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//===- StripSymbols.cpp - Strip symbols and debug info from a module ------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The StripSymbols transformation implements code stripping. Specifically, it
// can delete:
//
// * names for virtual registers
// * symbols for internal globals and functions
// * debug information
//
// Note that this transformation makes code much less readable, so it should
// only be used in situations where the 'strip' utility would be used, such as
// reducing code size or making it harder to reverse engineer code.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/StripSymbols.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/TypeFinder.h"
#include "llvm/IR/ValueSymbolTable.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
namespace {
class StripSymbols : public ModulePass {
bool OnlyDebugInfo;
public:
static char ID; // Pass identification, replacement for typeid
explicit StripSymbols(bool ODI = false)
: ModulePass(ID), OnlyDebugInfo(ODI) {
initializeStripSymbolsPass(*PassRegistry::getPassRegistry());
}
bool runOnModule(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
};
class StripNonDebugSymbols : public ModulePass {
public:
static char ID; // Pass identification, replacement for typeid
explicit StripNonDebugSymbols()
: ModulePass(ID) {
initializeStripNonDebugSymbolsPass(*PassRegistry::getPassRegistry());
}
bool runOnModule(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
};
class StripDebugDeclare : public ModulePass {
public:
static char ID; // Pass identification, replacement for typeid
explicit StripDebugDeclare()
: ModulePass(ID) {
initializeStripDebugDeclarePass(*PassRegistry::getPassRegistry());
}
bool runOnModule(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
};
class StripDeadDebugInfo : public ModulePass {
public:
static char ID; // Pass identification, replacement for typeid
explicit StripDeadDebugInfo()
: ModulePass(ID) {
initializeStripDeadDebugInfoPass(*PassRegistry::getPassRegistry());
}
bool runOnModule(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
};
}
char StripSymbols::ID = 0;
INITIALIZE_PASS(StripSymbols, "strip",
"Strip all symbols from a module", false, false)
ModulePass *llvm::createStripSymbolsPass(bool OnlyDebugInfo) {
return new StripSymbols(OnlyDebugInfo);
}
char StripNonDebugSymbols::ID = 0;
INITIALIZE_PASS(StripNonDebugSymbols, "strip-nondebug",
"Strip all symbols, except dbg symbols, from a module",
false, false)
ModulePass *llvm::createStripNonDebugSymbolsPass() {
return new StripNonDebugSymbols();
}
char StripDebugDeclare::ID = 0;
INITIALIZE_PASS(StripDebugDeclare, "strip-debug-declare",
"Strip all llvm.dbg.declare intrinsics", false, false)
ModulePass *llvm::createStripDebugDeclarePass() {
return new StripDebugDeclare();
}
char StripDeadDebugInfo::ID = 0;
INITIALIZE_PASS(StripDeadDebugInfo, "strip-dead-debug-info",
"Strip debug info for unused symbols", false, false)
ModulePass *llvm::createStripDeadDebugInfoPass() {
return new StripDeadDebugInfo();
}
/// OnlyUsedBy - Return true if V is only used by Usr.
static bool OnlyUsedBy(Value *V, Value *Usr) {
for (User *U : V->users())
if (U != Usr)
return false;
return true;
}
static void RemoveDeadConstant(Constant *C) {
assert(C->use_empty() && "Constant is not dead!");
SmallPtrSet<Constant*, 4> Operands;
for (Value *Op : C->operands())
if (OnlyUsedBy(Op, C))
Operands.insert(cast<Constant>(Op));
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
if (!GV->hasLocalLinkage()) return; // Don't delete non-static globals.
GV->eraseFromParent();
} else if (!isa<Function>(C)) {
// FIXME: Why does the type of the constant matter here?
if (isa<StructType>(C->getType()) || isa<ArrayType>(C->getType()) ||
isa<VectorType>(C->getType()))
C->destroyConstant();
}
// If the constant referenced anything, see if we can delete it as well.
for (Constant *O : Operands)
RemoveDeadConstant(O);
}
// Strip the symbol table of its names.
