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llvm-project/llvm/lib/MC/MCAssembler.cpp

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//===- lib/MC/MCAssembler.cpp - Assembler Backend Implementation ----------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "llvm/MC/MCAssembler.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCCodeView.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDwarf.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFixupKindInfo.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <cstring>
#include <tuple>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "assembler"
namespace {
namespace stats {
STATISTIC(EmittedFragments, "Number of emitted assembler fragments - total");
STATISTIC(EmittedRelaxableFragments,
"Number of emitted assembler fragments - relaxable");
STATISTIC(EmittedDataFragments,
"Number of emitted assembler fragments - data");
STATISTIC(EmittedCompactEncodedInstFragments,
"Number of emitted assembler fragments - compact encoded inst");
STATISTIC(EmittedAlignFragments,
"Number of emitted assembler fragments - align");
STATISTIC(EmittedFillFragments,
"Number of emitted assembler fragments - fill");
STATISTIC(EmittedOrgFragments,
"Number of emitted assembler fragments - org");
STATISTIC(evaluateFixup, "Number of evaluated fixups");
STATISTIC(FragmentLayouts, "Number of fragment layouts");
STATISTIC(ObjectBytes, "Number of emitted object file bytes");
STATISTIC(RelaxationSteps, "Number of assembler layout and relaxation steps");
STATISTIC(RelaxedInstructions, "Number of relaxed instructions");
STATISTIC(PaddingFragmentsRelaxations,
"Number of Padding Fragments relaxations");
STATISTIC(PaddingFragmentsBytes,
"Total size of all padding from adding Fragments");
} // end namespace stats
} // end anonymous namespace
// FIXME FIXME FIXME: There are number of places in this file where we convert
// what is a 64-bit assembler value used for computation into a value in the
// object file, which may truncate it. We should detect that truncation where
// invalid and report errors back.
/* *** */
MCAssembler::MCAssembler(MCContext &Context,
std::unique_ptr<MCAsmBackend> Backend,
std::unique_ptr<MCCodeEmitter> Emitter,
std::unique_ptr<MCObjectWriter> Writer)
: Context(Context), Backend(std::move(Backend)),
Emitter(std::move(Emitter)), Writer(std::move(Writer)),
BundleAlignSize(0), RelaxAll(false), SubsectionsViaSymbols(false),
IncrementalLinkerCompatible(false), ELFHeaderEFlags(0) {
VersionInfo.Major = 0; // Major version == 0 for "none specified"
}
MCAssembler::~MCAssembler() = default;
void MCAssembler::reset() {
Sections.clear();
Symbols.clear();
IndirectSymbols.clear();
DataRegions.clear();
LinkerOptions.clear();
FileNames.clear();
ThumbFuncs.clear();
BundleAlignSize = 0;
RelaxAll = false;
SubsectionsViaSymbols = false;
IncrementalLinkerCompatible = false;
ELFHeaderEFlags = 0;
LOHContainer.reset();
VersionInfo.Major = 0;
// reset objects owned by us
if (getBackendPtr())
getBackendPtr()->reset();
if (getEmitterPtr())
getEmitterPtr()->reset();
if (getWriterPtr())
getWriterPtr()->reset();
getLOHContainer().reset();
}
bool MCAssembler::registerSection(MCSection &Section) {
if (Section.isRegistered())
return false;
Sections.push_back(&Section);
Section.setIsRegistered(true);
return true;
}
bool MCAssembler::isThumbFunc(const MCSymbol *Symbol) const {
if (ThumbFuncs.count(Symbol))
return true;
if (!Symbol->isVariable())
return false;
const MCExpr *Expr = Symbol->getVariableValue();
MCValue V;
if (!Expr->evaluateAsRelocatable(V, nullptr, nullptr))
return false;
if (V.getSymB() || V.getRefKind() != MCSymbolRefExpr::VK_None)
return false;
const MCSymbolRefExpr *Ref = V.getSymA();
if (!Ref)
return false;
if (Ref->getKind() != MCSymbolRefExpr::VK_None)
return false;
const MCSymbol &Sym = Ref->getSymbol();
if (!isThumbFunc(&Sym))
return false;
ThumbFuncs.insert(Symbol); // Cache it.
return true;
}
bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {
// Non-temporary labels should always be visible to the linker.
if (!Symbol.isTemporary())
return true;
// Absolute temporary labels are never visible.
