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LegacyPassManagers.h
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1 //===- LegacyPassManagers.h - Legacy Pass Infrastructure --------*- C++ -*-===//
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 file declares the LLVM Pass Manager infrastructure.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #ifndef LLVM_IR_LEGACYPASSMANAGERS_H
14 #define LLVM_IR_LEGACYPASSMANAGERS_H
15 
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/FoldingSet.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/Pass.h"
21 #include <vector>
22 
23 //===----------------------------------------------------------------------===//
24 // Overview:
25 // The Pass Manager Infrastructure manages passes. It's responsibilities are:
26 //
27 // o Manage optimization pass execution order
28 // o Make required Analysis information available before pass P is run
29 // o Release memory occupied by dead passes
30 // o If Analysis information is dirtied by a pass then regenerate Analysis
31 // information before it is consumed by another pass.
32 //
33 // Pass Manager Infrastructure uses multiple pass managers. They are
34 // PassManager, FunctionPassManager, MPPassManager, FPPassManager, BBPassManager.
35 // This class hierarchy uses multiple inheritance but pass managers do not
36 // derive from another pass manager.
37 //
38 // PassManager and FunctionPassManager are two top-level pass manager that
39 // represents the external interface of this entire pass manager infrastucture.
40 //
41 // Important classes :
42 //
43 // [o] class PMTopLevelManager;
44 //
45 // Two top level managers, PassManager and FunctionPassManager, derive from
46 // PMTopLevelManager. PMTopLevelManager manages information used by top level
47 // managers such as last user info.
48 //
49 // [o] class PMDataManager;
50 //
51 // PMDataManager manages information, e.g. list of available analysis info,
52 // used by a pass manager to manage execution order of passes. It also provides
53 // a place to implement common pass manager APIs. All pass managers derive from
54 // PMDataManager.
55 //
56 // [o] class BBPassManager : public FunctionPass, public PMDataManager;
57 //
58 // BBPassManager manages BasicBlockPasses.
59 //
60 // [o] class FunctionPassManager;
61 //
62 // This is a external interface used to manage FunctionPasses. This
63 // interface relies on FunctionPassManagerImpl to do all the tasks.
64 //
65 // [o] class FunctionPassManagerImpl : public ModulePass, PMDataManager,
66 // public PMTopLevelManager;
67 //
68 // FunctionPassManagerImpl is a top level manager. It manages FPPassManagers
69 //
70 // [o] class FPPassManager : public ModulePass, public PMDataManager;
71 //
72 // FPPassManager manages FunctionPasses and BBPassManagers
73 //
74 // [o] class MPPassManager : public Pass, public PMDataManager;
75 //
76 // MPPassManager manages ModulePasses and FPPassManagers
77 //
78 // [o] class PassManager;
79 //
80 // This is a external interface used by various tools to manages passes. It
81 // relies on PassManagerImpl to do all the tasks.
82 //
83 // [o] class PassManagerImpl : public Pass, public PMDataManager,
84 // public PMTopLevelManager
85 //
86 // PassManagerImpl is a top level pass manager responsible for managing
87 // MPPassManagers.
88 //===----------------------------------------------------------------------===//
89 
91 
92 namespace llvm {
93 template <typename T> class ArrayRef;
94 class Module;
95 class Pass;
96 class StringRef;
97 class Value;
98 class Timer;
99 class PMDataManager;
100 
101 // enums for debugging strings
103  EXECUTION_MSG, // "Executing Pass '" + PassName
104  MODIFICATION_MSG, // "Made Modification '" + PassName
105  FREEING_MSG, // " Freeing Pass '" + PassName
106  ON_BASICBLOCK_MSG, // "' on BasicBlock '" + InstructionName + "'...\n"
107  ON_FUNCTION_MSG, // "' on Function '" + FunctionName + "'...\n"
108  ON_MODULE_MSG, // "' on Module '" + ModuleName + "'...\n"
109  ON_REGION_MSG, // "' on Region '" + Msg + "'...\n'"
110  ON_LOOP_MSG, // "' on Loop '" + Msg + "'...\n'"
111  ON_CG_MSG // "' on Call Graph Nodes '" + Msg + "'...\n'"
112 };
113 
114 /// PassManagerPrettyStackEntry - This is used to print informative information
115 /// about what pass is running when/if a stack trace is generated.
