LLVM  6.0.0svn
SimplifyCFGPass.cpp
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1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements dead code elimination and basic block merging, along
11 // with a collection of other peephole control flow optimizations. For example:
12 //
13 // * Removes basic blocks with no predecessors.
14 // * Merges a basic block into its predecessor if there is only one and the
15 // predecessor only has one successor.
16 // * Eliminates PHI nodes for basic blocks with a single predecessor.
17 // * Eliminates a basic block that only contains an unconditional branch.
18 // * Changes invoke instructions to nounwind functions to be calls.
19 // * Change things like "if (x) if (y)" into "if (x&y)".
20 // * etc..
21 //
22 //===----------------------------------------------------------------------===//
23 
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/ADT/SmallVector.h"
26 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/CFG.h"
31 #include "llvm/IR/Attributes.h"
32 #include "llvm/IR/CFG.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/DataLayout.h"
35 #include "llvm/IR/Instructions.h"
36 #include "llvm/IR/IntrinsicInst.h"
37 #include "llvm/IR/Module.h"
38 #include "llvm/Pass.h"
40 #include "llvm/Transforms/Scalar.h"
43 #include <utility>
44 using namespace llvm;
45 
46 #define DEBUG_TYPE "simplifycfg"
47 
48 static cl::opt<unsigned>
49 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
50  cl::desc("Control the number of bonus instructions (default = 1)"));
51 
52 STATISTIC(NumSimpl, "Number of blocks simplified");
53 
54 /// If we have more than one empty (other than phi node) return blocks,
55 /// merge them together to promote recursive block merging.
57  bool Changed = false;
58 
59  BasicBlock *RetBlock = nullptr;
60 
61  // Scan all the blocks in the function, looking for empty return blocks.
62  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
63  BasicBlock &BB = *BBI++;
64 
65  // Only look at return blocks.
67  if (!Ret) continue;
68 
69  // Only look at the block if it is empty or the only other thing in it is a
70  // single PHI node that is the operand to the return.
71  if (Ret != &BB.front()) {
72  // Check for something else in the block.
74  --I;
75  // Skip over debug info.
76  while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
77  --I;
78  if (!isa<DbgInfoIntrinsic>(I) &&
79  (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
80  Ret->getOperand(0) != &*I))
81  continue;
82  }
83 
84  // If this is the first returning block, remember it and keep going.
85  if (!RetBlock) {
86  RetBlock = &BB;
87  continue;
88  }
89 
90  // Otherwise, we found a duplicate return block. Merge the two.
91  Changed = true;
92 
93  // Case when there is no input to the return or when the returned values
94  // agree is trivial. Note that they can't agree if there are phis in the
95  // blocks.
96  if (Ret->getNumOperands() == 0 ||
97  Ret->getOperand(0) ==
98  cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
99  BB.replaceAllUsesWith(RetBlock);
100  BB.eraseFromParent();
101  continue;
102  }
103 
104  // If the canonical return block has no PHI node, create one now.
105  PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
106  if (!RetBlockPHI) {
107  Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
108  pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
109  RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
110  std::distance(PB, PE), "merge",
111  &RetBlock->front());
112 
113  for (pred_iterator PI = PB; PI != PE; ++PI)
114  RetBlockPHI->addIncoming(InVal, *PI);
115  RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
116  }
117 
118  // Turn BB into a block that just unconditionally branches to the return
119  // block. This handles the case when the two return blocks have a common
120  // predecessor but that return different things.
121  RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
123  BranchInst::Create(RetBlock, &BB);
124  }
125 
126  return Changed;
127 }
128 
129 /// Call SimplifyCFG on all the blocks in the function,
130 /// iterating until no more changes are made.
132  AssumptionCache *AC,
133  unsigned BonusInstThreshold,
134  bool LateSimplifyCFG) {
135  bool Changed = false;
136  bool LocalChange = true;
137 
139  FindFunctionBackedges(F, Edges);
140  SmallPtrSet<BasicBlock *, 16> LoopHeaders;
141  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
142  LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
143 
144  while (LocalChange) {
145  LocalChange = false;
146 
147  // Loop over all of the basic blocks and remove them if they are unneeded.
148  for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
149  if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders, LateSimplifyCFG)) {
150  LocalChange = true;
151  ++NumSimpl;
152  }
153  }
154  Changed |= LocalChange;
155  }
156  return Changed;
157 }
158 
160  AssumptionCache *AC, int BonusInstThreshold,
161  bool LateSimplifyCFG) {
162  bool EverChanged = removeUnreachableBlocks(F);
163  EverChanged |= mergeEmptyReturnBlocks(F);
164  EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold,
165  LateSimplifyCFG);
166 
167  // If neither pass changed anything, we're done.
