348 lines
13 KiB
C++
348 lines
13 KiB
C++
//===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements classes used to handle lowerings specific to common
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// object file formats.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Target/TargetLoweringObjectFile.h"
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#include "llvm/BinaryFormat/Dwarf.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/DerivedTypes.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/GlobalVariable.h"
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#include "llvm/IR/Mangler.h"
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#include "llvm/MC/MCContext.h"
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#include "llvm/MC/MCExpr.h"
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#include "llvm/MC/MCStreamer.h"
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#include "llvm/MC/MCSymbol.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/Target/TargetOptions.h"
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using namespace llvm;
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//===----------------------------------------------------------------------===//
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// Generic Code
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//===----------------------------------------------------------------------===//
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/// Initialize - this method must be called before any actual lowering is
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/// done. This specifies the current context for codegen, and gives the
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/// lowering implementations a chance to set up their default sections.
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void TargetLoweringObjectFile::Initialize(MCContext &ctx,
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const TargetMachine &TM) {
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Ctx = &ctx;
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// `Initialize` can be called more than once.
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delete Mang;
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Mang = new Mangler();
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InitMCObjectFileInfo(TM.getTargetTriple(), TM.isPositionIndependent(), *Ctx,
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TM.getCodeModel() == CodeModel::Large);
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}
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TargetLoweringObjectFile::~TargetLoweringObjectFile() {
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delete Mang;
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}
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static bool isNullOrUndef(const Constant *C) {
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// Check that the constant isn't all zeros or undefs.
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if (C->isNullValue() || isa<UndefValue>(C))
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return true;
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if (!isa<ConstantAggregate>(C))
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return false;
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for (auto Operand : C->operand_values()) {
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if (!isNullOrUndef(cast<Constant>(Operand)))
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return false;
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}
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return true;
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}
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static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
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const Constant *C = GV->getInitializer();
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// Must have zero initializer.
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if (!isNullOrUndef(C))
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return false;
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// Leave constant zeros in readonly constant sections, so they can be shared.
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if (GV->isConstant())
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return false;
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// If the global has an explicit section specified, don't put it in BSS.
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if (GV->hasSection())
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return false;
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// If -nozero-initialized-in-bss is specified, don't ever use BSS.
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if (NoZerosInBSS)
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return false;
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// Otherwise, put it in BSS!
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return true;
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}
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/// IsNullTerminatedString - Return true if the specified constant (which is
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/// known to have a type that is an array of 1/2/4 byte elements) ends with a
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/// nul value and contains no other nuls in it. Note that this is more general
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/// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
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static bool IsNullTerminatedString(const Constant *C) {
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// First check: is we have constant array terminated with zero
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if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
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unsigned NumElts = CDS->getNumElements();
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assert(NumElts != 0 && "Can't have an empty CDS");
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if (CDS->getElementAsInteger(NumElts-1) != 0)
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return false; // Not null terminated.
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// Verify that the null doesn't occur anywhere else in the string.
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for (unsigned i = 0; i != NumElts-1; ++i)
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if (CDS->getElementAsInteger(i) == 0)
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return false;
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return true;
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}
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// Another possibility: [1 x i8] zeroinitializer
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if (isa<ConstantAggregateZero>(C))
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return cast<ArrayType>(C->getType())->getNumElements() == 1;
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return false;
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}
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MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
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const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const {
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assert(!Suffix.empty());
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SmallString<60> NameStr;
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NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
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TM.getNameWithPrefix(NameStr, GV, *Mang);
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NameStr.append(Suffix.begin(), Suffix.end());
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return Ctx->getOrCreateSymbol(NameStr);
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}
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MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
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const GlobalValue *GV, const TargetMachine &TM,
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MachineModuleInfo *MMI) const {
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return TM.getSymbol(GV);
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}
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void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
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const DataLayout &,
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const MCSymbol *Sym) const {
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}
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/// getKindForGlobal - This is a top-level target-independent classifier for
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/// a global variable. Given an global variable and information from TM, it
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/// classifies the global in a variety of ways that make various target
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/// implementations simpler. The target implementation is free to ignore this
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/// extra info of course.
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SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
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const TargetMachine &TM){
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assert(!GO->isDeclaration() && !GO->hasAvailableExternallyLinkage() &&
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"Can only be used for global definitions");
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Reloc::Model ReloModel = TM.getRelocationModel();
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// Early exit - functions should be always in text sections.
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const auto *GVar = dyn_cast<GlobalVariable>(GO);
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if (!GVar)
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return SectionKind::getText();
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// Handle thread-local data first.
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if (GVar->isThreadLocal()) {
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if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
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return SectionKind::getThreadBSS();
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return SectionKind::getThreadData();
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}
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// Variables with common linkage always get classified as common.
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if (GVar->hasCommonLinkage())
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return SectionKind::getCommon();
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// Variable can be easily put to BSS section.
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if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
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if (GVar->hasLocalLinkage())
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return SectionKind::getBSSLocal();
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else if (GVar->hasExternalLinkage())
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return SectionKind::getBSSExtern();
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return SectionKind::getBSS();
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}
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const Constant *C = GVar->getInitializer();
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// If the global is marked constant, we can put it into a mergable section,
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// a mergable string section, or general .data if it contains relocations.
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if (GVar->isConstant()) {
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// If the initializer for the global contains something that requires a
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// relocation, then we may have to drop this into a writable data section
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// even though it is marked const.
