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/Transforms/IPO.h"
33 #include <algorithm>
34 #include <cassert>
35 #include <utility>
36 
37 using namespace llvm;
38 
39 #define DEBUG_TYPE "constmerge"
40 
41 STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
42 
43 /// Find values that are marked as llvm.used.
44 static void FindUsedValues(GlobalVariable *LLVMUsed,
45                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
46   if (!LLVMUsed) return;
47   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
48 
49   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
50     Value *Operand = Inits->getOperand(i)->stripPointerCasts();
51     GlobalValue *GV = cast<GlobalValue>(Operand);
52     UsedValues.insert(GV);
53   }
54 }
55 
56 // True if A is better than B.
57 static bool IsBetterCanonical(const GlobalVariable &A,
58                               const GlobalVariable &B) {
59   if (!A.hasLocalLinkage() && B.hasLocalLinkage())
60     return true;
61 
62   if (A.hasLocalLinkage() && !B.hasLocalLinkage())
63     return false;
64 
65   return A.hasGlobalUnnamedAddr();
66 }
67 
68 static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
69   SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
70   GV->getAllMetadata(MDs);
71   for (const auto &V : MDs)
72     if (V.first != LLVMContext::MD_dbg)
73       return true;
74   return false;
75 }
76 
77 static void copyDebugLocMetadata(const GlobalVariable *From,
78                                  GlobalVariable *To) {
79   SmallVector<DIGlobalVariableExpression *, 1> MDs;
80   From->getDebugInfo(MDs);
81   for (auto *MD : MDs)
82     To->addDebugInfo(MD);
83 }
84 
85 static Align getAlign(GlobalVariable *GV) {
86   return GV->getAlign().value_or(
87       GV->getParent()->getDataLayout().getPreferredAlign(GV));
88 }
89 
90 static bool
91 isUnmergeableGlobal(GlobalVariable *GV,
92                     const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
93   // Only process constants with initializers in the default address space.
94   return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
95          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
96          // Don't touch thread-local variables.
97          GV->isThreadLocal() ||
98          // Don't touch values marked with attribute(used).
99          UsedGlobals.count(GV);
100 }
101 
102 enum class CanMerge { No, Yes };
103 static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
104   if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
105     return CanMerge::No;
106   if (hasMetadataOtherThanDebugLoc(Old))
107     return CanMerge::No;
108   assert(!hasMetadataOtherThanDebugLoc(New));
109   if (!Old->hasGlobalUnnamedAddr())
110     New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
111   return CanMerge::Yes;
112 }
113 
114 static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
115   Constant *NewConstant = New;
116 
117   LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
118                     << New->getName() << "\n");
119 
120   // Bump the alignment if necessary.
121   if (Old->getAlign() || New->getAlign())
122     New->setAlignment(std::max(getAlign(Old), getAlign(New)));
123 
124   copyDebugLocMetadata(Old, New);
125   Old->replaceAllUsesWith(NewConstant);
126 
127   // Delete the global value from the module.
128   assert(Old->hasLocalLinkage() &&
129          "Refusing to delete an externally visible global variable.");
130   Old->eraseFromParent();
131 }
132 
133 static bool mergeConstants(Module &M) {
134   // Find all the globals that are marked "used".  These cannot be merged.
135   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
136   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
137   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
138 
139   // Map unique constants to globals.
140   DenseMap<Constant *, GlobalVariable *> CMap;
141 
142   SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
143       SameContentReplacements;
144 
145   size_t ChangesMade = 0;
146   size_t OldChangesMade = 0;
147 
148   // Iterate constant merging while we are still making progress.  Merging two
149   // constants together may allow us to merge other constants together if the
150   // second level constants have initializers which point to the globals that
151   // were just merged.
152   while (true) {
153     // Find the canonical constants others will be merged with.
154     for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
155       // If this GV is dead, remove it.
156       GV.removeDeadConstantUsers();
157       if (GV.use_empty() && GV.hasLocalLinkage()) {
158         GV.eraseFromParent();
159         ++ChangesMade;
160         continue;
161       }
162 
163       if (isUnmergeableGlobal(&GV, UsedGlobals))
164         continue;
165 
166       // This transformation is legal for weak ODR globals in the sense it
167       // doesn't change semantics, but we really don't want to perform it
168       // anyway; it's likely to pessimize code generation, and some tools
169       // (like the Darwin linker in cases involving CFString) don't expect it.
170       if (GV.isWeakForLinker())
171         continue;
172 
173       // Don't touch globals with metadata other then !dbg.
174       if (hasMetadataOtherThanDebugLoc(&GV))
175         continue;
176 
177       Constant *Init = GV.getInitializer();
178 
179       // Check to see if the initializer is already known.
180       GlobalVariable *&Slot = CMap[Init];
181 
182       // If this is the first constant we find or if the old one is local,
183       // replace with the current one. If the current is externally visible
184       // it cannot be replace, but can be the canonical constant we merge with.
185       bool FirstConstantFound = !Slot;
186       if (FirstConstantFound || IsBetterCanonical(GV, *Slot)) {
187         Slot = &GV;
188         LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV.getName()
189                           << (FirstConstantFound ? "\n" : " (updated)\n"));
190       }
191     }
192 
193     // Identify all globals that can be merged together, filling in the
194     // SameContentReplacements vector. We cannot do the replacement in this pass
195     // because doing so may cause initializers of other globals to be rewritten,
196     // invalidating the Constant* pointers in CMap.
197     for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
198       if (isUnmergeableGlobal(&GV, UsedGlobals))
199         continue;
200 
201       // We can only replace constant with local linkage.
202       if (!GV.hasLocalLinkage())
203         continue;
204 
205       Constant *Init = GV.getInitializer();
206 
207       // Check to see if the initializer is already known.
208       auto Found = CMap.find(Init);
209       if (Found == CMap.end())
210         continue;
211 
212       GlobalVariable *Slot = Found->second;
213       if (Slot == &GV)
214         continue;
215 
216       if (makeMergeable(&GV, Slot) == CanMerge::No)
217         continue;
218 
219       // Make all uses of the duplicate constant use the canonical version.
220       LLVM_DEBUG(dbgs() << "Will replace: @" << GV.getName() << " -> @"
221                         << Slot->getName() << "\n");
222       SameContentReplacements.push_back(std::make_pair(&GV, Slot));
223     }
224 
225     // Now that we have figured out which replacements must be made, do them all
226     // now.  This avoid invalidating the pointers in CMap, which are unneeded
227     // now.
228     for (unsigned i = 0, e = SameContentReplacements.size(); i != e; ++i) {
229       GlobalVariable *Old = SameContentReplacements[i].first;
230       GlobalVariable *New = SameContentReplacements[i].second;
231       replace(M, Old, New);
232       ++ChangesMade;
233       ++NumIdenticalMerged;
234     }
235 
236     if (ChangesMade == OldChangesMade)
237       break;
238     OldChangesMade = ChangesMade;
239 
240     SameContentReplacements.clear();
241     CMap.clear();
242   }
243 
244   return ChangesMade;
245 }
246 
247 PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
248   if (!mergeConstants(M))
249     return PreservedAnalyses::all();
250   return PreservedAnalyses::none();
251 }
252