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llvm-project/llvm/lib/MC/MCFragment.cpp
Rafael Auler 6d0528636a Rebase: [Facebook] [MC] Introduce NeverAlign fragment type 
Summary:
Introduce NeverAlign fragment type.

The intended usage of this fragment is to insert it before a pair of
macro-op fusion eligible instructions. NeverAlign fragment ensures that
the next fragment (first instruction in the pair) does not end at a
given alignment boundary by emitting a minimal size nop if necessary.

In effect, it ensures that a pair of macro-fusible instructions is not
split by a given alignment boundary, which is a precondition for
macro-op fusion in modern Intel Cores (64B = cache line size, see Intel
Architecture Optimization Reference Manual, 2.3.2.1 Legacy Decode
Pipeline: Macro-Fusion).

This patch introduces functionality used by BOLT when emitting code with
MacroFusion alignment already in place.

The use case is different from BoundaryAlign and instruction bundling:
- BoundaryAlign can be extended to perform the desired alignment for the
first instruction in the macro-op fusion pair (D101817). However, this
approach has higher overhead due to reliance on relaxation as
BoundaryAlign requires in the general case - see
https://reviews.llvm.org/D97982#2710638.
- Instruction bundling: the intent of NeverAlign fragment is to prevent
the first instruction in a pair ending at a given alignment boundary, by
inserting at most one minimum size nop. It's OK if either instruction
crosses the cache line. Padding both instructions using bundles to not
cross the alignment boundary would result in excessive padding. There's
no straightforward way to request instruction bundling to avoid a given
end alignment for the first instruction in the bundle.

LLVM: https://reviews.llvm.org/D97982

Manual rebase conflict history:
https://phabricator.intern.facebook.com/D30142613

Test Plan: sandcastle

Reviewers: #llvm-bolt

Subscribers: phabricatorlinter

Differential Revision: https://phabricator.intern.facebook.com/D31361547
2022-07-11 09:31:52 -07:00