117  Pass *P;
118  Value *V;
119  Module *M;
120 
121 public:
123  : P(p), V(nullptr), M(nullptr) {} // When P is releaseMemory'd.
125  : P(p), V(&v), M(nullptr) {} // When P is run on V
127  : P(p), V(nullptr), M(&m) {} // When P is run on M
128 
129  /// print - Emit information about this stack frame to OS.
130  void print(raw_ostream &OS) const override;
131 };
132 
133 //===----------------------------------------------------------------------===//
134 // PMStack
135 //
136 /// PMStack - This class implements a stack data structure of PMDataManager
137 /// pointers.
138 ///
139 /// Top level pass managers (see PassManager.cpp) maintain active Pass Managers
140 /// using PMStack. Each Pass implements assignPassManager() to connect itself
141 /// with appropriate manager. assignPassManager() walks PMStack to find
142 /// suitable manager.
143 class PMStack {
144 public:
145  typedef std::vector<PMDataManager *>::const_reverse_iterator iterator;
146  iterator begin() const { return S.rbegin(); }
147  iterator end() const { return S.rend(); }
148 
149  void pop();
150  PMDataManager *top() const { return S.back(); }
151  void push(PMDataManager *PM);
152  bool empty() const { return S.empty(); }
153 
154  void dump() const;
155 
156 private:
157  std::vector<PMDataManager *> S;
158 };
159 
160 //===----------------------------------------------------------------------===//
161 // PMTopLevelManager
162 //
163 /// PMTopLevelManager manages LastUser info and collects common APIs used by
164 /// top level pass managers.
166 protected:
167  explicit PMTopLevelManager(PMDataManager *PMDM);
168 
169  unsigned getNumContainedManagers() const {
170  return (unsigned)PassManagers.size();
171  }
172 
173  void initializeAllAnalysisInfo();
174 
175 private:
176  virtual PMDataManager *getAsPMDataManager() = 0;
177  virtual PassManagerType getTopLevelPassManagerType() = 0;
178 
179 public:
180  /// Schedule pass P for execution. Make sure that passes required by
181  /// P are run before P is run. Update analysis info maintained by
182  /// the manager. Remove dead passes. This is a recursive function.
183  void schedulePass(Pass *P);
184 
185  /// Set pass P as the last user of the given analysis passes.
186  void setLastUser(ArrayRef<Pass*> AnalysisPasses, Pass *P);
187 
188  /// Collect passes whose last user is P
189  void collectLastUses(SmallVectorImpl<Pass *> &LastUses, Pass *P);
190 
191  /// Find the pass that implements Analysis AID. Search immutable
192  /// passes and all pass managers. If desired pass is not found
193  /// then return NULL.
194  Pass *findAnalysisPass(AnalysisID AID);
195 
196  /// Retrieve the PassInfo for an analysis.
197  const PassInfo *findAnalysisPassInfo(AnalysisID AID) const;
198 
199  /// Find analysis usage information for the pass P.
200  AnalysisUsage *findAnalysisUsage(Pass *P);
201 
202  virtual ~PMTopLevelManager();
203 
204  /// Add immutable pass and initialize it.
205  void addImmutablePass(ImmutablePass *P);
206 
208  return ImmutablePasses;
209  }
210 
211  void addPassManager(PMDataManager *Manager) {
212  PassManagers.push_back(Manager);
213  }
214 
215  // Add Manager into the list of managers that are not directly
216  // maintained by this top level pass manager
217  inline void addIndirectPassManager(PMDataManager *Manager) {
218  IndirectPassManagers.push_back(Manager);
219  }
220 
221  // Print passes managed by this top level manager.
222  void dumpPasses() const;
223  void dumpArguments() const;
224 
225  // Active Pass Managers
227 
228 protected:
229  /// Collection of pass managers
231 
232 private:
233  /// Collection of pass managers that are not directly maintained
234  /// by this pass manager
235  SmallVector<PMDataManager *, 8> IndirectPassManagers;
236 
237  // Map to keep track of last user of the analysis pass.