if (!Symbol.isInSection())
return false;
if (Symbol.isUsedInReloc())
return true;
return false;
}
const MCSymbol *MCAssembler::getAtom(const MCSymbol &S) const {
// Linker visible symbols define atoms.
if (isSymbolLinkerVisible(S))
return &S;
// Absolute and undefined symbols have no defining atom.
if (!S.isInSection())
return nullptr;
// Non-linker visible symbols in sections which can't be atomized have no
// defining atom.
if (!getContext().getAsmInfo()->isSectionAtomizableBySymbols(
*S.getFragment()->getParent()))
return nullptr;
// Otherwise, return the atom for the containing fragment.
return S.getFragment()->getAtom();
}
bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
const MCFixup &Fixup, const MCFragment *DF,
MCValue &Target, uint64_t &Value) const {
++stats::evaluateFixup;
// FIXME: This code has some duplication with recordRelocation. We should
// probably merge the two into a single callback that tries to evaluate a
// fixup and records a relocation if one is needed.
// On error claim to have completely evaluated the fixup, to prevent any
// further processing from being done.
const MCExpr *Expr = Fixup.getValue();
MCContext &Ctx = getContext();
Value = 0;
if (!Expr->evaluateAsRelocatable(Target, &Layout, &Fixup)) {
Ctx.reportError(Fixup.getLoc(), "expected relocatable expression");
return true;
}
if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
if (RefB->getKind() != MCSymbolRefExpr::VK_None) {
Ctx.reportError(Fixup.getLoc(),
"unsupported subtraction of qualified symbol");
return true;
}
}
assert(getBackendPtr() && "Expected assembler backend");
bool IsPCRel = getBackendPtr()->getFixupKindInfo(Fixup.getKind()).Flags &
MCFixupKindInfo::FKF_IsPCRel;
bool IsResolved = false;
if (IsPCRel) {
if (Target.getSymB()) {
IsResolved = false;
} else if (!Target.getSymA()) {
IsResolved = false;
} else {
const MCSymbolRefExpr *A = Target.getSymA();
const MCSymbol &SA = A->getSymbol();
if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {
IsResolved = false;
} else if (auto *Writer = getWriterPtr()) {
IsResolved = Writer->isSymbolRefDifferenceFullyResolvedImpl(
*this, SA, *DF, false, true);
}
}
} else {
IsResolved = Target.isAbsolute();
}
Value = Target.getConstant();
if (const MCSymbolRefExpr *A = Target.getSymA()) {
const MCSymbol &Sym = A->getSymbol();
if (Sym.isDefined())
Value += Layout.getSymbolOffset(Sym);
}
if (const MCSymbolRefExpr *B = Target.getSymB()) {
const MCSymbol &Sym = B->getSymbol();
if (Sym.isDefined())
Value -= Layout.getSymbolOffset(Sym);
}
bool ShouldAlignPC = getBackend().getFixupKindInfo(Fixup.getKind()).Flags &
MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
assert((ShouldAlignPC ? IsPCRel : true) &&
"FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");
if (IsPCRel) {
uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();
// A number of ARM fixups in Thumb mode require that the effective PC
// address be determined as the 32-bit aligned version of the actual offset.
if (ShouldAlignPC) Offset &= ~0x3;
Value -= Offset;
}
// Let the backend force a relocation if needed.
if (IsResolved && getBackend().shouldForceRelocation(*this, Fixup, Target))
IsResolved = false;
return IsResolved;
}
uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
const MCFragment &F) const {
assert(getBackendPtr() && "Requires assembler backend");
switch (F.getKind()) {
case MCFragment::FT_Data:
return cast<MCDataFragment>(F).getContents().size();
case MCFragment::FT_Relaxable:
return cast<MCRelaxableFragment>(F).getContents().size();
case MCFragment::FT_CompactEncodedInst:
return cast<MCCompactEncodedInstFragment>(F).getContents().size();
case MCFragment::FT_Fill: {
auto &FF = cast<MCFillFragment>(F);
int64_t Size = 0;
if (!FF.getSize().evaluateAsAbsolute(Size, Layout))
getContext().reportError(FF.getLoc(),
"expected assembly-time absolute expression");
if (Size < 0) {
getContext().reportError(FF.getLoc(), "invalid number of bytes");
return 0;
}
return Size;
}
case MCFragment::FT_LEB:
return cast<MCLEBFragment>(F).getContents().size();
case MCFragment::FT_Padding:
return cast<MCPaddingFragment>(F).getSize();
case MCFragment::FT_SymbolId:
return 4;
case MCFragment::FT_Align: {
const MCAlignFragment &AF = cast<MCAlignFragment>(F);
unsigned Offset = Layout.getFragmentOffset(&AF);
unsigned Size = OffsetToAlignment(Offset, AF.getAlignment());
// If we are padding with nops, force the padding to be larger than the
// minimum nop size.
