LLVM  4.0.0
GlobalSplit.cpp
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1 //===- GlobalSplit.cpp - global variable splitter -------------------------===//
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 pass uses inrange annotations on GEP indices to split globals where
11 // beneficial. Clang currently attaches these annotations to references to
12 // virtual table globals under the Itanium ABI for the benefit of the
13 // whole-program virtual call optimization and control flow integrity passes.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "llvm/Transforms/IPO.h"
19 #include "llvm/ADT/StringExtras.h"
20 #include "llvm/IR/Constants.h"
21 #include "llvm/IR/GlobalVariable.h"
22 #include "llvm/IR/Intrinsics.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/IR/Operator.h"
25 #include "llvm/Pass.h"
26 
27 #include <set>
28 
29 using namespace llvm;
30 
31 namespace {
32 
33 bool splitGlobal(GlobalVariable &GV) {
34  // If the address of the global is taken outside of the module, we cannot
35  // apply this transformation.
36  if (!GV.hasLocalLinkage())
37  return false;
38 
39  // We currently only know how to split ConstantStructs.
40  auto *Init = dyn_cast_or_null<ConstantStruct>(GV.getInitializer());
41  if (!Init)
42  return false;
43 
44  // Verify that each user of the global is an inrange getelementptr constant.
45  // From this it follows that any loads from or stores to that global must use
46  // a pointer derived from an inrange getelementptr constant, which is
47  // sufficient to allow us to apply the splitting transform.
48  for (User *U : GV.users()) {
49  if (!isa<Constant>(U))
50  return false;
51 
52  auto *GEP = dyn_cast<GEPOperator>(U);
53  if (!GEP || !GEP->getInRangeIndex() || *GEP->getInRangeIndex() != 1 ||
54  !isa<ConstantInt>(GEP->getOperand(1)) ||
55  !cast<ConstantInt>(GEP->getOperand(1))->isZero() ||
56  !isa<ConstantInt>(GEP->getOperand(2)))
57  return false;
58  }
59 
62 
63  const DataLayout &DL = GV.getParent()->getDataLayout();
64  const StructLayout *SL = DL.getStructLayout(Init->getType());
65 
67 
68  std::vector<GlobalVariable *> SplitGlobals(Init->getNumOperands());
69  for (unsigned I = 0; I != Init->getNumOperands(); ++I) {
70  // Build a global representing this split piece.
71  auto *SplitGV =
72  new GlobalVariable(*GV.getParent(), Init->getOperand(I)->getType(),
74  Init->getOperand(I), GV.getName() + "." + utostr(I));
75  SplitGlobals[I] = SplitGV;
76 
77  unsigned SplitBegin = SL->getElementOffset(I);
78  unsigned SplitEnd = (I == Init->getNumOperands() - 1)
79  ? SL->getSizeInBytes()
80  : SL->getElementOffset(I + 1);
81 
82  // Rebuild type metadata, adjusting by the split offset.
83  // FIXME: See if we can use DW_OP_piece to preserve debug metadata here.
84  for (MDNode *Type : Types) {
85  uint64_t ByteOffset = cast<ConstantInt>(
86  cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
87  ->getZExtValue();
88  if (ByteOffset < SplitBegin || ByteOffset >= SplitEnd)
89  continue;
90  SplitGV->addMetadata(
92  *MDNode::get(GV.getContext(),
94  ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
95  Type->getOperand(1)}));
96  }
97  }
98 
99  for (User *U : GV.users()) {
100  auto *GEP = cast<GEPOperator>(U);
101  unsigned I = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
102  if (I >= SplitGlobals.size())
103  continue;
104 
106  Ops.push_back(ConstantInt::get(Int32Ty, 0));
107  for (unsigned I = 3; I != GEP->getNumOperands(); ++I)
108  Ops.push_back(GEP->getOperand(I));
109 
110  auto *NewGEP = ConstantExpr::getGetElementPtr(
111  SplitGlobals[I]->getInitializer()->getType(), SplitGlobals[I], Ops,
112  GEP->isInBounds());
113  GEP->replaceAllUsesWith(NewGEP);
114  }
115 
116  // Finally, remove the original global. Any remaining uses refer to invalid
117  // elements of the global, so replace with undef.
118  if (!GV.use_empty())
120  GV.eraseFromParent();
121  return true;
122 }
123 
124 bool splitGlobals(Module &M) {
125  // First, see if the module uses either of the llvm.type.test or
126  // llvm.type.checked.load intrinsics, which indicates that splitting globals
127  // may be beneficial.
