LLVM 19.0.0git
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"
16#include "llvm/IR/Dominators.h"
18
19using namespace llvm;
20
21// The max number of basic blocks explored during reachability analysis between
22// two basic blocks. This is kept reasonably small to limit compile time when
23// repeatedly used by clients of this analysis (such as captureTracking).
25 "dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
26 cl::desc("Max number of BBs to explore for reachability analysis"),
27 cl::init(32));
28
29/// FindFunctionBackedges - Analyze the specified function to find all of the
30/// loop backedges in the function and return them. This is a relatively cheap
31/// (compared to computing dominators and loop info) analysis.
32///
33/// The output is added to Result, as pairs of <from,to> edge info.
35 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
36 const BasicBlock *BB = &F.getEntryBlock();
37 if (succ_empty(BB))
38 return;
39
43
44 Visited.insert(BB);
45 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
46 InStack.insert(BB);
47 do {
48 std::pair<const BasicBlock *, const_succ_iterator> &Top = VisitStack.back();
49 const BasicBlock *ParentBB = Top.first;
50 const_succ_iterator &I = Top.second;
51
52 bool FoundNew = false;
53 while (I != succ_end(ParentBB)) {
54 BB = *I++;
55 if (Visited.insert(BB).second) {
56 FoundNew = true;
57 break;
58 }
59 // Successor is in VisitStack, it's a back edge.
60 if (InStack.count(BB))
61 Result.push_back(std::make_pair(ParentBB, BB));
62 }
63
64 if (FoundNew) {
65 // Go down one level if there is a unvisited successor.
66 InStack.insert(BB);
67 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
68 } else {
69 // Go up one level.
70 InStack.erase(VisitStack.pop_back_val().first);
71 }
72 } while (!VisitStack.empty());
73}
74
75/// GetSuccessorNumber - Search for the specified successor of basic block BB
76/// and return its position in the terminator instruction's list of
77/// successors. It is an error to call this with a block that is not a
78/// successor.
80 const BasicBlock *Succ) {
81 const Instruction *Term = BB->getTerminator();
82#ifndef NDEBUG
83 unsigned e = Term->getNumSuccessors();
84#endif
85 for (unsigned i = 0; ; ++i) {
86 assert(i != e && "Didn't find edge?");
87 if (Term->getSuccessor(i) == Succ)
88 return i;
89 }
90}
91
92/// isCriticalEdge - Return true if the specified edge is a critical edge.
93/// Critical edges are edges from a block with multiple successors to a block
94/// with multiple predecessors.
95bool llvm::isCriticalEdge(const Instruction *TI, unsigned SuccNum,
96 bool AllowIdenticalEdges) {
97 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
98 return isCriticalEdge(TI, TI->getSuccessor(SuccNum), AllowIdenticalEdges);
99}
100
101bool llvm::isCriticalEdge(const Instruction *TI, const BasicBlock *Dest,
102 bool AllowIdenticalEdges) {
103 assert(TI->isTerminator() && "Must be a terminator to have successors!");
104 if (TI->getNumSuccessors() == 1) return false;
105
107 "No edge between TI's block and Dest.");
108
109 const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
110
111 // If there is more than one predecessor, this is a critical edge...
112 assert(I != E && "No preds, but we have an edge to the block?");
113 const BasicBlock *FirstPred = *I;
114 ++I; // Skip one edge due to the incoming arc from TI.
115 if (!AllowIdenticalEdges)
116 return I != E;
117
118 // If AllowIdenticalEdges is true, then we allow this edge to be considered
119 // non-critical iff all preds come from TI's block.
120 for (; I != E; ++I)
121 if (*I != FirstPred)
122 return true;
123 return false;
124}
125
126// LoopInfo contains a mapping from basic block to the innermost loop. Find
127// the outermost loop in the loop nest that contains BB.