if (Size > 0 && AF.hasEmitNops()) {
while (Size % getBackend().getMinimumNopSize())
Size += AF.getAlignment();
}
if (Size > AF.getMaxBytesToEmit())
return 0;
return Size;
}
case MCFragment::FT_Org: {
const MCOrgFragment &OF = cast<MCOrgFragment>(F);
MCValue Value;
if (!OF.getOffset().evaluateAsValue(Value, Layout)) {
getContext().reportError(OF.getLoc(),
"expected assembly-time absolute expression");
return 0;
}
uint64_t FragmentOffset = Layout.getFragmentOffset(&OF);
int64_t TargetLocation = Value.getConstant();
if (const MCSymbolRefExpr *A = Value.getSymA()) {
uint64_t Val;
if (!Layout.getSymbolOffset(A->getSymbol(), Val)) {
getContext().reportError(OF.getLoc(), "expected absolute expression");
return 0;
}
TargetLocation += Val;
}
int64_t Size = TargetLocation - FragmentOffset;
if (Size < 0 || Size >= 0x40000000) {
getContext().reportError(
OF.getLoc(), "invalid .org offset '" + Twine(TargetLocation) +
"' (at offset '" + Twine(FragmentOffset) + "')");
return 0;
}
return Size;
}
case MCFragment::FT_Dwarf:
return cast<MCDwarfLineAddrFragment>(F).getContents().size();
case MCFragment::FT_DwarfFrame:
return cast<MCDwarfCallFrameFragment>(F).getContents().size();
case MCFragment::FT_CVInlineLines:
return cast<MCCVInlineLineTableFragment>(F).getContents().size();
case MCFragment::FT_CVDefRange:
return cast<MCCVDefRangeFragment>(F).getContents().size();
case MCFragment::FT_Dummy:
llvm_unreachable("Should not have been added");
}
llvm_unreachable("invalid fragment kind");
}
void MCAsmLayout::layoutFragment(MCFragment *F) {
MCFragment *Prev = F->getPrevNode();
// We should never try to recompute something which is valid.
assert(!isFragmentValid(F) && "Attempt to recompute a valid fragment!");
// We should never try to compute the fragment layout if its predecessor
// isn't valid.
assert((!Prev || isFragmentValid(Prev)) &&
"Attempt to compute fragment before its predecessor!");
++stats::FragmentLayouts;
// Compute fragment offset and size.
if (Prev)
F->Offset = Prev->Offset + getAssembler().computeFragmentSize(*this, *Prev);
else
F->Offset = 0;
LastValidFragment[F->getParent()] = F;
// If bundling is enabled and this fragment has instructions in it, it has to
// obey the bundling restrictions. With padding, we'll have:
//
//
// BundlePadding
// |||
// -------------------------------------
// Prev |##########| F |
// -------------------------------------
// ^
// |
// F->Offset
//
// The fragment's offset will point to after the padding, and its computed
// size won't include the padding.
//
// When the -mc-relax-all flag is used, we optimize bundling by writting the
// padding directly into fragments when the instructions are emitted inside
// the streamer. When the fragment is larger than the bundle size, we need to
// ensure that it's bundle aligned. This means that if we end up with
// multiple fragments, we must emit bundle padding between fragments.
//
// ".align N" is an example of a directive that introduces multiple
// fragments. We could add a special case to handle ".align N" by emitting
// within-fragment padding (which would produce less padding when N is less
// than the bundle size), but for now we don't.