238  // LastUser->second is the last user of Lastuser->first.
239  DenseMap<Pass *, Pass *> LastUser;
240 
241  // Map to keep track of passes that are last used by a pass.
242  // This inverse map is initialized at PM->run() based on
243  // LastUser map.
244  DenseMap<Pass *, SmallPtrSet<Pass *, 8> > InversedLastUser;
245 
246  /// Immutable passes are managed by top level manager.
247  SmallVector<ImmutablePass *, 16> ImmutablePasses;
248 
249  /// Map from ID to immutable passes.
251 
252 
253  /// A wrapper around AnalysisUsage for the purpose of uniqueing. The wrapper
254  /// is used to avoid needing to make AnalysisUsage itself a folding set node.
255  struct AUFoldingSetNode : public FoldingSetNode {
256  AnalysisUsage AU;
257  AUFoldingSetNode(const AnalysisUsage &AU) : AU(AU) {}
258  void Profile(FoldingSetNodeID &ID) const {
259  Profile(ID, AU);
260  }
261  static void Profile(FoldingSetNodeID &ID, const AnalysisUsage &AU) {
262  // TODO: We could consider sorting the dependency arrays within the
263  // AnalysisUsage (since they are conceptually unordered).
264  ID.AddBoolean(AU.getPreservesAll());
265  auto ProfileVec = [&](const SmallVectorImpl<AnalysisID>& Vec) {
266  ID.AddInteger(Vec.size());
267  for(AnalysisID AID : Vec)
268  ID.AddPointer(AID);
269  };
270  ProfileVec(AU.getRequiredSet());
271  ProfileVec(AU.getRequiredTransitiveSet());
272  ProfileVec(AU.getPreservedSet());
273  ProfileVec(AU.getUsedSet());
274  }
275  };
276 
277  // Contains all of the unique combinations of AnalysisUsage. This is helpful
278  // when we have multiple instances of the same pass since they'll usually
279  // have the same analysis usage and can share storage.
280  FoldingSet<AUFoldingSetNode> UniqueAnalysisUsages;
281 
282  // Allocator used for allocating UAFoldingSetNodes. This handles deletion of
283  // all allocated nodes in one fell swoop.
284  SpecificBumpPtrAllocator<AUFoldingSetNode> AUFoldingSetNodeAllocator;
285 
286  // Maps from a pass to it's associated entry in UniqueAnalysisUsages. Does
287  // not own the storage associated with either key or value..
289 
290  /// Collection of PassInfo objects found via analysis IDs and in this top
291  /// level manager. This is used to memoize queries to the pass registry.
292  /// FIXME: This is an egregious hack because querying the pass registry is
293  /// either slow or racy.
294  mutable DenseMap<AnalysisID, const PassInfo *> AnalysisPassInfos;
295 };
296 
297 //===----------------------------------------------------------------------===//
298 // PMDataManager
299 
300 /// PMDataManager provides the common place to manage the analysis data
301 /// used by pass managers.
303 public:
304  explicit PMDataManager() : TPM(nullptr), Depth(0) {
305  initializeAnalysisInfo();
306  }
307 
308  virtual ~PMDataManager();
309 
310  virtual Pass *getAsPass() = 0;
311 
312  /// Augment AvailableAnalysis by adding analysis made available by pass P.
313  void recordAvailableAnalysis(Pass *P);
314 
315  /// verifyPreservedAnalysis -- Verify analysis presreved by pass P.
316  void verifyPreservedAnalysis(Pass *P);
317 
318  /// Remove Analysis that is not preserved by the pass
319  void removeNotPreservedAnalysis(Pass *P);
320 
321  /// Remove dead passes used by P.
322  void removeDeadPasses(Pass *P, StringRef Msg,
323  enum PassDebuggingString);
324 
325  /// Remove P.
326  void freePass(Pass *P, StringRef Msg,
327  enum PassDebuggingString);
328 
329  /// Add pass P into the PassVector. Update
330  /// AvailableAnalysis appropriately if ProcessAnalysis is true.