128static const Loop *getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB) {
129 const Loop *L = LI->getLoopFor(BB);
130 return L ? L->getOutermostLoop() : nullptr;
131}
132
134 SmallVectorImpl<BasicBlock *> &Worklist, const BasicBlock *StopBB,
135 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
136 const LoopInfo *LI) {
137 // When the stop block is unreachable, it's dominated from everywhere,
138 // regardless of whether there's a path between the two blocks.
139 if (DT && !DT->isReachableFromEntry(StopBB))
140 DT = nullptr;
141
142 // We can't skip directly from a block that dominates the stop block if the
143 // exclusion block is potentially in between.
144 if (ExclusionSet && !ExclusionSet->empty())
145 DT = nullptr;
146
147 // Normally any block in a loop is reachable from any other block in a loop,
148 // however excluded blocks might partition the body of a loop to make that
149 // untrue.
150 SmallPtrSet<const Loop *, 8> LoopsWithHoles;
151 if (LI && ExclusionSet) {
152 for (auto *BB : *ExclusionSet) {
153 if (const Loop *L = getOutermostLoop(LI, BB))
154 LoopsWithHoles.insert(L);
155 }
156 }
157
158 const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
159
160 unsigned Limit = DefaultMaxBBsToExplore;
162 do {
163 BasicBlock *BB = Worklist.pop_back_val();
164 if (!Visited.insert(BB).second)
165 continue;
166 if (BB == StopBB)
167 return true;
168 if (ExclusionSet && ExclusionSet->count(BB))
169 continue;
170 if (DT && DT->dominates(BB, StopBB))
171 return true;
172
173 const Loop *Outer = nullptr;
174 if (LI) {
175 Outer = getOutermostLoop(LI, BB);
176 // If we're in a loop with a hole, not all blocks in the loop are
177 // reachable from all other blocks. That implies we can't simply jump to
178 // the loop's exit blocks, as that exit might need to pass through an
179 // excluded block. Clear Outer so we process BB's successors.
180 if (LoopsWithHoles.count(Outer))
181 Outer = nullptr;
182 if (StopLoop && Outer == StopLoop)
183 return true;
184 }
185
186 if (!--Limit) {
187 // We haven't been able to prove it one way or the other. Conservatively
188 // answer true -- that there is potentially a path.
189 return true;
190 }
191
192 if (Outer) {
193 // All blocks in a single loop are reachable from all other blocks. From
194 // any of these blocks, we can skip directly to the exits of the loop,
195 // ignoring any other blocks inside the loop body.
196 Outer->getExitBlocks(Worklist);
197 } else {
198 Worklist.append(succ_begin(BB), succ_end(BB));
199 }
200 } while (!Worklist.empty());
201
202 // We have exhausted all possible paths and are certain that 'To' can not be
203 // reached from 'From'.
204 return false;
205}
206
208 const BasicBlock *A, const BasicBlock *B,
209 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
210 const LoopInfo *LI) {
211 assert(A->getParent() == B->getParent() &&
212 "This analysis is function-local!");
213
214 if (DT) {
216 return false;
217 if (!ExclusionSet || ExclusionSet->empty()) {
218 if (A->isEntryBlock() && DT->isReachableFromEntry(B))
219 return true;
220 if (B->isEntryBlock() && DT->isReachableFromEntry(A))
221 return false;
222 }
223 }
224
226 Worklist.push_back(const_cast<BasicBlock*>(A));
227
228 return isPotentiallyReachableFromMany(Worklist, B, ExclusionSet, DT, LI);
229}
230
232 const Instruction *A, const Instruction *B,
233 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet, const DominatorTree *DT,
234 const LoopInfo *LI) {
235 assert(A->getParent()->getParent() == B->getParent()->getParent() &&
236 "This analysis is function-local!");
237
238 if (A->getParent() == B->getParent()) {
239 // The same block case is special because it's the only time we're looking
240 // within a single block to see which instruction comes first. Once we
241 // start looking at multiple blocks, the first instruction of the block is
242 // reachable, so we only need to determine reachability between whole
243 // blocks.