//
if (Assembler.isBundlingEnabled() && F->hasInstructions()) {
assert(isa<MCEncodedFragment>(F) &&
"Only MCEncodedFragment implementations have instructions");
uint64_t FSize = Assembler.computeFragmentSize(*this, *F);
if (!Assembler.getRelaxAll() && FSize > Assembler.getBundleAlignSize())
report_fatal_error("Fragment can't be larger than a bundle size");
uint64_t RequiredBundlePadding = computeBundlePadding(Assembler, F,
F->Offset, FSize);
if (RequiredBundlePadding > UINT8_MAX)
report_fatal_error("Padding cannot exceed 255 bytes");
F->setBundlePadding(static_cast<uint8_t>(RequiredBundlePadding));
F->Offset += RequiredBundlePadding;
}
}
void MCAssembler::registerSymbol(const MCSymbol &Symbol, bool *Created) {
bool New = !Symbol.isRegistered();
if (Created)
*Created = New;
if (New) {
Symbol.setIsRegistered(true);
Symbols.push_back(&Symbol);
}
}
void MCAssembler::writeFragmentPadding(const MCFragment &F, uint64_t FSize,
MCObjectWriter *OW) const {
assert(getBackendPtr() && "Expected assembler backend");
// Should NOP padding be written out before this fragment?
unsigned BundlePadding = F.getBundlePadding();
if (BundlePadding > 0) {
assert(isBundlingEnabled() &&
"Writing bundle padding with disabled bundling");
assert(F.hasInstructions() &&
"Writing bundle padding for a fragment without instructions");
unsigned TotalLength = BundlePadding + static_cast<unsigned>(FSize);
if (F.alignToBundleEnd() && TotalLength > getBundleAlignSize()) {
// If the padding itself crosses a bundle boundary, it must be emitted
// in 2 pieces, since even nop instructions must not cross boundaries.
// v--------------v <- BundleAlignSize
// v---------v <- BundlePadding
// ----------------------------
// | Prev |####|####| F |
// ----------------------------
// ^-------------------^ <- TotalLength
unsigned DistanceToBoundary = TotalLength - getBundleAlignSize();
if (!getBackend().writeNopData(DistanceToBoundary, OW))
report_fatal_error("unable to write NOP sequence of " +
Twine(DistanceToBoundary) + " bytes");
BundlePadding -= DistanceToBoundary;
}
if (!getBackend().writeNopData(BundlePadding, OW))
report_fatal_error("unable to write NOP sequence of " +
Twine(BundlePadding) + " bytes");
}
}
/// Write the fragment \p F to the output file.
static void writeFragment(const MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment &F) {
MCObjectWriter *OW = Asm.getWriterPtr();
assert(OW && "Need ObjectWriter to write fragment");
// FIXME: Embed in fragments instead?
uint64_t FragmentSize = Asm.computeFragmentSize(Layout, F);
Asm.writeFragmentPadding(F, FragmentSize, OW);
// This variable (and its dummy usage) is to participate in the assert at
// the end of the function.
uint64_t Start = OW->getStream().tell();
(void) Start;
++stats::EmittedFragments;
switch (F.getKind()) {
case MCFragment::FT_Align: {
++stats::EmittedAlignFragments;
const MCAlignFragment &AF = cast<MCAlignFragment>(F);
assert(AF.getValueSize() && "Invalid virtual align in concrete fragment!");
uint64_t Count = FragmentSize / AF.getValueSize();
// FIXME: This error shouldn't actually occur (the front end should emit
// multiple .align directives to enforce the semantics it wants), but is
// severe enough that we want to report it. How to handle this?
if (Count * AF.getValueSize() != FragmentSize)
report_fatal_error("undefined .align directive, value size '" +
Twine(AF.getValueSize()) +
"' is not a divisor of padding size '" +
Twine(FragmentSize) + "'");
// See if we are aligning with nops, and if so do that first to try to fill
// the Count bytes. Then if that did not fill any bytes or there are any
// bytes left to fill use the Value and ValueSize to fill the rest.
// If we are aligning with nops, ask that target to emit the right data.
if (AF.hasEmitNops()) {
if (!Asm.getBackend().writeNopData(Count, OW))
report_fatal_error("unable to write nop sequence of " +
Twine(Count) + " bytes");
break;
}
// Otherwise, write out in multiples of the value size.
