LLVM 19.0.0git
ConstantMerge.cpp
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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
20#include "llvm/ADT/DenseMap.h"
23#include "llvm/ADT/Statistic.h"
24#include "llvm/IR/Constants.h"
25#include "llvm/IR/DataLayout.h"
27#include "llvm/IR/GlobalValue.h"
29#include "llvm/IR/LLVMContext.h"
30#include "llvm/IR/Module.h"
32#include "llvm/Transforms/IPO.h"
33#include <algorithm>
34#include <cassert>
35#include <utility>
36
37using namespace llvm;
38
39#define DEBUG_TYPE "constmerge"
40
41STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
42
43/// Find values that are marked as llvm.used.
44static void FindUsedValues(GlobalVariable *LLVMUsed,
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.
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
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
78 GlobalVariable *To) {
80 From->getDebugInfo(MDs);
81 for (auto *MD : MDs)
82 To->addDebugInfo(MD);
83}
84
86 return GV->getAlign().value_or(
88}
89
90static bool
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
102enum class CanMerge { No, Yes };
104 if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
105 return CanMerge::No;
107 return CanMerge::No;
109 if (!Old->hasGlobalUnnamedAddr())
110 New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
111 return CanMerge::Yes;
112}
113
114static 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
133static bool mergeConstants(Module &M) {
134 // Find all the globals that are marked "used". These cannot be merged.
136 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
137 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
138
139 // Map unique constants to globals.
141
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.
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
248 if (!mergeConstants(M))
249 return PreservedAnalyses::all();
251}
BlockVerifier::State From
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
static GCRegistry::Add< ErlangGC > A("erlang", "erlang-compatible garbage collector")
static bool IsBetterCanonical(const GlobalVariable &A, const GlobalVariable &B)
static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV)
static void copyDebugLocMetadata(const GlobalVariable *From, GlobalVariable *To)
static bool mergeConstants(Module &M)
static Align getAlign(GlobalVariable *GV)
static bool isUnmergeableGlobal(GlobalVariable *GV, const SmallPtrSetImpl< const GlobalValue * > &UsedGlobals)
static void FindUsedValues(GlobalVariable *LLVMUsed, SmallPtrSetImpl< const GlobalValue * > &UsedValues)
Find values that are marked as llvm.used.
CanMerge
static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New)
static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New)
This file contains the declarations for the subclasses of Constant, which represent the different fla...
#define LLVM_DEBUG(X)
Definition: Debug.h:101
This file defines the DenseMap class.
Module.h This file contains the declarations for the Module class.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file defines the SmallPtrSet class.
This file defines the SmallVector class.
This file defines the 'Statistic' class, which is designed to be an easy way to expose various metric...
#define STATISTIC(VARNAME, DESC)
Definition: Statistic.h:167
A container for analyses that lazily runs them and caches their results.
Definition: PassManager.h:321
ConstantArray - Constant Array Declarations.
Definition: Constants.h:424
PreservedAnalyses run(Module &M, ModuleAnalysisManager &)
This is an important base class in LLVM.
Definition: Constant.h:41
Align getPreferredAlign(const GlobalVariable *GV) const
Returns the preferred alignment of the specified global.
iterator find(const_arg_type_t< KeyT > Val)
Definition: DenseMap.h:155
iterator end()
Definition: DenseMap.h:84
MaybeAlign getAlign() const
Returns the alignment of the given variable or function.
Definition: GlobalObject.h:80
void getAllMetadata(SmallVectorImpl< std::pair< unsigned, MDNode * > > &MDs) const
Appends all metadata attached to this value to MDs, sorting by KindID.
Definition: Metadata.cpp:1477
bool hasSection() const
Check if this global has a custom object file section.
Definition: GlobalObject.h:110
bool isThreadLocal() const
If the value is "Thread Local", its value isn't shared by the threads.
Definition: GlobalValue.h:262
bool hasLocalLinkage() const
Definition: GlobalValue.h:527
Module * getParent()
Get the module that this global value is contained inside of...
Definition: GlobalValue.h:655
PointerType * getType() const
Global values are always pointers.
Definition: GlobalValue.h:293
bool hasGlobalUnnamedAddr() const
Definition: GlobalValue.h:214
const Constant * getInitializer() const
getInitializer - Return the initializer for this global variable.
bool isConstant() const
If the value is a global constant, its value is immutable throughout the runtime execution of the pro...
void eraseFromParent()
eraseFromParent - This method unlinks 'this' from the containing module and deletes it.
Definition: Globals.cpp:477
void addDebugInfo(DIGlobalVariableExpression *GV)
Attach a DIGlobalVariableExpression.
Definition: Metadata.cpp:1843
bool hasDefinitiveInitializer() const
hasDefinitiveInitializer - Whether the global variable has an initializer, and any other instances of...
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.h:293
unsigned getAddressSpace() const
Return the address space of the Pointer type.
Definition: DerivedTypes.h:679
A set of analyses that are preserved following a run of a transformation pass.
Definition: Analysis.h:109
static PreservedAnalyses none()
Convenience factory function for the empty preserved set.
Definition: Analysis.h:112
static PreservedAnalyses all()
Construct a special preserved set that preserves all passes.
Definition: Analysis.h:115
A templated base class for SmallPtrSet which provides the typesafe interface that is common across al...
Definition: SmallPtrSet.h:321
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:360
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:342
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements.
Definition: SmallPtrSet.h:427
size_t size() const
Definition: SmallVector.h:91
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
Value * getOperand(unsigned i) const
Definition: User.h:169
unsigned getNumOperands() const
Definition: User.h:191
LLVM Value Representation.
Definition: Value.h:74
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:534
const Value * stripPointerCasts() const
Strip off pointer casts, all-zero GEPs and address space casts.
Definition: Value.cpp:693
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:309
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
iterator_range< early_inc_iterator_impl< detail::IterOfRange< RangeT > > > make_early_inc_range(RangeT &&Range)
Make a range that does early increment to allow mutation of the underlying range without disrupting i...
Definition: STLExtras.h:656
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
This struct is a compact representation of a valid (non-zero power of two) alignment.
Definition: Alignment.h:39