1//===- RelLookupTableConverterPass - Rel Table Conv -----------------------===//
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 implements relative lookup table converter that converts
10// lookup tables to relative lookup tables to make them PIC-friendly.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Transforms/Utils/RelLookupTableConverter.h"
15#include "llvm/Analysis/ConstantFolding.h"
16#include "llvm/Analysis/TargetTransformInfo.h"
17#include "llvm/IR/BasicBlock.h"
18#include "llvm/IR/IRBuilder.h"
19#include "llvm/IR/Instructions.h"
20#include "llvm/IR/Module.h"
21
22using namespace llvm;
23
24static bool shouldConvertToRelLookupTable(Module &M, GlobalVariable &GV) {
25 // If lookup table has more than one user,
26 // do not generate a relative lookup table.
27 // This is to simplify the analysis that needs to be done for this pass.
28 // TODO: Add support for lookup tables with multiple uses.
29 // For ex, this can happen when a function that uses a lookup table gets
30 // inlined into multiple call sites.
31 if (!GV.hasInitializer() ||
32 !GV.isConstant() ||
33 !GV.hasOneUse())
34 return false;
35
36 GetElementPtrInst *GEP =
37 dyn_cast<GetElementPtrInst>(Val: GV.use_begin()->getUser());
38 if (!GEP || !GEP->hasOneUse() ||
39 GV.getValueType() != GEP->getSourceElementType())
40 return false;
41
42 LoadInst *Load = dyn_cast<LoadInst>(Val: GEP->use_begin()->getUser());
43 if (!Load || !Load->hasOneUse() ||
44 Load->getType() != GEP->getResultElementType())
45 return false;
46
47 // If the original lookup table does not have local linkage and is
48 // not dso_local, do not generate a relative lookup table.
49 // This optimization creates a relative lookup table that consists of
50 // offsets between the start of the lookup table and its elements.
51 // To be able to generate these offsets, relative lookup table and
52 // its elements should have internal linkage and be dso_local, which means
53 // that they should resolve to symbols within the same linkage unit.
54 if (!GV.hasLocalLinkage() ||
55 !GV.isDSOLocal() ||
56 !GV.isImplicitDSOLocal())
57 return false;
58
59 ConstantArray *Array = dyn_cast<ConstantArray>(Val: GV.getInitializer());
60 if (!Array)
61 return false;
62
63 // If values are not 64-bit pointers, do not generate a relative lookup table.
64 const DataLayout &DL = M.getDataLayout();
65 Type *ElemType = Array->getType()->getElementType();
66 if (!ElemType->isPointerTy() || DL.getPointerTypeSizeInBits(ElemType) != 64)
67 return false;
68
69 for (const Use &Op : Array->operands()) {
70 Constant *ConstOp = cast<Constant>(Val: &Op);
71 GlobalValue *GVOp;
72 APInt Offset;
73
74 // If an operand is not a constant offset from a lookup table,
75 // do not generate a relative lookup table.
76 if (!IsConstantOffsetFromGlobal(C: ConstOp, GV&: GVOp, Offset, DL))
77 return false;
78
79 // If operand is mutable, do not generate a relative lookup table.
80 auto *GlovalVarOp = dyn_cast<GlobalVariable>(Val: GVOp);
81 if (!GlovalVarOp || !GlovalVarOp->isConstant())
82 return false;
83
84 if (!GlovalVarOp->hasLocalLinkage() ||
85 !GlovalVarOp->isDSOLocal() ||
86 !GlovalVarOp->isImplicitDSOLocal())
87 return false;
88 }
89
90 return true;
91}
92
93static GlobalVariable *createRelLookupTable(Function &Func,
94 GlobalVariable &LookupTable) {
95 Module &M = *Func.getParent();
96 ConstantArray *LookupTableArr =
97 cast<ConstantArray>(Val: LookupTable.getInitializer());
98 unsigned NumElts = LookupTableArr->getType()->getNumElements();
99 ArrayType *IntArrayTy =
100 ArrayType::get(ElementType: Type::getInt32Ty(C&: M.getContext()), NumElements: NumElts);
101
102 GlobalVariable *RelLookupTable = new GlobalVariable(
103 M, IntArrayTy, LookupTable.isConstant(), LookupTable.getLinkage(),
104 nullptr, "reltable." + Func.getName(), &LookupTable,
105 LookupTable.getThreadLocalMode(), LookupTable.getAddressSpace(),
106 LookupTable.isExternallyInitialized());
107
108 uint64_t Idx = 0;
109 SmallVector<Constant *, 64> RelLookupTableContents(NumElts);
110
111 for (Use &Operand : LookupTableArr->operands()) {
