LLVM  10.0.0svn
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(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
91  return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
92 }
93 
94 bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
95  bool AllowIdenticalEdges) {
96  assert(TI->isTerminator() && "Must be a terminator to have successors!");
97  if (TI->getNumSuccessors() == 1) return false;
98 
99  assert(find(predecessors(Dest), TI->getParent()) != pred_end(Dest) &&
100  "No edge between TI's block and Dest.");
101 
102  const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
103 
104  // If there is more than one predecessor, this is a critical edge...
105  assert(I != E && "No preds, but we have an edge to the block?");
106  const BasicBlock *FirstPred = *I;
107  ++I; // Skip one edge due to the incoming arc from TI.
108  if (!AllowIdenticalEdges)
109  return I != E;
110 
111  // If AllowIdenticalEdges is true, then we allow this edge to be considered
112  // non-critical iff all preds come from TI's block.
113  for (; I != E; ++I)
114  if (*I != FirstPred)
115  return true;
116  return false;
117 }
118 
119 // LoopInfo contains a mapping from basic block to the innermost loop. Find
120 // the outermost loop in the loop nest that contains BB.
121 static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
122  const Loop *L = LI->getLoopFor(BB);
123  if (L) {
124  while (const Loop *Parent = L->getParentLoop())
125  L = Parent;
126  }
127  return L;
128 }
129 
131  SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
132  const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
133  const LoopInfo *LI) {
134  // When the stop block is unreachable, it's dominated from everywhere,
135  // regardless of whether there's a path between the two blocks.
136  if (DT && !DT->isReachableFromEntry(StopBB))
137  DT = nullptr;
138 
139  // We can't skip directly from a block that dominates the stop block if the
140  // exclusion block is potentially in between.
141  if (ExclusionSet && !ExclusionSet->empty())
142  DT = nullptr;
143 
144  // Normally any block in a loop is reachable from any other block in a loop,
145  // however excluded blocks might partition the body of a loop to make that
146  // untrue.
147  SmallPtrSet<const Loop *, 8> LoopsWithHoles;
148  if (LI && ExclusionSet) {
149  for (auto BB : *ExclusionSet) {
150  if (const Loop *L = getOutermostLoop(LI, BB))
151  LoopsWithHoles.insert(L);
152  }
153  }
154 
155  const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
156 
157  // Limit the number of blocks we visit. The goal is to avoid run-away compile
158  // times on large CFGs without hampering sensible code. Arbitrarily chosen.
159  unsigned Limit = 32;
161  do {
162  BasicBlock *BB = Worklist.pop_back_val();
163  if (!Visited.insert(BB).second)
164  continue;
165  if (BB == StopBB)
166  return true;
167  if (ExclusionSet && ExclusionSet->count(BB))
168  continue;
169  if (DT && DT->dominates(BB, StopBB))
170  return true;
171 
172  const Loop *Outer = nullptr;
173  if (LI) {
174  Outer = getOutermostLoop(LI, BB);
175  // If we're in a loop with a hole, not all blocks in the loop are
176  // reachable from all other blocks. That implies we can't simply jump to
177  // the loop's exit blocks, as that exit might need to pass through an
178  // excluded block. Clear Outer so we process BB's successors.
179  if (LoopsWithHoles.count(Outer))
180  Outer = nullptr;
181  if (StopLoop && Outer == StopLoop)
182  return true;
183  }
184 
185  if (!--Limit) {
186  // We haven't been able to prove it one way or the other. Conservatively
187  // answer true -- that there is potentially a path.
188  return true;
189  }
190 
191  if (Outer) {
192  // All blocks in a single loop are reachable from all other blocks. From
193  // any of these blocks, we can skip directly to the exits of the loop,
194  // ignoring any other blocks inside the loop body.
195  Outer->getExitBlocks(Worklist);
196  } else {
197  Worklist.append(succ_begin(BB), succ_end(BB));
198  }
199  } while (!Worklist.empty());
200 
201  // We have exhausted all possible paths and are certain that 'To' can not be
202  // reached from 'From'.
203  return false;
204 }
205 
207  const DominatorTree *DT, const LoopInfo *LI) {
208  assert(A->getParent() == B->getParent() &&
209  "This analysis is function-local!");
210 
212  Worklist.push_back(const_cast<BasicBlock*>(A));
213 
214  return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
215  nullptr, DT, LI);
216 }
217 
219  const Instruction *A, const Instruction *B,
220  const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
221  const LoopInfo *LI) {
222  assert(A->getParent()->getParent() == B->getParent()->getParent() &&
223  "This analysis is function-local!");
224 
226 
227  if (A->getParent() == B->getParent()) {
228  // The same block case is special because it's the only time we're looking
229  // within a single block to see which instruction comes first. Once we
230  // start looking at multiple blocks, the first instruction of the block is
231  // reachable, so we only need to determine reachability between whole
232  // blocks.
233  BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
234 
235  // If the block is in a loop then we can reach any instruction in the block
236  // from any other instruction in the block by going around a backedge.
237  if (LI && LI->getLoopFor(BB) != nullptr)
238  return true;
239 
240  // Linear scan, start at 'A', see whether we hit 'B' or the end first.
241  for (BasicBlock::const_iterator I = A->getIterator(), E = BB->end(); I != E;
242  ++I) {
243  if (&*I == B)
244  return true;
245  }
246 
247  // Can't be in a loop if it's the entry block -- the entry block may not
248  // have predecessors.
249  if (BB == &BB->getParent()->getEntryBlock())
250  return false;
251 
252  // Otherwise, continue doing the normal per-BB CFG walk.
253  Worklist.append(succ_begin(BB), succ_end(BB));
254 
255  if (Worklist.empty()) {
256  // We've proven that there's no path!
257  return false;
258  }
259  } else {
260  Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
261  }
262 
263  if (DT) {
264  if (DT->isReachableFromEntry(A->getParent()) &&
265  !DT->isReachableFromEntry(B->getParent()))
266  return false;
267  if (!ExclusionSet || ExclusionSet->empty()) {
268  if (A->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
270  return true;
271  if (B->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
273  return false;
274  }
275  }
276 
278  Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
279 }
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:218
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:928
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:121
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
auto find(R &&Range, const T &Val) -> decltype(adl_begin(Range))
Provide wrappers to std::find which take ranges instead of having to pass begin/end explicitly...
Definition: STLExtras.h:1198
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
pred_range predecessors(BasicBlock *BB)
Definition: CFG.h:124
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:106
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:509
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