168  if (!EverChanged) return false;
169 
170  // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
171  // removeUnreachableBlocks is needed to nuke them, which means we should
172  // iterate between the two optimizations. We structure the code like this to
173  // avoid rerunning iterativelySimplifyCFG if the second pass of
174  // removeUnreachableBlocks doesn't do anything.
175  if (!removeUnreachableBlocks(F))
176  return true;
177 
178  do {
179  EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold,
180  LateSimplifyCFG);
181  EverChanged |= removeUnreachableBlocks(F);
182  } while (EverChanged);
183 
184  return true;
185 }
186 
188  : BonusInstThreshold(UserBonusInstThreshold),
189  LateSimplifyCFG(true) {}
190 
191 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold, bool LateSimplifyCFG)
192  : BonusInstThreshold(BonusInstThreshold),
193  LateSimplifyCFG(LateSimplifyCFG) {}
194 
197  auto &TTI = AM.getResult<TargetIRAnalysis>(F);
198  auto &AC = AM.getResult<AssumptionAnalysis>(F);
199 
200  if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold, LateSimplifyCFG))
201  return PreservedAnalyses::all();
203  PA.preserve<GlobalsAA>();
204  return PA;
205 }
206 
207 namespace {
208 struct BaseCFGSimplifyPass : public FunctionPass {
209  unsigned BonusInstThreshold;
210  std::function<bool(const Function &)> PredicateFtor;
211  bool LateSimplifyCFG;
212 
213  BaseCFGSimplifyPass(int T, bool LateSimplifyCFG,
214  std::function<bool(const Function &)> Ftor,
215  char &ID)
216  : FunctionPass(ID), PredicateFtor(std::move(Ftor)),
217  LateSimplifyCFG(LateSimplifyCFG) {
218  BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
219  }
220  bool runOnFunction(Function &F) override {
221  if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
222  return false;
223 
224  AssumptionCache *AC =
225  &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
226  const TargetTransformInfo &TTI =
227  getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
228  return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold, LateSimplifyCFG);
229  }
230 
231  void getAnalysisUsage(AnalysisUsage &AU) const override {
235  }
236 };
237 
238 struct CFGSimplifyPass : public BaseCFGSimplifyPass {
239  static char ID; // Pass identification, replacement for typeid
240 
241  CFGSimplifyPass(int T = -1,
242  std::function<bool(const Function &)> Ftor = nullptr)
243  : BaseCFGSimplifyPass(T, false, Ftor, ID) {
245  }
246 };
247 
248 struct LateCFGSimplifyPass : public BaseCFGSimplifyPass {
249  static char ID; // Pass identification, replacement for typeid
250 
251  LateCFGSimplifyPass(int T = -1,
252  std::function<bool(const Function &)> Ftor = nullptr)
253  : BaseCFGSimplifyPass(T, true, Ftor, ID) {
255  }
256 };
257 }
258 
259 char CFGSimplifyPass::ID = 0;
260 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
261  false)
264 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
265  false)
266 
267 char LateCFGSimplifyPass::ID = 0;
268 INITIALIZE_PASS_BEGIN(LateCFGSimplifyPass, "latesimplifycfg",
269  "Simplify the CFG more aggressively", false, false)
272 INITIALIZE_PASS_END(LateCFGSimplifyPass, "latesimplifycfg",
273  "Simplify the CFG more aggressively", false, false)
274 
275 // Public interface to the CFGSimplification pass
276 FunctionPass *
278  std::function<bool(const Function &)> Ftor) {
279  return new CFGSimplifyPass(Threshold, std::move(Ftor));
280 }
281 
282 // Public interface to the LateCFGSimplification pass
283 FunctionPass *
285  std::function<bool(const Function &)> Ftor) {
286  return new LateCFGSimplifyPass(Threshold, std::move(Ftor));
287 }
Legacy wrapper pass to provide the GlobalsAAResult object.
This file provides the interface for the pass responsible for both simplifying and canonicalizing the...
Return a value (possibly void), from a function.
SymbolTableList< Instruction >::iterator eraseFromParent()
This method unlinks &#39;this&#39; from the containing basic block and deletes it.
Definition: Instruction.cpp:69
static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, AssumptionCache *AC, unsigned BonusInstThreshold, bool LateSimplifyCFG)
Call SimplifyCFG on all the blocks in the function, iterating until no more changes are made...