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//===- lib/MC/MCFragment.cpp - Assembler Fragment Implementation ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//
#include "llvm/MC/MCFragment.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <utility>
using namespace llvm;
MCAsmLayout::MCAsmLayout(MCAssembler &Asm) : Assembler(Asm) {
// Compute the section layout order. Virtual sections must go last.
for (MCSection &Sec : Asm)
if (!Sec.isVirtualSection())
SectionOrder.push_back(&Sec);
for (MCSection &Sec : Asm)
if (Sec.isVirtualSection())
SectionOrder.push_back(&Sec);
}
bool MCAsmLayout::isFragmentValid(const MCFragment *F) const {
const MCSection *Sec = F->getParent();
const MCFragment *LastValid = LastValidFragment.lookup(Sec);
if (!LastValid)
return false;
assert(LastValid->getParent() == Sec);
return F->getLayoutOrder() <= LastValid->getLayoutOrder();
}
bool MCAsmLayout::canGetFragmentOffset(const MCFragment *F) const {
MCSection *Sec = F->getParent();
MCSection::iterator I;
if (MCFragment *LastValid = LastValidFragment[Sec]) {
// Fragment already valid, offset is available.
if (F->getLayoutOrder() <= LastValid->getLayoutOrder())
return true;
I = ++MCSection::iterator(LastValid);
} else
I = Sec->begin();
// A fragment ordered before F is currently being laid out.
const MCFragment *FirstInvalidFragment = &*I;
if (FirstInvalidFragment->IsBeingLaidOut)
return false;
return true;
}
void MCAsmLayout::invalidateFragmentsFrom(MCFragment *F) {
// If this fragment wasn't already valid, we don't need to do anything.
if (!isFragmentValid(F))
return;
// Otherwise, reset the last valid fragment to the previous fragment
// (if this is the first fragment, it will be NULL).
LastValidFragment[F->getParent()] = F->getPrevNode();
}
void MCAsmLayout::ensureValid(const MCFragment *F) const {
MCSection *Sec = F->getParent();
MCSection::iterator I;
if (MCFragment *Cur = LastValidFragment[Sec])
I = ++MCSection::iterator(Cur);
else
I = Sec->begin();
// Advance the layout position until the fragment is valid.
while (!isFragmentValid(F)) {
assert(I != Sec->end() && "Layout bookkeeping error");
const_cast<MCAsmLayout *>(this)->layoutFragment(&*I);
++I;
}
}
uint64_t MCAsmLayout::getFragmentOffset(const MCFragment *F) const {
ensureValid(F);
assert(F->Offset != ~UINT64_C(0) && "Address not set!");
return F->Offset;
}
// Simple getSymbolOffset helper for the non-variable case.
static bool getLabelOffset(const MCAsmLayout &Layout, const MCSymbol &S,
bool ReportError, uint64_t &Val) {
if (!S.getFragment()) {
if (ReportError)
report_fatal_error("unable to evaluate offset to undefined symbol '" +
S.getName() + "'");
return false;
}
Val = Layout.getFragmentOffset(S.getFragment()) + S.getOffset();
return true;
}
static bool getSymbolOffsetImpl(const MCAsmLayout &Layout, const MCSymbol &S,
bool ReportError, uint64_t &Val) {
if (!S.isVariable())
return getLabelOffset(Layout, S, ReportError, Val);
// If SD is a variable, evaluate it.
MCValue Target;
if (!S.getVariableValue()->evaluateAsValue(Target, Layout))
report_fatal_error("unable to evaluate offset for variable '" +
S.getName() + "'");
uint64_t Offset = Target.getConstant();
const MCSymbolRefExpr *A = Target.getSymA();
if (A) {
uint64_t ValA;
// FIXME: On most platforms, `Target`'s component symbols are labels from
// having been simplified during evaluation, but on Mach-O they can be
// variables due to PR19203. This, and the line below for `B` can be
// restored to call `getLabelOffset` when PR19203 is fixed.
if (!getSymbolOffsetImpl(Layout, A->getSymbol(), ReportError, ValA))
return false;
Offset += ValA;
}
const MCSymbolRefExpr *B = Target.getSymB();
if (B) {
uint64_t ValB;
if (!getSymbolOffsetImpl(Layout, B->getSymbol(), ReportError, ValB))
return false;
Offset -= ValB;
}
Val = Offset;
return true;
}
bool MCAsmLayout::getSymbolOffset(const MCSymbol &S, uint64_t &Val) const {
return getSymbolOffsetImpl(*this, S, false, Val);
}
uint64_t MCAsmLayout::getSymbolOffset(const MCSymbol &S) const {
uint64_t Val;
getSymbolOffsetImpl(*this, S, true, Val);
return Val;
}