//
static void StripSymtab(ValueSymbolTable &ST, bool PreserveDbgInfo) {
for (ValueSymbolTable::iterator VI = ST.begin(), VE = ST.end(); VI != VE; ) {
Value *V = VI->getValue();
++VI;
if (!isa<GlobalValue>(V) || cast<GlobalValue>(V)->hasLocalLinkage()) {
if (!PreserveDbgInfo || !V->getName().startswith("llvm.dbg"))
// Set name to "", removing from symbol table!
V->setName("");
}
}
}
// Strip any named types of their names.
static void StripTypeNames(Module &M, bool PreserveDbgInfo) {
TypeFinder StructTypes;
StructTypes.run(M, false);
for (unsigned i = 0, e = StructTypes.size(); i != e; ++i) {
StructType *STy = StructTypes[i];
if (STy->isLiteral() || STy->getName().empty()) continue;
if (PreserveDbgInfo && STy->getName().startswith("llvm.dbg"))
continue;
STy->setName("");
}
}
/// Find values that are marked as llvm.used.
static void findUsedValues(GlobalVariable *LLVMUsed,
SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
if (!LLVMUsed) return;
UsedValues.insert(LLVMUsed);
ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
if (GlobalValue *GV =
dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
UsedValues.insert(GV);
}
/// StripSymbolNames - Strip symbol names.
static bool StripSymbolNames(Module &M, bool PreserveDbgInfo) {
SmallPtrSet<const GlobalValue*, 8> llvmUsedValues;
findUsedValues(M.getGlobalVariable("llvm.used"), llvmUsedValues);
findUsedValues(M.getGlobalVariable("llvm.compiler.used"), llvmUsedValues);
for (GlobalVariable &GV : M.globals()) {
if (GV.hasLocalLinkage() && !llvmUsedValues.contains(&GV))
if (!PreserveDbgInfo || !GV.getName().startswith("llvm.dbg"))
GV.setName(""); // Internal symbols can't participate in linkage
}
for (Function &I : M) {
if (I.hasLocalLinkage() && !llvmUsedValues.contains(&I))
if (!PreserveDbgInfo || !I.getName().startswith("llvm.dbg"))
I.setName(""); // Internal symbols can't participate in linkage
if (auto *Symtab = I.getValueSymbolTable())
StripSymtab(*Symtab, PreserveDbgInfo);
}
// Remove all names from types.
StripTypeNames(M, PreserveDbgInfo);
return true;
}
bool StripSymbols::runOnModule(Module &M) {
if (skipModule(M))
return false;
bool Changed = false;
Changed |= StripDebugInfo(M);
if (!OnlyDebugInfo)
Changed |= StripSymbolNames(M, false);
return Changed;
}
bool StripNonDebugSymbols::runOnModule(Module &M) {
if (skipModule(M))
return false;
return StripSymbolNames(M, true);
}
static bool stripDebugDeclareImpl(Module &M) {
Function *Declare = M.getFunction("llvm.dbg.declare");
std::vector<Constant*> DeadConstants;
if (Declare) {
while (!Declare->use_empty()) {
CallInst *CI = cast<CallInst>(Declare->user_back());
Value *Arg1 = CI->getArgOperand(0);
Value *Arg2 = CI->getArgOperand(1);
assert(CI->use_empty() && "llvm.dbg intrinsic should have void result");
CI->eraseFromParent();
if (Arg1->use_empty()) {
if (Constant *C = dyn_cast<Constant>(Arg1))
DeadConstants.push_back(C);
else
RecursivelyDeleteTriviallyDeadInstructions(Arg1);
}
if (Arg2->use_empty())
if (Constant *C = dyn_cast<Constant>(Arg2))
DeadConstants.push_back(C);
}
Declare->eraseFromParent();
}
while (!DeadConstants.empty()) {
Constant *C = DeadConstants.back();
DeadConstants.pop_back();
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
if (GV->hasLocalLinkage())
RemoveDeadConstant(GV);
} else
RemoveDeadConstant(C);
}
return true;
}
bool StripDebugDeclare::runOnModule(Module &M) {
if (skipModule(M))
return false;
return stripDebugDeclareImpl(M);
}
static bool stripDeadDebugInfoImpl(Module &M) {
bool Changed = false;
LLVMContext &C = M.getContext();
// Find all debug info in F. This is actually overkill in terms of what we
// want to do, but we want to try and be as resilient as possible in the face
// of potential debug info changes by using the formal interfaces given to us
// as much as possible.
