer = nullptr;
166  if (LI) {
167  Outer = getOutermostLoop(LI, BB);
168  // If we're in a loop with a hole, not all blocks in the loop are
169  // reachable from all other blocks. That implies we can't simply jump to
170  // the loop's exit blocks, as that exit might need to pass through an
171  // excluded block. Clear Outer so we process BB's successors.
172  if (LoopsWithHoles.count(Outer))
173  Outer = nullptr;
174  if (StopLoop && Outer == StopLoop)
175  return true;
176  }
177 
178  if (!--Limit) {
179  // We haven't been able to prove it one way or the other. Conservatively
180  // answer true -- that there is potentially a path.
181  return true;
182  }
183 
184  if (Outer) {
185  // All blocks in a single loop are reachable from all other blocks. From
186  // any of these blocks, we can skip directly to the exits of the loop,
187  // ignoring any other blocks inside the loop body.
188  Outer->getExitBlocks(Worklist);
189  } else {
190  Worklist.append(succ_begin(BB), succ_end(BB));
191  }
192  } while (!Worklist.empty());
193 
194  // We have exhausted all possible paths and are certain that 'To' can not be
195  // reached from 'From'.
196  return false;
197 }
198 
200  const DominatorTree *DT, const LoopInfo *LI) {
201  assert(A->getParent() == B->getParent() &&
202  "This analysis is function-local!");
203 
205  Worklist.push_back(const_cast<BasicBlock*>(A));
206 
207  return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
208  nullptr, DT, LI);
209 }
210 
212  const Instruction *A, const Instruction *B,
213  const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
214  const LoopInfo *LI) {
215  assert(A->getParent()->getParent() == B->getParent()->getParent() &&
216  "This analysis is function-local!");
217 
219 
220  if (A->getParent() == B->getParent()) {
221  // The same block case is special because it's the only time we're looking
222  // within a single block to see which instruction comes first. Once we
223  // start looking at multiple blocks, the first instruction of the block is
224  // reachable, so we only need to determine reachability between whole
225  // blocks.
226  BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
227 
228  // If the block is in a loop then we can reach any instruction in the block
229  // from any other instruction in the block by going around a backedge.
230  if (LI && LI->getLoopFor(BB) != nullptr)
231  return true;
232 
233  // Linear scan, start at 'A', see whether we hit 'B' or the end first.
234  for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
235  ++I) {
236  if (&*I == B)
237  return true;
238  }
239 
240  // Can't be in a loop if it's the entry block -- the entry block may not
241  // have predecessors.
242  if (BB == &BB->getParent()->getEntryBlock())
243  return false;
244 
245  // Otherwise, continue doing the normal per-BB CFG walk.
246  Worklist.append(succ_begin(BB), succ_end(BB));
247 
248  if (Worklist.empty()) {
249  // We've proven that there's no path!
250  return false;
251  }
252  } else {
253  Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
254  }
255 
256  if (DT) {
257  if (DT->isReachableFromEntry(A->getParent()) &&
258  !DT->isReachableFromEntry(B->getParent()))
259  return false;
260  if (!ExclusionSet || ExclusionSet->empty()) {
261  if (A->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
263  return true;
264  if (B->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
266  return false;
267  }
268  }
269 
271  Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
272 }
This class represents lattice values for constants.
Definition: AllocatorList.h:23
BasicBlock * getSuccessor(unsigned Idx) const
Return the specified successor. This instruction must be a terminator.
bool isPotentiallyReachable(const Instruction *From, const Instruction *To, const SmallPtrSetImpl< BasicBlock *> *ExclusionSet=nullptr, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether instruction &#39;To&#39; is reachable from &#39;From&#39;, without passing through any blocks in Ex...
Definition: CFG.cpp:211
bool isTerminator() const
Definition: Instruction.h:128
F(f)
const Instruction * getTerminator() const LLVM_READONLY
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.cpp:137
bool isReachableFromEntry(const Use &U) const
Provide an overload for a Use.
Definition: Dominators.cpp:299
A templated base class for SmallPtrSet which provides the typesafe interface that is common across al...
Definition: SmallPtrSet.h:343
LoopT * getLoopFor(const BlockT *BB) const
Return the inner most loop that BB lives in.
Definition: LoopInfo.h:897
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APFloat.h:41
Interval::succ_iterator succ_begin(Interval *I)
succ_begin/succ_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:102
void getExitBlocks(SmallVectorImpl< BlockT *> &ExitBlocks) const
Return all of the successor blocks of this loop.
Definition: LoopInfoImpl.h:62
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:144
unsigned getNumSuccessors() const
Return the number of successors that this instruction has.
Interval::succ_iterator succ_end(Interval *I)
Definition: Interval.h:105
const BasicBlock & getEntryBlock() const
Definition: Function.h:664
static const Loop * getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB)
Definition: CFG.cpp:114
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
LLVM Basic Block Representation.
Definition: BasicBlock.h:57
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
LLVM_NODISCARD bool empty() const
Definition: SmallPtrSet.h:91
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:370
Interval::pred_iterator pred_begin(Interval *I)
pred_begin/pred_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:112
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:115
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:381
self_iterator getIterator()
Definition: ilist_node.h:81
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:88
bool succ_empty(const Instruction *I)
Definition: CFG.h:253
Iterator for intrusive lists based on ilist_node.
bool erase(PtrType Ptr)
erase - If the set contains the specified pointer, remove it and return true, otherwise return false...
Definition: SmallPtrSet.h:377
iterator end()
Definition: BasicBlock.h:270
This is a &#39;vector&#39; (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:837
bool dominates(const Instruction *Def, const Use &U) const
Return true if Def dominates a use in User.
Definition: Dominators.cpp:248
LLVM_NODISCARD T pop_back_val()
Definition: SmallVector.h:374
void append(in_iter in_start, in_iter in_end)
Add the specified range to the end of the SmallVector.
Definition: SmallVector.h:387
LoopT * getParentLoop() const
Definition: LoopInfo.h:103
unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ)
Search for the specified successor of basic block BB and return its position in the terminator instru...
Definition: CFG.cpp:72
LLVM_NODISCARD bool empty() const
Definition: SmallVector.h:55
Represents a single loop in the control flow graph.
Definition: LoopInfo.h:506
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:106
#define I(x, y, z)
Definition: MD5.cpp:58
void FindFunctionBackedges(const Function &F, SmallVectorImpl< std::pair< const BasicBlock *, const BasicBlock *> > &Result)
Analyze the specified function to find all of the loop backedges in the function and return them...
Definition: CFG.cpp:27
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
bool isPotentiallyReachableFromMany(SmallVectorImpl< BasicBlock *> &Worklist, BasicBlock *StopBB, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether there is at least one path from a block in &#39;Worklist&#39; to &#39;StopBB&#39;, returning true if uncertain.
const BasicBlock * getParent() const
Definition: Instruction.h:66