112 Constant *Element = cast<Constant>(Val&: Operand);
113 Type *IntPtrTy = M.getDataLayout().getIntPtrType(C&: M.getContext());
114 Constant *Base = llvm::ConstantExpr::getPtrToInt(C: RelLookupTable, Ty: IntPtrTy);
115 Constant *Target = llvm::ConstantExpr::getPtrToInt(C: Element, Ty: IntPtrTy);
116 Constant *Sub = llvm::ConstantExpr::getSub(C1: Target, C2: Base);
117 Constant *RelOffset =
118 llvm::ConstantExpr::getTrunc(C: Sub, Ty: Type::getInt32Ty(C&: M.getContext()));
119 RelLookupTableContents[Idx++] = RelOffset;
120 }
121
122 Constant *Initializer =
123 ConstantArray::get(T: IntArrayTy, V: RelLookupTableContents);
124 RelLookupTable->setInitializer(Initializer);
125 RelLookupTable->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
126 RelLookupTable->setAlignment(llvm::Align(4));
127 return RelLookupTable;
128}
129
130static void convertToRelLookupTable(GlobalVariable &LookupTable) {
131 GetElementPtrInst *GEP =
132 cast<GetElementPtrInst>(Val: LookupTable.use_begin()->getUser());
133 LoadInst *Load = cast<LoadInst>(Val: GEP->use_begin()->getUser());
134
135 Module &M = *LookupTable.getParent();
136 BasicBlock *BB = GEP->getParent();
137 IRBuilder<> Builder(BB);
138 Function &Func = *BB->getParent();
139
140 // Generate an array that consists of relative offsets.
141 GlobalVariable *RelLookupTable = createRelLookupTable(Func, LookupTable);
142
143 // Place new instruction sequence before GEP.
144 Builder.SetInsertPoint(GEP);
145 Value *Index = GEP->getOperand(i_nocapture: 2);
146 IntegerType *IntTy = cast<IntegerType>(Val: Index->getType());
147 Value *Offset =
148 Builder.CreateShl(LHS: Index, RHS: ConstantInt::get(Ty: IntTy, V: 2), Name: "reltable.shift");
149
150 // Insert the call to load.relative intrinsic before LOAD.
151 // GEP might not be immediately followed by a LOAD, like it can be hoisted
152 // outside the loop or another instruction might be inserted them in between.
153 Builder.SetInsertPoint(Load);
154 Function *LoadRelIntrinsic = llvm::Intrinsic::getDeclaration(
155 M: &M, Intrinsic::id: load_relative, Tys: {Index->getType()});
156
157 // Create a call to load.relative intrinsic that computes the target address
158 // by adding base address (lookup table address) and relative offset.
159 Value *Result = Builder.CreateCall(Callee: LoadRelIntrinsic, Args: {RelLookupTable, Offset},
160 Name: "reltable.intrinsic");
161
162 // Replace load instruction with the new generated instruction sequence.
163 Load->replaceAllUsesWith(V: Result);
164 // Remove Load and GEP instructions.
165 Load->eraseFromParent();
166 GEP->eraseFromParent();
167}
168
169// Convert lookup tables to relative lookup tables in the module.
170static bool convertToRelativeLookupTables(
171 Module &M, function_ref<TargetTransformInfo &(Function &)> GetTTI) {
172 for (Function &F : M) {
173 if (F.isDeclaration())
174 continue;
175
176 // Check if we have a target that supports relative lookup tables.
177 if (!GetTTI(F).shouldBuildRelLookupTables())
178 return false;
179
180 // We assume that the result is independent of the checked function.
181 break;
182 }
183
184 bool Changed = false;
185
186 for (GlobalVariable &GV : llvm::make_early_inc_range(Range: M.globals())) {
187 if (!shouldConvertToRelLookupTable(M, GV))
188 continue;
189
190 convertToRelLookupTable(LookupTable&: GV);
191
192 // Remove the original lookup table.
193 GV.eraseFromParent();
194
195 Changed = true;
196 }
197
198 return Changed;
199}
200
201PreservedAnalyses RelLookupTableConverterPass::run(Module &M,
202 ModuleAnalysisManager &AM) {
203 FunctionAnalysisManager &FAM =
204 AM.getResult<FunctionAnalysisManagerModuleProxy>(IR&: M).getManager();
205
206 auto GetTTI = [&](Function &F) -> TargetTransformInfo & {
207 return FAM.getResult<TargetIRAnalysis>(IR&: F);
208 };
209
210 if (!convertToRelativeLookupTables(M, GetTTI))
211 return PreservedAnalyses::all();
212
213 PreservedAnalyses PA;
214 PA.preserveSet<CFGAnalyses>();
215 return PA;
216}
217

source code of llvm/lib/Transforms/Utils/RelLookupTableConverter.cpp