for (uint64_t i = 0; i != Count; ++i) {
switch (AF.getValueSize()) {
default: llvm_unreachable("Invalid size!");
case 1: OW->write8 (uint8_t (AF.getValue())); break;
case 2: OW->write16(uint16_t(AF.getValue())); break;
case 4: OW->write32(uint32_t(AF.getValue())); break;
case 8: OW->write64(uint64_t(AF.getValue())); break;
}
}
break;
}
case MCFragment::FT_Data:
++stats::EmittedDataFragments;
OW->writeBytes(cast<MCDataFragment>(F).getContents());
break;
case MCFragment::FT_Relaxable:
++stats::EmittedRelaxableFragments;
OW->writeBytes(cast<MCRelaxableFragment>(F).getContents());
break;
case MCFragment::FT_CompactEncodedInst:
++stats::EmittedCompactEncodedInstFragments;
OW->writeBytes(cast<MCCompactEncodedInstFragment>(F).getContents());
break;
case MCFragment::FT_Fill: {
++stats::EmittedFillFragments;
const MCFillFragment &FF = cast<MCFillFragment>(F);
uint8_t V = FF.getValue();
const unsigned MaxChunkSize = 16;
char Data[MaxChunkSize];
memcpy(Data, &V, 1);
for (unsigned I = 1; I < MaxChunkSize; ++I)
Data[I] = Data[0];
uint64_t Size = FragmentSize;
for (unsigned ChunkSize = MaxChunkSize; ChunkSize; ChunkSize /= 2) {
StringRef Ref(Data, ChunkSize);
for (uint64_t I = 0, E = Size / ChunkSize; I != E; ++I)
OW->writeBytes(Ref);
Size = Size % ChunkSize;
}
break;
}
case MCFragment::FT_LEB: {
const MCLEBFragment &LF = cast<MCLEBFragment>(F);
OW->writeBytes(LF.getContents());
break;
}
case MCFragment::FT_Padding: {
if (!Asm.getBackend().writeNopData(FragmentSize, OW))
report_fatal_error("unable to write nop sequence of " +
Twine(FragmentSize) + " bytes");
break;
}
case MCFragment::FT_SymbolId: {
const MCSymbolIdFragment &SF = cast<MCSymbolIdFragment>(F);
OW->write32(SF.getSymbol()->getIndex());
break;
}
case MCFragment::FT_Org: {
++stats::EmittedOrgFragments;
const MCOrgFragment &OF = cast<MCOrgFragment>(F);
for (uint64_t i = 0, e = FragmentSize; i != e; ++i)
OW->write8(uint8_t(OF.getValue()));
break;
}
case MCFragment::FT_Dwarf: {
const MCDwarfLineAddrFragment &OF = cast<MCDwarfLineAddrFragment>(F);
OW->writeBytes(OF.getContents());
break;
}
case MCFragment::FT_DwarfFrame: {
const MCDwarfCallFrameFragment &CF = cast<MCDwarfCallFrameFragment>(F);
OW->writeBytes(CF.getContents());
break;
}
case MCFragment::FT_CVInlineLines: {
const auto &OF = cast<MCCVInlineLineTableFragment>(F);
OW->writeBytes(OF.getContents());
break;
}
case MCFragment::FT_CVDefRange: {
const auto &DRF = cast<MCCVDefRangeFragment>(F);
OW->writeBytes(DRF.getContents());
break;
}
case MCFragment::FT_Dummy:
llvm_unreachable("Should not have been added");
}
assert(OW->getStream().tell() - Start == FragmentSize &&
"The stream should advance by fragment size");
}
void MCAssembler::writeSectionData(const MCSection *Sec,
const MCAsmLayout &Layout) const {
assert(getBackendPtr() && "Expected assembler backend");
// Ignore virtual sections.
if (Sec->isVirtualSection()) {
assert(Layout.getSectionFileSize(Sec) == 0 && "Invalid size for section!");
// Check that contents are only things legal inside a virtual section.
for (const MCFragment &F : *Sec) {
switch (F.getKind()) {
default: llvm_unreachable("Invalid fragment in virtual section!");
case MCFragment::FT_Data: {
// Check that we aren't trying to write a non-zero contents (or fixups)
// into a virtual section. This is to support clients which use standard
// directives to fill the contents of virtual sections.
const MCDataFragment &DF = cast<MCDataFragment>(F);
if (DF.fixup_begin() != DF.fixup_end())
report_fatal_error("cannot have fixups in virtual section!");
for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
if (DF.getContents()[i]) {
if (auto *ELFSec = dyn_cast<const MCSectionELF>(Sec))
report_fatal_error("non-zero initializer found in section '" +
ELFSec->getSectionName() + "'");
else
report_fatal_error("non-zero initializer found in virtual section");
}
break;
}
case MCFragment::FT_Align:
// Check that we aren't trying to write a non-zero value into a virtual
// section.
