1//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
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 defines the interface to a pass that merges duplicate global
10// constants together into a single constant that is shared. This is useful
11// because some passes (ie TraceValues) insert a lot of string constants into
12// the program, regardless of whether or not an existing string is available.
13//
14// Algorithm: ConstantMerge is designed to build up a map of available constants
15// and eliminate duplicates when it is initialized.
16//
17//===----------------------------------------------------------------------===//
18
19#include "llvm/Transforms/IPO/ConstantMerge.h"
20#include "llvm/ADT/DenseMap.h"
21#include "llvm/ADT/SmallPtrSet.h"
22#include "llvm/ADT/SmallVector.h"
23#include "llvm/ADT/Statistic.h"
24#include "llvm/IR/Constants.h"
25#include "llvm/IR/DataLayout.h"
26#include "llvm/IR/DerivedTypes.h"
27#include "llvm/IR/GlobalValue.h"
28#include "llvm/IR/GlobalVariable.h"
29#include "llvm/IR/LLVMContext.h"
30#include "llvm/IR/Module.h"
31#include "llvm/Support/Casting.h"
32#include "llvm/Support/Debug.h"
33#include "llvm/Transforms/IPO.h"
34#include <algorithm>
35#include <cassert>
36#include <utility>
37
38using namespace llvm;
39
40#define DEBUG_TYPE "constmerge"
41
42STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
43
44/// Find values that are marked as llvm.used.
45static void FindUsedValues(GlobalVariable *LLVMUsed,
46 SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
47 if (!LLVMUsed) return;
48 ConstantArray *Inits = cast<ConstantArray>(Val: LLVMUsed->getInitializer());
49
50 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
51 Value *Operand = Inits->getOperand(i_nocapture: i)->stripPointerCasts();
52 GlobalValue *GV = cast<GlobalValue>(Val: Operand);
53 UsedValues.insert(Ptr: GV);
54 }
55}
56
57// True if A is better than B.
58static bool IsBetterCanonical(const GlobalVariable &A,
59 const GlobalVariable &B) {
60 if (!A.hasLocalLinkage() && B.hasLocalLinkage())
61 return true;
62
63 if (A.hasLocalLinkage() && !B.hasLocalLinkage())
64 return false;
65
66 return A.hasGlobalUnnamedAddr();
67}
68
69static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
70 SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
71 GV->getAllMetadata(MDs);
72 for (const auto &V : MDs)
73 if (V.first != LLVMContext::MD_dbg)
74 return true;
75 return false;
76}
77
78static void copyDebugLocMetadata(const GlobalVariable *From,
79 GlobalVariable *To) {
80 SmallVector<DIGlobalVariableExpression *, 1> MDs;
81 From->getDebugInfo(GVs&: MDs);
82 for (auto *MD : MDs)
83 To->addDebugInfo(GV: MD);
84}
85
86static Align getAlign(GlobalVariable *GV) {
87 return GV->getAlign().value_or(
88 u: GV->getDataLayout().getPreferredAlign(GV));
89}
90
91static bool
92isUnmergeableGlobal(GlobalVariable *GV,
93 const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
94 // Only process constants with initializers in the default address space.
95 return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
96 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
97 // Don't touch thread-local variables.
98 GV->isThreadLocal() ||
99 // Don't touch values marked with attribute(used).
100 UsedGlobals.count(Ptr: GV);
101}
102
103enum class CanMerge { No, Yes };
104static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
105 if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
106 return CanMerge::No;
107 if (hasMetadataOtherThanDebugLoc(GV: Old))
108 return CanMerge::No;
109 assert(!hasMetadataOtherThanDebugLoc(New));
110 if (!Old->hasGlobalUnnamedAddr())
111 New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
112 return CanMerge::Yes;
113}
114
115static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
116 Constant *NewConstant = New;
117
118 LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
119 << New->getName() << "\n");
120
121 // Bump the alignment if necessary.
122 if (Old->getAlign() || New->getAlign())
123 New->setAlignment(std::max(a: getAlign(GV: Old), b: getAlign(GV: New)));
124
125 copyDebugLocMetadata(From: Old, To: New);
126 Old->replaceAllUsesWith(V: NewConstant);
127
128 // Delete the global value from the module.
129 assert(Old->hasLocalLinkage() &&
130 "Refusing to delete an externally visible global variable.");
131 Old->eraseFromParent();
132}
133
134static bool mergeConstants(Module &M) {
135 // Find all the globals that are marked "used". These cannot be merged.