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
void addIncoming(Value *V, BasicBlock *BB)
Add an incoming value to the end of the PHI list.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
PassT::Result & getResult(IRUnitT &IR, ExtraArgTs... ExtraArgs)
Get the result of an analysis pass for a given IR unit.
Definition: PassManager.h:687
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
This is the interface for a simple mod/ref and alias analysis over globals.
LLVM_ATTRIBUTE_ALWAYS_INLINE size_type size() const
Definition: SmallVector.h:136
iterator end()
Definition: Function.h:590
Simplify the CFG more aggressively
An immutable pass that tracks lazily created AssumptionCache objects.
static cl::opt< unsigned > UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1), cl::desc("Control the number of bonus instructions (default = 1)"))
A cache of .assume calls within a function.
Analysis pass providing the TargetTransformInfo.
STATISTIC(NumFunctions, "Total number of functions")
F(f)
Simplify the CFG
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:252
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:51
Definition: BitVector.h:920
This file contains the simple types necessary to represent the attributes associated with functions a...
FunctionPass * createCFGSimplificationPass(int Threshold=-1, std::function< bool(const Function &)> Ftor=nullptr)
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:245
bool SimplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI, unsigned BonusInstThreshold, AssumptionCache *AC=nullptr, SmallPtrSetImpl< BasicBlock *> *LoopHeaders=nullptr, bool LateSimplifyCFG=false)
This function is used to do simplification of a CFG.
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:428
iterator begin()
Definition: Function.h:588
Value * getOperand(unsigned i) const
Definition: User.h:154
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:406
simplifycfg
Wrapper pass for TargetTransformInfo.
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:153
LLVM Basic Block Representation.
Definition: BasicBlock.h:59
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
latesimplifycfg
void initializeLateCFGSimplifyPassPass(PassRegistry &)
This file contains the declarations for the subclasses of Constant, which represent the different fla...
const Instruction & front() const
Definition: BasicBlock.h:264
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:363
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:113
Represent the analysis usage information of a pass.
Analysis pass providing a never-invalidated alias analysis result.
static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, AssumptionCache *AC, int BonusInstThreshold, bool LateSimplifyCFG)
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:116
FunctionPass * createLateCFGSimplificationPass(int Threshold=-1, std::function< bool(const Function &)> Ftor=nullptr)
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:159
INITIALIZE_PASS_END(RegBankSelect, DEBUG_TYPE, "Assign register bank of generic virtual registers", false, false) RegBankSelect
A function analysis which provides an AssumptionCache.
Iterator for intrusive lists based on ilist_node.
unsigned getNumOperands() const
Definition: User.h:176
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:410
This pass provides access to the codegen interfaces that are needed for IR-level transformations.
bool removeUnreachableBlocks(Function &F, LazyValueInfo *LVI=nullptr)
Remove all blocks that can not be reached from the function&#39;s entry.
Definition: Local.cpp:1677
This is a &#39;vector&#39; (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:864
Module.h This file contains the declarations for the Module class.
static BranchInst * Create(BasicBlock *IfTrue, Instruction *InsertBefore=nullptr)
static PHINode * Create(Type *Ty, unsigned NumReservedValues, const Twine &NameStr="", Instruction *InsertBefore=nullptr)
Constructors - NumReservedValues is a hint for the number of incoming edges that this phi node will h...
void setOperand(unsigned i, Value *Val)
Definition: User.h:159
Basic Alias true
INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, false) INITIALIZE_PASS_END(CFGSimplifyPass
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Run the pass over the function.
SymbolTableList< BasicBlock >::iterator eraseFromParent()
Unlink &#39;this&#39; from the containing function and delete it.
Definition: BasicBlock.cpp:97
#define I(x, y, z)
Definition: MD5.cpp:58
LLVM_NODISCARD std::enable_if<!is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:323
void preserve()
Mark an analysis as preserved.
Definition: PassManager.h:174
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
static bool mergeEmptyReturnBlocks(Function &F)
If we have more than one empty (other than phi node) return blocks, merge them together to promote re...
static int const Threshold
TODO: Write a new FunctionPass AliasAnalysis so that it can keep a cache.
void initializeCFGSimplifyPassPass(PassRegistry &)
aarch64 promote const
SimplifyCFGPass()
Construct a pass with the default thresholds and switch optimizations.
LLVM Value Representation.
Definition: Value.h:73
print Print MemDeps of function
A container for analyses that lazily runs them and caches their results.
This pass exposes codegen information to IR-level passes.
const TerminatorInst * 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:120