331  void add(Pass *P, bool ProcessAnalysis = true);
332 
333  /// Add RequiredPass into list of lower level passes required by pass P.
334  /// RequiredPass is run on the fly by Pass Manager when P requests it
335  /// through getAnalysis interface.
336  virtual void addLowerLevelRequiredPass(Pass *P, Pass *RequiredPass);
337 
338  virtual Pass *getOnTheFlyPass(Pass *P, AnalysisID PI, Function &F);
339 
340  /// Initialize available analysis information.
342  AvailableAnalysis.clear();
343  for (unsigned i = 0; i < PMT_Last; ++i)
344  InheritedAnalysis[i] = nullptr;
345  }
346 
347  // Return true if P preserves high level analysis used by other
348  // passes that are managed by this manager.
349  bool preserveHigherLevelAnalysis(Pass *P);
350 
351  /// Populate UsedPasses with analysis pass that are used or required by pass
352  /// P and are available. Populate ReqPassNotAvailable with analysis pass that
353  /// are required by pass P but are not available.
354  void collectRequiredAndUsedAnalyses(
355  SmallVectorImpl<Pass *> &UsedPasses,
356  SmallVectorImpl<AnalysisID> &ReqPassNotAvailable, Pass *P);
357 
358  /// All Required analyses should be available to the pass as it runs! Here
359  /// we fill in the AnalysisImpls member of the pass so that it can
360  /// successfully use the getAnalysis() method to retrieve the
361  /// implementations it needs.
362  void initializeAnalysisImpl(Pass *P);
363 
364  /// Find the pass that implements Analysis AID. If desired pass is not found
365  /// then return NULL.
366  Pass *findAnalysisPass(AnalysisID AID, bool Direction);
367 
368  // Access toplevel manager
371 
372  unsigned getDepth() const { return Depth; }
373  void setDepth(unsigned newDepth) { Depth = newDepth; }
374 
375  // Print routines used by debug-pass
376  void dumpLastUses(Pass *P, unsigned Offset) const;
377  void dumpPassArguments() const;
378  void dumpPassInfo(Pass *P, enum PassDebuggingString S1,
379  enum PassDebuggingString S2, StringRef Msg);
380  void dumpRequiredSet(const Pass *P) const;
381  void dumpPreservedSet(const Pass *P) const;
382  void dumpUsedSet(const Pass *P) const;
383 
384  unsigned getNumContainedPasses() const {
385  return (unsigned)PassVector.size();
386  }
387 
389  assert ( 0 && "Invalid use of getPassManagerType");
390  return PMT_Unknown;
391  }
392 
394  return &AvailableAnalysis;
395  }
396 
397  // Collect AvailableAnalysis from all the active Pass Managers.
399  unsigned Index = 0;
400  for (PMStack::iterator I = PMS.begin(), E = PMS.end();
401  I != E; ++I)
402  InheritedAnalysis[Index++] = (*I)->getAvailableAnalysis();
403  }
404 
405  /// Set the initial size of the module if the user has specified that they
406  /// want remarks for size.
407  /// Returns 0 if the remark was not requested.
408  unsigned initSizeRemarkInfo(
409  Module &M,
410  StringMap<std::pair<unsigned, unsigned>> &FunctionToInstrCount);
411 
412  /// Emit a remark signifying that the number of IR instructions in the module
413  /// changed.
414  /// \p F is optionally passed by passes which run on Functions, and thus
415  /// always know whether or not a non-empty function is available.
416  ///
417  /// \p FunctionToInstrCount maps the name of a \p Function to a pair. The
418  /// first member of the pair is the IR count of the \p Function before running
419  /// \p P, and the second member is the IR count of the \p Function after
420  /// running \p P.
421  void emitInstrCountChangedRemark(
422  Pass *P, Module &M, int64_t Delta, unsigned CountBefore,
423  StringMap<std::pair<unsigned, unsigned>> &FunctionToInstrCount,
424  Function *F = nullptr);
425 
426 protected:
427  // Top level manager.