244 BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
245
246 // If the block is in a loop then we can reach any instruction in the block
247 // from any other instruction in the block by going around a backedge.
248 if (LI && LI->getLoopFor(BB) != nullptr)
249 return true;
250
251 // If A comes before B, then B is definitively reachable from A.
252 if (A == B || A->comesBefore(B))
253 return true;
254
255 // Can't be in a loop if it's the entry block -- the entry block may not
256 // have predecessors.
257 if (BB->isEntryBlock())
258 return false;
259
260 // Otherwise, continue doing the normal per-BB CFG walk.
262 Worklist.append(succ_begin(BB), succ_end(BB));
263 if (Worklist.empty()) {
264 // We've proven that there's no path!
265 return false;
266 }
267
268 return isPotentiallyReachableFromMany(Worklist, B->getParent(),
269 ExclusionSet, DT, LI);
270 }
271
273 A->getParent(), B->getParent(), ExclusionSet, DT, LI);
274}
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
static GCRegistry::Add< ErlangGC > A("erlang", "erlang-compatible garbage collector")
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
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"), cl::init(32))
static const Loop * getOutermostLoop(const LoopInfo *LI, const BasicBlock *BB)
Definition: CFG.cpp:128
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
bool isEntryBlock() const
Return true if this is the entry block of the containing function.
Definition: BasicBlock.cpp:607
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.h:229
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree.
Definition: Dominators.h:162
bool isReachableFromEntry(const Use &U) const
Provide an overload for a Use.
Definition: Dominators.cpp:321
bool dominates(const BasicBlock *BB, const Use &U) const
Return true if the (end of the) basic block BB dominates the use U.
Definition: Dominators.cpp:122
unsigned getNumSuccessors() const LLVM_READONLY
Return the number of successors that this instruction has.
const BasicBlock * getParent() const
Definition: Instruction.h:150
BasicBlock * getSuccessor(unsigned Idx) const LLVM_READONLY
Return the specified successor. This instruction must be a terminator.
bool isTerminator() const
Definition: Instruction.h:253
LoopT * getLoopFor(const BlockT *BB) const
Return the inner most loop that BB lives in.
Represents a single loop in the control flow graph.
Definition: LoopInfo.h:44
A templated base class for SmallPtrSet which provides the typesafe interface that is common across al...
Definition: SmallPtrSet.h:321
bool erase(PtrType Ptr)
erase - If the set contains the specified pointer, remove it and return true, otherwise return false.
Definition: SmallPtrSet.h:356
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:360
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:342
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:427
bool empty() const
Definition: SmallVector.h:94
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: SmallVector.h:586
void append(ItTy in_start, ItTy in_end)
Add the specified range to the end of the SmallVector.
Definition: SmallVector.h:696
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:450
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
Interval::succ_iterator succ_end(Interval *I)
Definition: Interval.h:102
bool succ_empty(const Instruction *I)
Definition: CFG.h:255
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:79
bool isPotentiallyReachableFromMany(SmallVectorImpl< BasicBlock * > &Worklist, const BasicBlock *StopBB, const SmallPtrSetImpl< BasicBlock * > *ExclusionSet, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether there is at least one path from a block in 'Worklist' to 'StopBB' without passing t...
Definition: CFG.cpp:133
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:99
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:112
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:109
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:95
auto predecessors(const MachineBasicBlock *BB)
bool is_contained(R &&Range, const E &Element)
Returns true if Element is found in Range.
Definition: STLExtras.h:1888
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:34
bool isPotentiallyReachable(const Instruction *From, const Instruction *To, const SmallPtrSetImpl< BasicBlock * > *ExclusionSet=nullptr, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether instruction 'To' is reachable from 'From', without passing through any blocks in Ex...
Definition: CFG.cpp:231