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/GlobalSplit.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/StringExtras.h"
20 #include "llvm/IR/Constant.h"
21 #include "llvm/IR/Constants.h"
22 #include "llvm/IR/DataLayout.h"
23 #include "llvm/IR/Function.h"
24 #include "llvm/IR/GlobalValue.h"
25 #include "llvm/IR/GlobalVariable.h"
26 #include "llvm/IR/Intrinsics.h"
27 #include "llvm/IR/LLVMContext.h"
28 #include "llvm/IR/Metadata.h"
29 #include "llvm/IR/Module.h"
30 #include "llvm/IR/Operator.h"
31 #include "llvm/IR/Type.h"
32 #include "llvm/IR/User.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Support/Casting.h"
35 #include "llvm/Transforms/IPO.h"
36 #include <cstdint>
37 #include <vector>
38
39 using namespace llvm;
40
splitGlobal(GlobalVariable & GV)41 static bool splitGlobal(GlobalVariable &GV) {
42 // If the address of the global is taken outside of the module, we cannot
43 // apply this transformation.
44 if (!GV.hasLocalLinkage())
45 return false;
46
47 // We currently only know how to split ConstantStructs.
48 auto *Init = dyn_cast_or_null<ConstantStruct>(GV.getInitializer());
49 if (!Init)
50 return false;
51
52 // Verify that each user of the global is an inrange getelementptr constant.
53 // From this it follows that any loads from or stores to that global must use
54 // a pointer derived from an inrange getelementptr constant, which is
55 // sufficient to allow us to apply the splitting transform.
56 for (User *U : GV.users()) {
57 if (!isa<Constant>(U))
58 return false;
59
60 auto *GEP = dyn_cast<GEPOperator>(U);
61 if (!GEP || !GEP->getInRangeIndex() || *GEP->getInRangeIndex() != 1 ||
62 !isa<ConstantInt>(GEP->getOperand(1)) ||
63 !cast<ConstantInt>(GEP->getOperand(1))->isZero() ||
64 !isa<ConstantInt>(GEP->getOperand(2)))
65 return false;
66 }
67
68 SmallVector<MDNode *, 2> Types;
69 GV.getMetadata(LLVMContext::MD_type, Types);
70
71 const DataLayout &DL = GV.getParent()->getDataLayout();
72 const StructLayout *SL = DL.getStructLayout(Init->getType());
73
74 IntegerType *Int32Ty = Type::getInt32Ty(GV.getContext());
75
76 std::vector<GlobalVariable *> SplitGlobals(Init->getNumOperands());
77 for (unsigned I = 0; I != Init->getNumOperands(); ++I) {
78 // Build a global representing this split piece.
79 auto *SplitGV =
80 new GlobalVariable(*GV.getParent(), Init->getOperand(I)->getType(),
81 GV.isConstant(), GlobalValue::PrivateLinkage,
82 Init->getOperand(I), GV.getName() + "." + utostr(I));
83 SplitGlobals[I] = SplitGV;
84
85 unsigned SplitBegin = SL->getElementOffset(I);
86 unsigned SplitEnd = (I == Init->getNumOperands() - 1)
87 ? SL->getSizeInBytes()
88 : SL->getElementOffset(I + 1);
89
90 // Rebuild type metadata, adjusting by the split offset.
91 // FIXME: See if we can use DW_OP_piece to preserve debug metadata here.
92 for (MDNode *Type : Types) {
93 uint64_t ByteOffset = cast<ConstantInt>(
94 cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
95 ->getZExtValue();
96 // Type metadata may be attached one byte after the end of the vtable, for
97 // classes without virtual methods in Itanium ABI. AFAIK, it is never
98 // attached to the first byte of a vtable. Subtract one to get the right
99 // slice.
100 // This is making an assumption that vtable groups are the only kinds of
101 // global variables that !type metadata can be attached to, and that they
102 // are either Itanium ABI vtable groups or contain a single vtable (i.e.
103 // Microsoft ABI vtables).
104 uint64_t AttachedTo = (ByteOffset == 0) ? ByteOffset : ByteOffset - 1;
105 if (AttachedTo < SplitBegin || AttachedTo >= SplitEnd)
106 continue;
107 SplitGV->addMetadata(
108 LLVMContext::MD_type,
109 *MDNode::get(GV.getContext(),
110 {ConstantAsMetadata::get(
111 ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
112 Type->getOperand(1)}));
113 }
114 }
115
116 for (User *U : GV.users()) {
117 auto *GEP = cast<GEPOperator>(U);
118 unsigned I = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
119 if (I >= SplitGlobals.size())
120 continue;
121
122 SmallVector<Value *, 4> Ops;
123 Ops.push_back(ConstantInt::get(Int32Ty, 0));
124 for (unsigned I = 3; I != GEP->getNumOperands(); ++I)
125 Ops.push_back(GEP->getOperand(I));
126
127 auto *NewGEP = ConstantExpr::getGetElementPtr(
128 SplitGlobals[I]->getInitializer()->getType(), SplitGlobals[I], Ops,
129 GEP->isInBounds());
130 GEP->replaceAllUsesWith(NewGEP);
131 }
132
133 // Finally, remove the original global. Any remaining uses refer to invalid
134 // elements of the global, so replace with undef.
135 if (!GV.use_empty())
136 GV.replaceAllUsesWith(UndefValue::get(GV.getType()));
137 GV.eraseFromParent();
138 return true;
139 }
140
splitGlobals(Module & M)141 static bool splitGlobals(Module &M) {
142 // First, see if the module uses either of the llvm.type.test or
143 // llvm.type.checked.load intrinsics, which indicates that splitting globals
144 // may be beneficial.
145 Function *TypeTestFunc =
146 M.getFunction(Intrinsic::getName(Intrinsic::type_test));
147 Function *TypeCheckedLoadFunc =
148 M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
149 if ((!TypeTestFunc || TypeTestFunc->use_empty()) &&
150 (!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
151 return false;
152
153 bool Changed = false;
154 for (auto I = M.global_begin(); I != M.global_end();) {
155 GlobalVariable &GV = *I;
156 ++I;
157 Changed |= splitGlobal(GV);
158 }
159 return Changed;
160 }
161
162 namespace {
163
164 struct GlobalSplit : public ModulePass {
165 static char ID;
166
GlobalSplit__anon28937fe50111::GlobalSplit167 GlobalSplit() : ModulePass(ID) {
168 initializeGlobalSplitPass(*PassRegistry::getPassRegistry());
169 }
170
runOnModule__anon28937fe50111::GlobalSplit171 bool runOnModule(Module &M) override {
172 if (skipModule(M))
173 return false;
174
175 return splitGlobals(M);
176 }
177 };
178
179 } // end anonymous namespace
180
181 char GlobalSplit::ID = 0;
182
183 INITIALIZE_PASS(GlobalSplit, "globalsplit", "Global splitter", false, false)
184
createGlobalSplitPass()185 ModulePass *llvm::createGlobalSplitPass() {
186 return new GlobalSplit;
187 }
188
run(Module & M,ModuleAnalysisManager & AM)189 PreservedAnalyses GlobalSplitPass::run(Module &M, ModuleAnalysisManager &AM) {
190 if (!splitGlobals(M))
191 return PreservedAnalyses::all();
192 return PreservedAnalyses::none();
193 }
194