128  Function *TypeTestFunc =
129  M.getFunction(Intrinsic::getName(Intrinsic::type_test));
130  Function *TypeCheckedLoadFunc =
131  M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
132  if ((!TypeTestFunc || TypeTestFunc->use_empty()) &&
133  (!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
134  return false;
135 
136  bool Changed = false;
137  for (auto I = M.global_begin(); I != M.global_end();) {
138  GlobalVariable &GV = *I;
139  ++I;
140  Changed |= splitGlobal(GV);
141  }
142  return Changed;
143 }
144 
145 struct GlobalSplit : public ModulePass {
146  static char ID;
147  GlobalSplit() : ModulePass(ID) {
149  }
150  bool runOnModule(Module &M) {
151  if (skipModule(M))
152  return false;
153 
154  return splitGlobals(M);
155  }
156 };
157 
158 }
159 
160 INITIALIZE_PASS(GlobalSplit, "globalsplit", "Global splitter", false, false)
161 char GlobalSplit::ID = 0;
162 
164  return new GlobalSplit;
165 }
166 
168  if (!splitGlobals(M))
169  return PreservedAnalyses::all();
170  return PreservedAnalyses::none();
171 }
PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM)
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:102
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:52
static Constant * getGetElementPtr(Type *Ty, Constant *C, ArrayRef< Constant * > IdxList, bool InBounds=false, Optional< unsigned > InRangeIndex=None, Type *OnlyIfReducedTy=nullptr)
Getelementptr form.
Definition: Constants.h:1126
void initializeGlobalSplitPass(PassRegistry &)
Like Internal, but omit from symbol table.
Definition: GlobalValue.h:57
Metadata node.
Definition: Metadata.h:830
FunctionType * getType(LLVMContext &Context, ID id, ArrayRef< Type * > Tys=None)
Return the function type for an intrinsic.
Definition: Function.cpp:905
Hexagon Common GEP
const Constant * getInitializer() const
getInitializer - Return the initializer for this global variable.
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:191
Used to lazily calculate structure layout information for a target machine, based on the DataLayout s...
Definition: DataLayout.h:496
StringRef getName(ID id)
Return the LLVM name for an intrinsic, such as "llvm.ppc.altivec.lvx".
Definition: Function.cpp:555
const StructLayout * getStructLayout(StructType *Ty) const
Returns a StructLayout object, indicating the alignment of the struct, its size, and the offsets of i...
Definition: DataLayout.cpp:566
void eraseFromParent() override
eraseFromParent - This method unlinks 'this' from the containing module and deletes it...
Definition: Globals.cpp:319
global_iterator global_begin()
Definition: Module.h:518
static std::string utostr(uint64_t X, bool isNeg=false)
Definition: StringExtras.h:79
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:401
static ConstantAsMetadata * get(Constant *C)
Definition: Metadata.h:392
static PreservedAnalyses none()
Convenience factory function for the empty preserved set.
Definition: PassManager.h:110
Function * getFunction(StringRef Name) const
Look up the specified function in the module symbol table.
Definition: Module.cpp:196
uint64_t getElementOffset(unsigned Idx) const
Definition: DataLayout.h:517
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:107
The instances of the Type class are immutable: once they are created, they are never changed...
Definition: Type.h:45
This file contains the declarations for the subclasses of Constant, which represent the different fla...
Class to represent integer types.
Definition: DerivedTypes.h:39
static UndefValue * get(Type *T)
Static factory methods - Return an 'undef' object of the specified type.
Definition: Constants.cpp:1337
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: PassManager.h:113
LLVMContext & getContext() const
All values hold a context through their type.
Definition: Value.cpp:654
ModulePass * createGlobalSplitPass()
This pass splits globals into pieces for the benefit of whole-program devirtualization and control-fl...
#define INITIALIZE_PASS(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:36
global_iterator global_end()
Definition: Module.h:520
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:843
Module.h This file contains the declarations for the Module class.
uint64_t getSizeInBytes() const
Definition: DataLayout.h:503
static Constant * get(Type *Ty, uint64_t V, bool isSigned=false)
If Ty is a vector type, return a Constant with a splat of the given value.
Definition: Constants.cpp:558
bool isConstant() const
If the value is a global constant, its value is immutable throughout the runtime execution of the pro...
iterator_range< user_iterator > users()
Definition: Value.h:370
MDNode * getMetadata(unsigned KindID) const
Get the current metadata attachments for the given kind, if any.
Definition: Metadata.cpp:1391
static MDTuple * get(LLVMContext &Context, ArrayRef< Metadata * > MDs)
Definition: Metadata.h:1132
static bool isZero(Value *V, const DataLayout &DL, DominatorTree *DT, AssumptionCache *AC)
Definition: Lint.cpp:528
PointerType * getType() const
Global values are always pointers.
Definition: GlobalValue.h:259
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.cpp:384
static IntegerType * getInt32Ty(LLVMContext &C)
Definition: Type.cpp:169
#define I(x, y, z)
Definition: MD5.cpp:54
ModulePass class - This class is used to implement unstructured interprocedural optimizations and ana...
Definition: Pass.h:235
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:287
bool hasLocalLinkage() const
Definition: GlobalValue.h:415
bool use_empty() const
Definition: Value.h:299
Module * getParent()
Get the module that this global value is contained inside of...
Definition: GlobalValue.h:537
A container for analyses that lazily runs them and caches their results.
IntegerType * Int32Ty