429 
430  // Collection of pass that are managed by this manager
432 
433  // Collection of Analysis provided by Parent pass manager and
434  // used by current pass manager. At at time there can not be more
435  // then PMT_Last active pass mangers.
437 
438  /// isPassDebuggingExecutionsOrMore - Return true if -debug-pass=Executions
439  /// or higher is specified.
440  bool isPassDebuggingExecutionsOrMore() const;
441 
442 private:
443  void dumpAnalysisUsage(StringRef Msg, const Pass *P,
444  const AnalysisUsage::VectorType &Set) const;
445 
446  // Set of available Analysis. This information is used while scheduling
447  // pass. If a pass requires an analysis which is not available then
448  // the required analysis pass is scheduled to run before the pass itself is
449  // scheduled to run.
450  DenseMap<AnalysisID, Pass*> AvailableAnalysis;
451 
452  // Collection of higher level analysis used by the pass managed by
453  // this manager.
454  SmallVector<Pass *, 16> HigherLevelAnalysis;
455 
456  unsigned Depth;
457 };
458 
459 //===----------------------------------------------------------------------===//
460 // FPPassManager
461 //
462 /// FPPassManager manages BBPassManagers and FunctionPasses.
463 /// It batches all function passes and basic block pass managers together and
464 /// sequence them to process one function at a time before processing next
465 /// function.
466 class FPPassManager : public ModulePass, public PMDataManager {
467 public:
468  static char ID;
469  explicit FPPassManager()
470  : ModulePass(ID), PMDataManager() { }
471 
472  /// run - Execute all of the passes scheduled for execution. Keep track of
473  /// whether any of the passes modifies the module, and if so, return true.
474  bool runOnFunction(Function &F);
475  bool runOnModule(Module &M) override;
476 
477  /// cleanup - After running all passes, clean up pass manager cache.
478  void cleanup();
479 
480  /// doInitialization - Overrides ModulePass doInitialization for global
481  /// initialization tasks
482  ///
484 
485  /// doInitialization - Run all of the initializers for the function passes.
486  ///
487  bool doInitialization(Module &M) override;
488 
489  /// doFinalization - Overrides ModulePass doFinalization for global
490  /// finalization tasks
491  ///
493 
494  /// doFinalization - Run all of the finalizers for the function passes.
495  ///
496  bool doFinalization(Module &M) override;
497 
498  PMDataManager *getAsPMDataManager() override { return this; }
499  Pass *getAsPass() override { return this; }
500 
501  /// Pass Manager itself does not invalidate any analysis info.
502  void getAnalysisUsage(AnalysisUsage &Info) const override {
503  Info.setPreservesAll();
504  }
505 
506  // Print passes managed by this manager
507  void dumpPassStructure(unsigned Offset) override;
508 
509  StringRef getPassName() const override { return "Function Pass Manager"; }
510 
512  assert ( N < PassVector.size() && "Pass number out of range!");
513  FunctionPass *FP = static_cast<FunctionPass *>(PassVector[N]);
514  return FP;
515  }
516 
519  }
520 };
521 
522 }
523 
524 #endif
PMTopLevelManager * TPM
Pass interface - Implemented by all &#39;passes&#39;.
Definition: Pass.h:80
void AddPointer(const void *Ptr)
Add* - Add various data types to Bit data.
Definition: FoldingSet.cpp:51
StringRef getPassName() const override
getPassName - Return a nice clean name for a pass.
PassManagerType
Different types of internal pass managers.
Definition: Pass.h:53
PassManagerPrettyStackEntry(Pass *p, Value &v)
This class represents lattice values for constants.
Definition: AllocatorList.h:23
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:66
std::vector< PMDataManager * >::const_reverse_iterator iterator
F(f)
PMDataManager * getAsPMDataManager() override
const VectorType & getUsedSet() const
static void cleanup(BlockFrequencyInfoImplBase &BFI)
Clear all memory not needed downstream.
const VectorType & getRequiredSet() const
PMTopLevelManager manages LastUser info and collects common APIs used by top level pass managers...
void setDepth(unsigned newDepth)
void addPassManager(PMDataManager *Manager)
iterator begin() const
virtual bool doFinalization(Module &)
doFinalization - Virtual method overriden by subclasses to do any necessary clean up after all passes...