assert((cast<MCAlignFragment>(F).getValueSize() == 0 ||
cast<MCAlignFragment>(F).getValue() == 0) &&
"Invalid align in virtual section!");
break;
case MCFragment::FT_Fill:
assert((cast<MCFillFragment>(F).getValue() == 0) &&
"Invalid fill in virtual section!");
break;
}
}
return;
}
uint64_t Start = getWriter().getStream().tell();
(void)Start;
for (const MCFragment &F : *Sec)
writeFragment(*this, Layout, F);
assert(getWriter().getStream().tell() - Start ==
Layout.getSectionAddressSize(Sec));
}
std::tuple<MCValue, uint64_t, bool>
MCAssembler::handleFixup(const MCAsmLayout &Layout, MCFragment &F,
const MCFixup &Fixup) {
// Evaluate the fixup.
MCValue Target;
uint64_t FixedValue;
bool IsResolved = evaluateFixup(Layout, Fixup, &F, Target, FixedValue);
if (!IsResolved) {
// The fixup was unresolved, we need a relocation. Inform the object
// writer of the relocation, and give it an opportunity to adjust the
// fixup value if need be.
getWriter().recordRelocation(*this, Layout, &F, Fixup, Target, FixedValue);
}
return std::make_tuple(Target, FixedValue, IsResolved);
}
void MCAssembler::layout(MCAsmLayout &Layout) {
assert(getBackendPtr() && "Expected assembler backend");
DEBUG_WITH_TYPE("mc-dump", {
errs() << "assembler backend - pre-layout\n--\n";
dump(); });
// Create dummy fragments and assign section ordinals.
unsigned SectionIndex = 0;
for (MCSection &Sec : *this) {
// Create dummy fragments to eliminate any empty sections, this simplifies
// layout.
if (Sec.getFragmentList().empty())
new MCDataFragment(&Sec);
Sec.setOrdinal(SectionIndex++);
}
// Assign layout order indices to sections and fragments.
for (unsigned i = 0, e = Layout.getSectionOrder().size(); i != e; ++i) {
MCSection *Sec = Layout.getSectionOrder()[i];
Sec->setLayoutOrder(i);
unsigned FragmentIndex = 0;
for (MCFragment &Frag : *Sec)
Frag.setLayoutOrder(FragmentIndex++);
}
// Layout until everything fits.
while (layoutOnce(Layout))
if (getContext().hadError())
return;
DEBUG_WITH_TYPE("mc-dump", {
errs() << "assembler backend - post-relaxation\n--\n";
dump(); });
// Finalize the layout, including fragment lowering.
finishLayout(Layout);
DEBUG_WITH_TYPE("mc-dump", {
errs() << "assembler backend - final-layout\n--\n";
dump(); });
// Allow the object writer a chance to perform post-layout binding (for
// example, to set the index fields in the symbol data).
getWriter().executePostLayoutBinding(*this, Layout);
// Evaluate and apply the fixups, generating relocation entries as necessary.
for (MCSection &Sec : *this) {
for (MCFragment &Frag : Sec) {
// Data and relaxable fragments both have fixups. So only process
// those here.
// FIXME: Is there a better way to do this? MCEncodedFragmentWithFixups
// being templated makes this tricky.
if (isa<MCEncodedFragment>(&Frag) &&
isa<MCCompactEncodedInstFragment>(&Frag))
continue;
if (!isa<MCEncodedFragment>(&Frag) && !isa<MCCVDefRangeFragment>(&Frag))
continue;
ArrayRef<MCFixup> Fixups;
MutableArrayRef<char> Contents;
if (auto *FragWithFixups = dyn_cast<MCDataFragment>(&Frag)) {
Fixups = FragWithFixups->getFixups();
Contents = FragWithFixups->getContents();
} else if (auto *FragWithFixups = dyn_cast<MCRelaxableFragment>(&Frag)) {
Fixups = FragWithFixups->getFixups();
Contents = FragWithFixups->getContents();
} else if (auto *FragWithFixups = dyn_cast<MCCVDefRangeFragment>(&Frag)) {
Fixups = FragWithFixups->getFixups();
Contents = FragWithFixups->getContents();
} else
llvm_unreachable("Unknown fragment with fixups!");
for (const MCFixup &Fixup : Fixups) {
uint64_t FixedValue;
bool IsResolved;
MCValue Target;
std::tie(Target, FixedValue, IsResolved) =
handleFixup(Layout, Frag, Fixup);
getBackend().applyFixup(*this, Fixup, Target, Contents, FixedValue,
IsResolved);
}
}
}
}
void MCAssembler::Finish() {
// Create the layout object.