DebugInfoFinder F;
F.processModule(M);
// For each compile unit, find the live set of global variables/functions and
// replace the current list of potentially dead global variables/functions
// with the live list.
SmallVector<Metadata *, 64> LiveGlobalVariables;
DenseSet<DIGlobalVariableExpression *> VisitedSet;
std::set<DIGlobalVariableExpression *> LiveGVs;
for (GlobalVariable &GV : M.globals()) {
SmallVector<DIGlobalVariableExpression *, 1> GVEs;
GV.getDebugInfo(GVEs);
for (auto *GVE : GVEs)
LiveGVs.insert(GVE);
}
std::set<DICompileUnit *> LiveCUs;
// Any CU referenced from a subprogram is live.
for (DISubprogram *SP : F.subprograms()) {
if (SP->getUnit())
LiveCUs.insert(SP->getUnit());
}
bool HasDeadCUs = false;
for (DICompileUnit *DIC : F.compile_units()) {
// Create our live global variable list.
bool GlobalVariableChange = false;
for (auto *DIG : DIC->getGlobalVariables()) {
if (DIG->getExpression() && DIG->getExpression()->isConstant())
LiveGVs.insert(DIG);
// Make sure we only visit each global variable only once.
if (!VisitedSet.insert(DIG).second)
continue;
// If a global variable references DIG, the global variable is live.
if (LiveGVs.count(DIG))
LiveGlobalVariables.push_back(DIG);
else
GlobalVariableChange = true;
}
if (!LiveGlobalVariables.empty())
LiveCUs.insert(DIC);
else if (!LiveCUs.count(DIC))
HasDeadCUs = true;
// If we found dead global variables, replace the current global
// variable list with our new live global variable list.
if (GlobalVariableChange) {
DIC->replaceGlobalVariables(MDTuple::get(C, LiveGlobalVariables));
Changed = true;
}
// Reset lists for the next iteration.
LiveGlobalVariables.clear();
}
if (HasDeadCUs) {
// Delete the old node and replace it with a new one
NamedMDNode *NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
NMD->clearOperands();
if (!LiveCUs.empty()) {
for (DICompileUnit *CU : LiveCUs)
NMD->addOperand(CU);
}
Changed = true;
}
return Changed;
}
/// Remove any debug info for global variables/functions in the given module for
/// which said global variable/function no longer exists (i.e. is null).
///
/// Debugging information is encoded in llvm IR using metadata. This is designed
/// such a way that debug info for symbols preserved even if symbols are
/// optimized away by the optimizer. This special pass removes debug info for
/// such symbols.
bool StripDeadDebugInfo::runOnModule(Module &M) {
if (skipModule(M))
return false;
return stripDeadDebugInfoImpl(M);
}
PreservedAnalyses StripSymbolsPass::run(Module &M, ModuleAnalysisManager &AM) {
StripDebugInfo(M);
StripSymbolNames(M, false);
return PreservedAnalyses::all();
}
PreservedAnalyses StripNonDebugSymbolsPass::run(Module &M,
ModuleAnalysisManager &AM) {
StripSymbolNames(M, true);
return PreservedAnalyses::all();
}
PreservedAnalyses StripDebugDeclarePass::run(Module &M,
ModuleAnalysisManager &AM) {
stripDebugDeclareImpl(M);
return PreservedAnalyses::all();
}
PreservedAnalyses StripDeadDebugInfoPass::run(Module &M,
ModuleAnalysisManager &AM) {
stripDeadDebugInfoImpl(M);
return PreservedAnalyses::all();
}