Definition: Pass.h:109
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APFloat.h:41
void initializeAnalysisInfo()
Initialize available analysis information.
void AddInteger(signed I)
Definition: FoldingSet.cpp:60
const VectorType & getPreservedSet() const
void populateInheritedAnalysis(PMStack &PMS)
iterator end() const
FPPassManager manages BBPassManagers and FunctionPasses.
PMStack - This class implements a stack data structure of PMDataManager pointers. ...
PassManagerPrettyStackEntry - This is used to print informative information about what pass is runnin...
PassManagerPrettyStackEntry(Pass *p, Module &m)
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: APInt.h:32
unsigned getNumContainedManagers() const
Analysis containing CSE Info
Definition: CSEInfo.cpp:20
virtual bool doInitialization(Module &)
doInitialization - Virtual method overridden by subclasses to do any necessary initialization before ...
Definition: Pass.h:105
FoldingSetNodeID - This class is used to gather all the unique data bits of a node.
Definition: FoldingSet.h:305
static bool runOnFunction(Function &F, bool PostInlining)
#define P(N)
void dump(const SparseBitVector< ElementSize > &LHS, raw_ostream &out)
void print(raw_ostream &OS) const override
print - Emit information about this stack frame to OS.
static void push(SmallVectorImpl< uint64_t > &R, StringRef Str)
void addIndirectPassManager(PMDataManager *Manager)
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
virtual PassManagerType getPassManagerType() const
void AddBoolean(bool B)
Definition: FoldingSet.h:324
Represent the analysis usage information of a pass.
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:284
SmallVectorImpl< ImmutablePass * > & getImmutablePasses()
PassInfo class - An instance of this class exists for every pass known by the system, and can be obtained from a live Pass by calling its getPassInfo() method.
Definition: PassInfo.h:30
PrettyStackTraceEntry - This class is used to represent a frame of the "pretty" stack trace that is d...
print lazy value Lazy Value Info Printer Pass
ImmutablePass class - This class is used to provide information that does not need to be run...
Definition: Pass.h:255
FPPassManager.
Definition: Pass.h:57
static uint64_t add(uint64_t LeftOp, uint64_t RightOp)
Definition: FileCheck.cpp:215
This is a &#39;vector&#39; (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:837
StringMap - This is an unconventional map that is specialized for handling keys that are "strings"...
Definition: StringMap.h:242
SmallVector< Pass *, 16 > PassVector
const void * AnalysisID
Definition: Pass.h:48
Pass * getAsPass() override
void getAnalysisUsage(AnalysisUsage &Info) const override
Pass Manager itself does not invalidate any analysis info.
DenseMap< AnalysisID, Pass * > * getAvailableAnalysis()
void setPreservesAll()
Set by analyses that do not transform their input at all.
const VectorType & getRequiredTransitiveSet() const
void setTopLevelManager(PMTopLevelManager *T)
SmallVector< PMDataManager *, 8 > PassManagers
Collection of pass managers.
Node - This class is used to maintain the singly linked bucket list in a folding set.
Definition: FoldingSet.h:135
FunctionPass * getContainedPass(unsigned N)
#define I(x, y, z)
Definition: MD5.cpp:58
#define N
ModulePass class - This class is used to implement unstructured interprocedural optimizations and ana...
Definition: Pass.h:224
PMDataManager provides the common place to manage the analysis data used by pass managers.
Loop::LoopBounds::Direction Direction
Definition: LoopInfo.cpp:226
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
PMDataManager * top() const
LLVM Value Representation.
Definition: Value.h:74
unsigned getNumContainedPasses() const
bool empty() const
PMTopLevelManager * getTopLevelManager()
This class implements an extremely fast bulk output stream that can only output to a stream...
Definition: raw_ostream.h:45
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:48
unsigned getDepth() const
bool getPreservesAll() const
Determine whether a pass said it does not transform its input at all.
PassManagerType getPassManagerType() const override