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2c21fe4650
If a section is rw, it is irrelevant if the dynamic linker will write to it or not. It looks like llvm implemented this because gcc was doing it. It looks like gcc implemented this in the hope that it would put all the relocated items close together and speed up the dynamic linker. There are two problem with this: * It doesn't work. Both bfd and gold will map .data.rel to .data and concatenate the input sections in the order they are seen. * If we want a feature like that, it can be implemented directly in the linker since it knowns where the dynamic relocations are. llvm-svn: 253436
331 lines
12 KiB
C++
331 lines
12 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/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/MCAsmInfo.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/Dwarf.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/TargetLowering.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/Target/TargetOptions.h"
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#include "llvm/Target/TargetSubtargetInfo.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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InitMCObjectFileInfo(TM.getTargetTriple(), TM.getRelocationModel(),
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TM.getCodeModel(), *Ctx);
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}
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TargetLoweringObjectFile::~TargetLoweringObjectFile() {
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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 (!C->isNullValue())
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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, Mangler &Mang,
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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, Mangler &Mang, const TargetMachine &TM,
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MachineModuleInfo *MMI) const {
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return TM.getSymbol(GV, Mang);
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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 GlobalValue *GV,
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const TargetMachine &TM){
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assert(!GV->isDeclaration() && !GV->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 GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
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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->hasUnnamedAddr())
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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 (GV->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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default:
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return SectionKind::getReadOnly();
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}
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} else {
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// In static relocation model, the linker will resolve all addresses, so
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// the relocation entries will actually be constants by the time the app
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// starts up. However, we can't put this into a mergable section, because
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// the linker doesn't take relocations into consideration when it tries to
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// merge entries in the section.
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if (ReloModel == Reloc::Static)
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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. If the initializer for the global is going
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// to require a runtime relocation by the dynamic linker, put it into a more
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// specific section to improve startup time of the app. This coalesces these
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// globals together onto fewer pages, improving the locality of the dynamic
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// linker.
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if (ReloModel == Reloc::Static)
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return SectionKind::getData();
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if (C->needsRelocation())
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return SectionKind::getData();
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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 *
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TargetLoweringObjectFile::SectionForGlobal(const GlobalValue *GV,
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SectionKind Kind, Mangler &Mang,
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const TargetMachine &TM) const {
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// Select section name.
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if (GV->hasSection())
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return getExplicitSectionGlobal(GV, Kind, Mang, TM);
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// Use default section depending on the 'type' of global
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return SelectSectionForGlobal(GV, Kind, Mang, TM);
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}
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MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
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const Function &F, Mangler &Mang, const TargetMachine &TM) const {
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return getSectionForConstant(F.getParent()->getDataLayout(),
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SectionKind::getReadOnly(), /*C=*/nullptr);
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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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if (F.isWeakForLinker())
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return true;
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return false;
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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) 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, Mangler &Mang,
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const TargetMachine &TM, MachineModuleInfo *MMI,
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MCStreamer &Streamer) const {
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const MCSymbolRefExpr *Ref =
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MCSymbolRefExpr::create(TM.getSymbol(GV, Mang), 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, Mangler &Mang,
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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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