1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
| //===- CtorUtils.cpp - Helpers for working with global_ctors ----*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines functions that are used to process llvm.global_ctors.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/CtorUtils.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "ctor_utils"
using namespace llvm;
/// Given a specified llvm.global_ctors list, remove the listed elements.
static void removeGlobalCtors(GlobalVariable *GCL, const BitVector &CtorsToRemove) {
// Filter out the initializer elements to remove.
ConstantArray *OldCA = cast<ConstantArray>(GCL->getInitializer());
SmallVector<Constant *, 10> CAList;
for (unsigned I = 0, E = OldCA->getNumOperands(); I < E; ++I)
if (!CtorsToRemove.test(I))
CAList.push_back(OldCA->getOperand(I));
// Create the new array initializer.
ArrayType *ATy =
ArrayType::get(OldCA->getType()->getElementType(), CAList.size());
Constant *CA = ConstantArray::get(ATy, CAList);
// If we didn't change the number of elements, don't create a new GV.
if (CA->getType() == OldCA->getType()) {
GCL->setInitializer(CA);
return;
}
// Create the new global and insert it next to the existing list.
GlobalVariable *NGV =
new GlobalVariable(CA->getType(), GCL->isConstant(), GCL->getLinkage(),
CA, "", GCL->getThreadLocalMode());
GCL->getParent()->getGlobalList().insert(GCL->getIterator(), NGV);
NGV->takeName(GCL);
// Nuke the old list, replacing any uses with the new one.
if (!GCL->use_empty()) {
Constant *V = NGV;
if (V->getType() != GCL->getType())
V = ConstantExpr::getBitCast(V, GCL->getType());
GCL->replaceAllUsesWith(V);
}
GCL->eraseFromParent();
}
/// Given a llvm.global_ctors list that we can understand,
/// return a list of the functions and null terminator as a vector.
static std::vector<Function *> parseGlobalCtors(GlobalVariable *GV) {
if (GV->getInitializer()->isNullValue())
return std::vector<Function *>();
ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
std::vector<Function *> Result;
Result.reserve(CA->getNumOperands());
for (auto &V : CA->operands()) {
ConstantStruct *CS = cast<ConstantStruct>(V);
Result.push_back(dyn_cast<Function>(CS->getOperand(1)));
}
return Result;
}
/// Find the llvm.global_ctors list, verifying that all initializers have an
/// init priority of 65535.
static GlobalVariable *findGlobalCtors(Module &M) {
GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
if (!GV)
return nullptr;
// Verify that the initializer is simple enough for us to handle. We are
// only allowed to optimize the initializer if it is unique.
if (!GV->hasUniqueInitializer())
return nullptr;
if (isa<ConstantAggregateZero>(GV->getInitializer()))
return GV;
ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
for (auto &V : CA->operands()) {
if (isa<ConstantAggregateZero>(V))
continue;
ConstantStruct *CS = cast<ConstantStruct>(V);
if (isa<ConstantPointerNull>(CS->getOperand(1)))
continue;
// Must have a function or null ptr.
if (!isa<Function>(CS->getOperand(1)))
return nullptr;
// Init priority must be standard.
ConstantInt *CI = cast<ConstantInt>(CS->getOperand(0));
if (CI->getZExtValue() != 65535)
return nullptr;
}
return GV;
}
/// Call "ShouldRemove" for every entry in M's global_ctor list and remove the
/// entries for which it returns true. Return true if anything changed.
bool llvm::optimizeGlobalCtorsList(
Module &M, function_ref<bool(Function *)> ShouldRemove) {
GlobalVariable *GlobalCtors = findGlobalCtors(M);
if (!GlobalCtors)
return false;
std::vector<Function *> Ctors = parseGlobalCtors(GlobalCtors);
if (Ctors.empty())
return false;
bool MadeChange = false;
// Loop over global ctors, optimizing them when we can.
unsigned NumCtors = Ctors.size();
BitVector CtorsToRemove(NumCtors);
for (unsigned i = 0; i != Ctors.size() && NumCtors > 0; ++i) {
Function *F = Ctors[i];
// Found a null terminator in the middle of the list, prune off the rest of
// the list.
if (!F)
continue;
LLVM_DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n");
// We cannot simplify external ctor functions.
if (F->empty())
continue;
// If we can evaluate the ctor at compile time, do.
if (ShouldRemove(F)) {
Ctors[i] = nullptr;
CtorsToRemove.set(i);
NumCtors--;
MadeChange = true;
continue;
}
}
if (!MadeChange)
return false;
removeGlobalCtors(GlobalCtors, CtorsToRemove);
return true;
}
|