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if (!C->needsRelocation()) {
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// If the global is required to have a unique address, it can't be put
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// into a mergable section: just drop it into the general read-only
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// section instead.
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if (!GVar->hasGlobalUnnamedAddr())
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return SectionKind::getReadOnly();
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// If initializer is a null-terminated string, put it in a "cstring"
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// section of the right width.
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if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
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if (IntegerType *ITy =
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dyn_cast<IntegerType>(ATy->getElementType())) {
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if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
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ITy->getBitWidth() == 32) &&
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IsNullTerminatedString(C)) {
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if (ITy->getBitWidth() == 8)
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return SectionKind::getMergeable1ByteCString();
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if (ITy->getBitWidth() == 16)
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return SectionKind::getMergeable2ByteCString();
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assert(ITy->getBitWidth() == 32 && "Unknown width");
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return SectionKind::getMergeable4ByteCString();
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}
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}
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}
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// Otherwise, just drop it into a mergable constant section. If we have
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// a section for this size, use it, otherwise use the arbitrary sized
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// mergable section.
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switch (
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GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
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case 4: return SectionKind::getMergeableConst4();
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case 8: return SectionKind::getMergeableConst8();
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case 16: return SectionKind::getMergeableConst16();
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case 32: return SectionKind::getMergeableConst32();
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default:
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return SectionKind::getReadOnly();
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}
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} else {
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// In static, ROPI and RWPI relocation models, the linker will resolve
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// all addresses, so the relocation entries will actually be constants by
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// the time the app starts up. However, we can't put this into a
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// mergable section, because the linker doesn't take relocations into
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// consideration when it tries to merge entries in the section.
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if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
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ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI)
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return SectionKind::getReadOnly();
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// Otherwise, the dynamic linker needs to fix it up, put it in the
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// writable data.rel section.
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return SectionKind::getReadOnlyWithRel();
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}
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}
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// Okay, this isn't a constant.
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return SectionKind::getData();
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}
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/// This method computes the appropriate section to emit the specified global
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/// variable or function definition. This should not be passed external (or
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/// available externally) globals.
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MCSection *TargetLoweringObjectFile::SectionForGlobal(
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const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
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// Select section name.
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if (GO->hasSection())
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return getExplicitSectionGlobal(GO, Kind, TM);
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if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
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auto Attrs = GVar->getAttributes();
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if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
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(Attrs.hasAttribute("data-section") && Kind.isData()) ||
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(Attrs.hasAttribute("rodata-section") && Kind.isReadOnly())) {
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return getExplicitSectionGlobal(GO, Kind, TM);
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}
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}
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if (auto *F = dyn_cast<Function>(GO)) {
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if (F->hasFnAttribute("implicit-section-name"))
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return getExplicitSectionGlobal(GO, Kind, TM);
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}
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// Use default section depending on the 'type' of global
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return SelectSectionForGlobal(GO, Kind, TM);
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}
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MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
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const Function &F, const TargetMachine &TM) const {
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unsigned Align = 0;
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return getSectionForConstant(F.getParent()->getDataLayout(),
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SectionKind::getReadOnly(), /*C=*/nullptr,
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Align);
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}
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bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
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bool UsesLabelDifference, const Function &F) const {
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// In PIC mode, we need to emit the jump table to the same section as the
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// function body itself, otherwise the label differences won't make sense.
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// FIXME: Need a better predicate for this: what about custom entries?
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if (UsesLabelDifference)
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return true;
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// We should also do if the section name is NULL or function is declared
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// in discardable section
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// FIXME: this isn't the right predicate, should be based on the MCSection
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// for the function.
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return F.isWeakForLinker();
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}
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/// Given a mergable constant with the specified size and relocation
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/// information, return a section that it should be placed in.
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MCSection *TargetLoweringObjectFile::getSectionForConstant(
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const DataLayout &DL, SectionKind Kind, const Constant *C,
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unsigned &Align) const {
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if (Kind.isReadOnly() && ReadOnlySection != nullptr)
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return ReadOnlySection;
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return DataSection;
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}
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/// getTTypeGlobalReference - Return an MCExpr to use for a
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/// reference to the specified global variable from exception
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/// handling information.
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const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
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const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM,
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MachineModuleInfo *MMI, MCStreamer &Streamer) const {
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const MCSymbolRefExpr *Ref =
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MCSymbolRefExpr::create(TM.getSymbol(GV), getContext());
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return getTTypeReference(Ref, Encoding, Streamer);
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}
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const MCExpr *TargetLoweringObjectFile::
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getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
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MCStreamer &Streamer) const {
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switch (Encoding & 0x70) {
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default:
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report_fatal_error("We do not support this DWARF encoding yet!");
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case dwarf::DW_EH_PE_absptr:
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// Do nothing special
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return Sym;
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case dwarf::DW_EH_PE_pcrel: {
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// Emit a label to the streamer for the current position. This gives us
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// .-foo addressing.
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MCSymbol *PCSym = getContext().createTempSymbol();
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Streamer.EmitLabel(PCSym);
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const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
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return MCBinaryExpr::createSub(Sym, PC, getContext());
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}
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}
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}
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const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
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// FIXME: It's not clear what, if any, default this should have - perhaps a
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// null return could mean 'no location' & we should just do that here.
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return MCSymbolRefExpr::create(Sym, *Ctx);
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}
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void TargetLoweringObjectFile::getNameWithPrefix(
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SmallVectorImpl<char> &OutName, const GlobalValue *GV,
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const TargetMachine &TM) const {
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Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
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}
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