MCAsmLayout Layout(*this);
layout(Layout);
raw_ostream &OS = getWriter().getStream();
uint64_t StartOffset = OS.tell();
// Write the object file.
getWriter().writeObject(*this, Layout);
stats::ObjectBytes += OS.tell() - StartOffset;
}
bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
const MCRelaxableFragment *DF,
const MCAsmLayout &Layout) const {
assert(getBackendPtr() && "Expected assembler backend");
MCValue Target;
uint64_t Value;
bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
if (Target.getSymA() &&
Target.getSymA()->getKind() == MCSymbolRefExpr::VK_X86_ABS8 &&
Fixup.getKind() == FK_Data_1)
return false;
return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
Layout);
}
bool MCAssembler::fragmentNeedsRelaxation(const MCRelaxableFragment *F,
const MCAsmLayout &Layout) const {
assert(getBackendPtr() && "Expected assembler backend");
// If this inst doesn't ever need relaxation, ignore it. This occurs when we
// are intentionally pushing out inst fragments, or because we relaxed a
// previous instruction to one that doesn't need relaxation.
if (!getBackend().mayNeedRelaxation(F->getInst()))
return false;
for (const MCFixup &Fixup : F->getFixups())
if (fixupNeedsRelaxation(Fixup, F, Layout))
return true;
return false;
}
bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
MCRelaxableFragment &F) {
assert(getEmitterPtr() &&
"Expected CodeEmitter defined for relaxInstruction");
if (!fragmentNeedsRelaxation(&F, Layout))
return false;
++stats::RelaxedInstructions;
// FIXME-PERF: We could immediately lower out instructions if we can tell
// they are fully resolved, to avoid retesting on later passes.
// Relax the fragment.
MCInst Relaxed;
getBackend().relaxInstruction(F.getInst(), F.getSubtargetInfo(), Relaxed);
// Encode the new instruction.
//
// FIXME-PERF: If it matters, we could let the target do this. It can
// probably do so more efficiently in many cases.
SmallVector<MCFixup, 4> Fixups;
SmallString<256> Code;
raw_svector_ostream VecOS(Code);
getEmitter().encodeInstruction(Relaxed, VecOS, Fixups, F.getSubtargetInfo());
// Update the fragment.
F.setInst(Relaxed);
F.getContents() = Code;
F.getFixups() = Fixups;
return true;
}
bool MCAssembler::relaxPaddingFragment(MCAsmLayout &Layout,
MCPaddingFragment &PF) {
assert(getBackendPtr() && "Expected assembler backend");
uint64_t OldSize = PF.getSize();
if (!getBackend().relaxFragment(&PF, Layout))
return false;
uint64_t NewSize = PF.getSize();
++stats::PaddingFragmentsRelaxations;
stats::PaddingFragmentsBytes += NewSize;
stats::PaddingFragmentsBytes -= OldSize;
return true;
}
bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
uint64_t OldSize = LF.getContents().size();
int64_t Value;
bool Abs = LF.getValue().evaluateKnownAbsolute(Value, Layout);
if (!Abs)
report_fatal_error("sleb128 and uleb128 expressions must be absolute");
SmallString<8> &Data = LF.getContents();
Data.clear();
raw_svector_ostream OSE(Data);
// The compiler can generate EH table assembly that is impossible to assemble
// without either adding padding to an LEB fragment or adding extra padding
// to a later alignment fragment. To accommodate such tables, relaxation can
// only increase an LEB fragment size here, not decrease it. See PR35809.
