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//===- GlobalSplit.cpp - global variable splitter -------------------------===//
//
// 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 pass uses inrange annotations on GEP indices to split globals where
// beneficial. Clang currently attaches these annotations to references to
// virtual table globals under the Itanium ABI for the benefit of the
// whole-program virtual call optimization and control flow integrity passes.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/GlobalSplit.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/IPO.h"
#include <cstdint>
#include <vector>
using namespace llvm;
static bool splitGlobal(GlobalVariable &GV) {
// If the address of the global is taken outside of the module, we cannot
// apply this transformation.
if (!GV.hasLocalLinkage())
return false;
// We currently only know how to split ConstantStructs.
auto *Init = dyn_cast_or_null<ConstantStruct>(GV.getInitializer());
if (!Init)
return false;
// Verify that each user of the global is an inrange getelementptr constant.
// From this it follows that any loads from or stores to that global must use
// a pointer derived from an inrange getelementptr constant, which is
// sufficient to allow us to apply the splitting transform.
for (User *U : GV.users()) {
if (!isa<Constant>(U))
return false;
auto *GEP = dyn_cast<GEPOperator>(U);
if (!GEP || !GEP->getInRangeIndex() || *GEP->getInRangeIndex() != 1 ||
!isa<ConstantInt>(GEP->getOperand(1)) ||
!cast<ConstantInt>(GEP->getOperand(1))->isZero() ||
!isa<ConstantInt>(GEP->getOperand(2)))
return false;
}
SmallVector<MDNode *, 2> Types;
GV.getMetadata(LLVMContext::MD_type, Types);
const DataLayout &DL = GV.getParent()->getDataLayout();
const StructLayout *SL = DL.getStructLayout(Init->getType());
IntegerType *Int32Ty = Type::getInt32Ty(GV.getContext());
std::vector<GlobalVariable *> SplitGlobals(Init->getNumOperands());
for (unsigned I = 0; I != Init->getNumOperands(); ++I) {
// Build a global representing this split piece.
auto *SplitGV =
new GlobalVariable(*GV.getParent(), Init->getOperand(I)->getType(),
GV.isConstant(), GlobalValue::PrivateLinkage,
Init->getOperand(I), GV.getName() + "." + utostr(I));
SplitGlobals[I] = SplitGV;
unsigned SplitBegin = SL->getElementOffset(I);
unsigned SplitEnd = (I == Init->getNumOperands() - 1)
? SL->getSizeInBytes()
: SL->getElementOffset(I + 1);
// Rebuild type metadata, adjusting by the split offset.
// FIXME: See if we can use DW_OP_piece to preserve debug metadata here.
for (MDNode *Type : Types) {
uint64_t ByteOffset = cast<ConstantInt>(
cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
->getZExtValue();
// Type metadata may be attached one byte after the end of the vtable, for
// classes without virtual methods in Itanium ABI. AFAIK, it is never
// attached to the first byte of a vtable. Subtract one to get the right
// slice.
// This is making an assumption that vtable groups are the only kinds of
// global variables that !type metadata can be attached to, and that they
// are either Itanium ABI vtable groups or contain a single vtable (i.e.
// Microsoft ABI vtables).
uint64_t AttachedTo = (ByteOffset == 0) ? ByteOffset : ByteOffset - 1;
if (AttachedTo < SplitBegin || AttachedTo >= SplitEnd)
continue;
SplitGV->addMetadata(
LLVMContext::MD_type,
*MDNode::get(GV.getContext(),
{ConstantAsMetadata::get(
ConstantInt::get(Int32Ty, ByteOffset - SplitBegin)),
Type->getOperand(1)}));
}
}
for (User *U : GV.users()) {
auto *GEP = cast<GEPOperator>(U);
unsigned I = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
if (I >= SplitGlobals.size())
continue;
SmallVector<Value *, 4> Ops;
Ops.push_back(ConstantInt::get(Int32Ty, 0));
for (unsigned I = 3; I != GEP->getNumOperands(); ++I)
Ops.push_back(GEP->getOperand(I));
auto *NewGEP = ConstantExpr::getGetElementPtr(
SplitGlobals[I]->getInitializer()->getType(), SplitGlobals[I], Ops,
GEP->isInBounds());
GEP->replaceAllUsesWith(NewGEP);
}
// Finally, remove the original global. Any remaining uses refer to invalid
// elements of the global, so replace with undef.
if (!GV.use_empty())
GV.replaceAllUsesWith(UndefValue::get(GV.getType()));
GV.eraseFromParent();
return true;
}
static bool splitGlobals(Module &M) {
// First, see if the module uses either of the llvm.type.test or
// llvm.type.checked.load intrinsics, which indicates that splitting globals
// may be beneficial.
Function *TypeTestFunc =
M.getFunction(Intrinsic::getName(Intrinsic::type_test));
Function *TypeCheckedLoadFunc =
M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
if ((!TypeTestFunc || TypeTestFunc->use_empty()) &&
(!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
return false;
bool Changed = false;
for (auto I = M.global_begin(); I != M.global_end();) {
GlobalVariable &GV = *I;
++I;
Changed |= splitGlobal(GV);
}
return Changed;
}
namespace {
struct GlobalSplit : public ModulePass {
static char ID;
GlobalSplit() : ModulePass(ID) {
initializeGlobalSplitPass(*PassRegistry::getPassRegistry());
}
bool runOnModule(Module &M) override {
if (skipModule(M))
return false;
return splitGlobals(M);
}
};
} // end anonymous namespace
char GlobalSplit::ID = 0;
INITIALIZE_PASS(GlobalSplit, "globalsplit", "Global splitter", false, false)
ModulePass *llvm::createGlobalSplitPass() {
return new GlobalSplit;
}
PreservedAnalyses GlobalSplitPass::run(Module &M, ModuleAnalysisManager &AM) {
if (!splitGlobals(M))
return PreservedAnalyses::all();
return PreservedAnalyses::none();
}
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