136 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
137 FindUsedValues(LLVMUsed: M.getGlobalVariable(Name: "llvm.used"), UsedValues&: UsedGlobals);
138 FindUsedValues(LLVMUsed: M.getGlobalVariable(Name: "llvm.compiler.used"), UsedValues&: UsedGlobals);
139
140 // Map unique constants to globals.
141 DenseMap<Constant *, GlobalVariable *> CMap;
142
143 SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
144 SameContentReplacements;
145
146 size_t ChangesMade = 0;
147 size_t OldChangesMade = 0;
148
149 // Iterate constant merging while we are still making progress. Merging two
150 // constants together may allow us to merge other constants together if the
151 // second level constants have initializers which point to the globals that
152 // were just merged.
153 while (true) {
154 // Find the canonical constants others will be merged with.
155 for (GlobalVariable &GV : llvm::make_early_inc_range(Range: M.globals())) {
156 // If this GV is dead, remove it.
157 GV.removeDeadConstantUsers();
158 if (GV.use_empty() && GV.hasLocalLinkage()) {
159 GV.eraseFromParent();
160 ++ChangesMade;
161 continue;
162 }
163
164 if (isUnmergeableGlobal(GV: &GV, UsedGlobals))
165 continue;
166
167 // This transformation is legal for weak ODR globals in the sense it
168 // doesn't change semantics, but we really don't want to perform it
169 // anyway; it's likely to pessimize code generation, and some tools
170 // (like the Darwin linker in cases involving CFString) don't expect it.
171 if (GV.isWeakForLinker())
172 continue;
173
174 // Don't touch globals with metadata other then !dbg.
175 if (hasMetadataOtherThanDebugLoc(GV: &GV))
176 continue;
177
178 Constant *Init = GV.getInitializer();
179
180 // Check to see if the initializer is already known.
181 GlobalVariable *&Slot = CMap[Init];
182
183 // If this is the first constant we find or if the old one is local,
184 // replace with the current one. If the current is externally visible
185 // it cannot be replace, but can be the canonical constant we merge with.
186 bool FirstConstantFound = !Slot;
187 if (FirstConstantFound || IsBetterCanonical(A: GV, B: *Slot)) {
188 Slot = &GV;
189 LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV.getName()
190 << (FirstConstantFound ? "\n" : " (updated)\n"));
191 }
192 }
193
194 // Identify all globals that can be merged together, filling in the
195 // SameContentReplacements vector. We cannot do the replacement in this pass
196 // because doing so may cause initializers of other globals to be rewritten,
197 // invalidating the Constant* pointers in CMap.
198 for (GlobalVariable &GV : llvm::make_early_inc_range(Range: M.globals())) {
199 if (isUnmergeableGlobal(GV: &GV, UsedGlobals))
200 continue;
201
202 // We can only replace constant with local linkage.
203 if (!GV.hasLocalLinkage())
204 continue;
205
206 Constant *Init = GV.getInitializer();
207
208 // Check to see if the initializer is already known.
209 auto Found = CMap.find(Val: Init);
210 if (Found == CMap.end())
211 continue;
212
213 GlobalVariable *Slot = Found->second;
214 if (Slot == &GV)
215 continue;
216
217 if (makeMergeable(Old: &GV, New: Slot) == CanMerge::No)
218 continue;
219
220 // Make all uses of the duplicate constant use the canonical version.
221 LLVM_DEBUG(dbgs() << "Will replace: @" << GV.getName() << " -> @"
222 << Slot->getName() << "\n");
223 SameContentReplacements.push_back(Elt: std::make_pair(x: &GV, y&: Slot));
224 }
225
226 // Now that we have figured out which replacements must be made, do them all
227 // now. This avoid invalidating the pointers in CMap, which are unneeded
228 // now.
229 for (const auto &[Old, New] : SameContentReplacements) {
230 replace(M, Old, New);
231 ++ChangesMade;
232 ++NumIdenticalMerged;
233 }
234
235 if (ChangesMade == OldChangesMade)
236 break;
237 OldChangesMade = ChangesMade;
238
239 SameContentReplacements.clear();
240 CMap.clear();
241 }
242
243 return ChangesMade;
244}
245
246PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
247 if (!mergeConstants(M))
248 return PreservedAnalyses::all();
249 return PreservedAnalyses::none();
250}
251