if (LF.isSigned())
encodeSLEB128(Value, OSE, OldSize);
else
encodeULEB128(Value, OSE, OldSize);
return OldSize != LF.getContents().size();
}
bool MCAssembler::relaxDwarfLineAddr(MCAsmLayout &Layout,
MCDwarfLineAddrFragment &DF) {
MCContext &Context = Layout.getAssembler().getContext();
uint64_t OldSize = DF.getContents().size();
int64_t AddrDelta;
bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
assert(Abs && "We created a line delta with an invalid expression");
(void) Abs;
int64_t LineDelta;
LineDelta = DF.getLineDelta();
SmallString<8> &Data = DF.getContents();
Data.clear();
raw_svector_ostream OSE(Data);
MCDwarfLineAddr::Encode(Context, getDWARFLinetableParams(), LineDelta,
AddrDelta, OSE);
return OldSize != Data.size();
}
bool MCAssembler::relaxDwarfCallFrameFragment(MCAsmLayout &Layout,
MCDwarfCallFrameFragment &DF) {
MCContext &Context = Layout.getAssembler().getContext();
uint64_t OldSize = DF.getContents().size();
int64_t AddrDelta;
bool Abs = DF.getAddrDelta().evaluateKnownAbsolute(AddrDelta, Layout);
assert(Abs && "We created call frame with an invalid expression");
(void) Abs;
SmallString<8> &Data = DF.getContents();
Data.clear();
raw_svector_ostream OSE(Data);
MCDwarfFrameEmitter::EncodeAdvanceLoc(Context, AddrDelta, OSE);
return OldSize != Data.size();
}
bool MCAssembler::relaxCVInlineLineTable(MCAsmLayout &Layout,
MCCVInlineLineTableFragment &F) {
unsigned OldSize = F.getContents().size();
getContext().getCVContext().encodeInlineLineTable(Layout, F);
return OldSize != F.getContents().size();
}
bool MCAssembler::relaxCVDefRange(MCAsmLayout &Layout,
MCCVDefRangeFragment &F) {
unsigned OldSize = F.getContents().size();
getContext().getCVContext().encodeDefRange(Layout, F);
return OldSize != F.getContents().size();
}
bool MCAssembler::layoutSectionOnce(MCAsmLayout &Layout, MCSection &Sec) {
// Holds the first fragment which needed relaxing during this layout. It will
// remain NULL if none were relaxed.
// When a fragment is relaxed, all the fragments following it should get
// invalidated because their offset is going to change.
MCFragment *FirstRelaxedFragment = nullptr;
// Attempt to relax all the fragments in the section.
for (MCSection::iterator I = Sec.begin(), IE = Sec.end(); I != IE; ++I) {
// Check if this is a fragment that needs relaxation.
bool RelaxedFrag = false;
switch(I->getKind()) {
default:
break;
case MCFragment::FT_Relaxable:
assert(!getRelaxAll() &&
"Did not expect a MCRelaxableFragment in RelaxAll mode");
RelaxedFrag = relaxInstruction(Layout, *cast<MCRelaxableFragment>(I));
break;
case MCFragment::FT_Dwarf:
RelaxedFrag = relaxDwarfLineAddr(Layout,
*cast<MCDwarfLineAddrFragment>(I));
break;
case MCFragment::FT_DwarfFrame:
RelaxedFrag =
relaxDwarfCallFrameFragment(Layout,
*cast<MCDwarfCallFrameFragment>(I));
break;
case MCFragment::FT_LEB:
RelaxedFrag = relaxLEB(Layout, *cast<MCLEBFragment>(I));
break;
case MCFragment::FT_Padding:
RelaxedFrag = relaxPaddingFragment(Layout, *cast<MCPaddingFragment>(I));
break;
case MCFragment::FT_CVInlineLines:
RelaxedFrag =
relaxCVInlineLineTable(Layout, *cast<MCCVInlineLineTableFragment>(I));
break;
case MCFragment::FT_CVDefRange:
RelaxedFrag = relaxCVDefRange(Layout, *cast<MCCVDefRangeFragment>(I));
break;
}
if (RelaxedFrag && !FirstRelaxedFragment)
FirstRelaxedFragment = &*I;
}
if (FirstRelaxedFragment) {
Layout.invalidateFragmentsFrom(FirstRelaxedFragment);
return true;
}
return false;
}
bool MCAssembler::layoutOnce(MCAsmLayout &Layout) {
++stats::RelaxationSteps;
bool WasRelaxed = false;
for (iterator it = begin(), ie = end(); it != ie; ++it) {
MCSection &Sec = *it;
while (layoutSectionOnce(Layout, Sec))
WasRelaxed = true;
}
return WasRelaxed;
}
void MCAssembler::finishLayout(MCAsmLayout &Layout) {
assert(getBackendPtr() && "Expected assembler backend");
// The layout is done. Mark every fragment as valid.
for (unsigned int i = 0, n = Layout.getSectionOrder().size(); i != n; ++i) {
MCSection &Section = *Layout.getSectionOrder()[i];
Layout.getFragmentOffset(&*Section.rbegin());
computeFragmentSize(Layout, *Section.rbegin());
}
getBackend().finishLayout(*this, Layout);
}
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