const MCSymbol *MCAsmLayout::getBaseSymbol(const MCSymbol &Symbol) const {
if (!Symbol.isVariable())
return &Symbol;
const MCExpr *Expr = Symbol.getVariableValue();
MCValue Value;
if (!Expr->evaluateAsValue(Value, *this)) {
Assembler.getContext().reportError(
Expr->getLoc(), "expression could not be evaluated");
return nullptr;
}
const MCSymbolRefExpr *RefB = Value.getSymB();
if (RefB) {
Assembler.getContext().reportError(
Expr->getLoc(), Twine("symbol '") + RefB->getSymbol().getName() +
"' could not be evaluated in a subtraction expression");
return nullptr;
}
const MCSymbolRefExpr *A = Value.getSymA();
if (!A)
return nullptr;
const MCSymbol &ASym = A->getSymbol();
const MCAssembler &Asm = getAssembler();
if (ASym.isCommon()) {
Asm.getContext().reportError(Expr->getLoc(),
"Common symbol '" + ASym.getName() +
"' cannot be used in assignment expr");
return nullptr;
}
return &ASym;
}
uint64_t MCAsmLayout::getSectionAddressSize(const MCSection *Sec) const {
// The size is the last fragment's end offset.
const MCFragment &F = Sec->getFragmentList().back();
return getFragmentOffset(&F) + getAssembler().computeFragmentSize(*this, F);
}
uint64_t MCAsmLayout::getSectionFileSize(const MCSection *Sec) const {
// Virtual sections have no file size.
if (Sec->isVirtualSection())
return 0;
// Otherwise, the file size is the same as the address space size.
return getSectionAddressSize(Sec);
}
uint64_t llvm::computeBundlePadding(const MCAssembler &Assembler,
const MCEncodedFragment *F,
uint64_t FOffset, uint64_t FSize) {
uint64_t BundleSize = Assembler.getBundleAlignSize();
assert(BundleSize > 0 &&
"computeBundlePadding should only be called if bundling is enabled");
uint64_t BundleMask = BundleSize - 1;
uint64_t OffsetInBundle = FOffset & BundleMask;
uint64_t EndOfFragment = OffsetInBundle + FSize;
// There are two kinds of bundling restrictions:
//
// 1) For alignToBundleEnd(), add padding to ensure that the fragment will
// *end* on a bundle boundary.
// 2) Otherwise, check if the fragment would cross a bundle boundary. If it
// would, add padding until the end of the bundle so that the fragment
// will start in a new one.
if (F->alignToBundleEnd()) {
// Three possibilities here:
//
// A) The fragment just happens to end at a bundle boundary, so we're good.
// B) The fragment ends before the current bundle boundary: pad it just
// enough to reach the boundary.
// C) The fragment ends after the current bundle boundary: pad it until it
// reaches the end of the next bundle boundary.
//
// Note: this code could be made shorter with some modulo trickery, but it's
// intentionally kept in its more explicit form for simplicity.
if (EndOfFragment == BundleSize)
return 0;
else if (EndOfFragment < BundleSize)
return BundleSize - EndOfFragment;
else { // EndOfFragment > BundleSize
return 2 * BundleSize - EndOfFragment;
}
} else if (OffsetInBundle > 0 && EndOfFragment > BundleSize)
return BundleSize - OffsetInBundle;
else
return 0;
}
/* *** */
void ilist_alloc_traits<MCFragment>::deleteNode(MCFragment *V) { V->destroy(); }
MCFragment::MCFragment(FragmentType Kind, bool HasInstructions,
MCSection *Parent)
: Parent(Parent), Atom(nullptr), Offset(~UINT64_C(0)), LayoutOrder(0),
Kind(Kind), IsBeingLaidOut(false), HasInstructions(HasInstructions) {
if (Parent && !isa<MCDummyFragment>(*this))
Parent->getFragmentList().push_back(this);
}
void MCFragment::destroy() {
// First check if we are the sentinal.
if (Kind == FragmentType(~0)) {
delete this;
return;
}
switch (Kind) {
case FT_Align:
delete cast<MCAlignFragment>(this);
return;
case FT_NeverAlign:
delete cast<MCNeverAlignFragment>(this);
return;
case FT_Data:
delete cast<MCDataFragment>(this);
return;
case FT_CompactEncodedInst:
delete cast<MCCompactEncodedInstFragment>(this);
return;
case FT_Fill:
delete cast<MCFillFragment>(this);
return;
case FT_Nops:
delete cast<MCNopsFragment>(this);
return;
case FT_Relaxable:
delete cast<MCRelaxableFragment>(this);
return;
case FT_Org:
delete cast<MCOrgFragment>(this);
return;
case FT_Dwarf:
delete cast<MCDwarfLineAddrFragment>(this);
return;
case FT_DwarfFrame:
delete cast<MCDwarfCallFrameFragment>(this);
return;
case FT_LEB:
delete cast<MCLEBFragment>(this);
return;
case FT_BoundaryAlign:
delete cast<MCBoundaryAlignFragment>(this);
return;
case FT_SymbolId:
delete cast<MCSymbolIdFragment>(this);
return;
case FT_CVInlineLines:
delete cast<MCCVInlineLineTableFragment>(this);
return;
case FT_CVDefRange:
delete cast<MCCVDefRangeFragment>(this);
return;
case FT_PseudoProbe:
delete cast<MCPseudoProbeAddrFragment>(this);
return;
case FT_Dummy:
delete cast<MCDummyFragment>(this);
return;
}
}
// Debugging methods
namespace llvm {
raw_ostream &operator<<(raw_ostream &OS, const MCFixup &AF) {
OS << "<MCFixup" << " Offset:" << AF.getOffset()
<< " Value:" << *AF.getValue()
<< " Kind:" << AF.getKind() << ">";
return OS;
}
} // end namespace llvm
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void MCFragment::dump() const {
raw_ostream &OS = errs();
OS << "<";
switch (getKind()) {
case MCFragment::FT_Align: OS << "MCAlignFragment"; break;
case MCFragment::FT_NeverAlign:
OS << "MCNeverAlignFragment";
break;
case MCFragment::FT_Data: OS << "MCDataFragment"; break;
case MCFragment::FT_CompactEncodedInst:
OS << "MCCompactEncodedInstFragment"; break;
case MCFragment::FT_Fill: OS << "MCFillFragment"; break;
case MCFragment::FT_Nops:
OS << "MCFNopsFragment";
break;
case MCFragment::FT_Relaxable: OS << "MCRelaxableFragment"; break;
case MCFragment::FT_Org: OS << "MCOrgFragment"; break;
case MCFragment::FT_Dwarf: OS << "MCDwarfFragment"; break;
case MCFragment::FT_DwarfFrame: OS << "MCDwarfCallFrameFragment"; break;
case MCFragment::FT_LEB: OS << "MCLEBFragment"; break;
case MCFragment::FT_BoundaryAlign: OS<<"MCBoundaryAlignFragment"; break;
case MCFragment::FT_SymbolId: OS << "MCSymbolIdFragment"; break;
case MCFragment::FT_CVInlineLines: OS << "MCCVInlineLineTableFragment"; break;
case MCFragment::FT_CVDefRange: OS << "MCCVDefRangeTableFragment"; break;
case MCFragment::FT_PseudoProbe:
OS << "MCPseudoProbe";
break;
case MCFragment::FT_Dummy: OS << "MCDummyFragment"; break;
}
OS << "<MCFragment " << (const void *)this << " LayoutOrder:" << LayoutOrder
<< " Offset:" << Offset << " HasInstructions:" << hasInstructions();
if (const auto *EF = dyn_cast<MCEncodedFragment>(this))
OS << " BundlePadding:" << static_cast<unsigned>(EF->getBundlePadding());
OS << ">";
switch (getKind()) {
case MCFragment::FT_Align: {
const auto *AF = cast<MCAlignFragment>(this);
if (AF->hasEmitNops())
OS << " (emit nops)";
OS << "\n ";
OS << " Alignment:" << AF->getAlignment().value()
<< " Value:" << AF->getValue() << " ValueSize:" << AF->getValueSize()
<< " MaxBytesToEmit:" << AF->getMaxBytesToEmit() << ">";
break;
}
case MCFragment::FT_NeverAlign: {
const MCNeverAlignFragment *NAF = cast<MCNeverAlignFragment>(this);
OS << "\n ";
OS << " Alignment:" << NAF->getAlignment() << ">";
break;
}
case MCFragment::FT_Data: {
const auto *DF = cast<MCDataFragment>(this);
OS << "\n ";
OS << " Contents:[";
const SmallVectorImpl<char> &Contents = DF->getContents();
for (unsigned i = 0, e = Contents.size(); i != e; ++i) {
if (i) OS << ",";
OS << hexdigit((Contents[i] >> 4) & 0xF) << hexdigit(Contents[i] & 0xF);
}
OS << "] (" << Contents.size() << " bytes)";
if (DF->fixup_begin() != DF->fixup_end()) {
OS << ",\n ";
OS << " Fixups:[";
for (MCDataFragment::const_fixup_iterator it = DF->fixup_begin(),
ie = DF->fixup_end(); it != ie; ++it) {
if (it != DF->fixup_begin()) OS << ",\n ";
OS << *it;
}
OS << "]";
}
break;
}
case MCFragment::FT_CompactEncodedInst: {
const auto *CEIF =
cast<MCCompactEncodedInstFragment>(this);
OS << "\n ";
OS << " Contents:[";
const SmallVectorImpl<char> &Contents = CEIF->getContents();
for (unsigned i = 0, e = Contents.size(); i != e; ++i) {
if (i) OS << ",";
OS << hexdigit((Contents[i] >> 4) & 0xF) << hexdigit(Contents[i] & 0xF);
}
OS << "] (" << Contents.size() << " bytes)";
break;
}
case MCFragment::FT_Fill: {
const auto *FF = cast<MCFillFragment>(this);
OS << " Value:" << static_cast<unsigned>(FF->getValue())
<< " ValueSize:" << static_cast<unsigned>(FF->getValueSize())
<< " NumValues:" << FF->getNumValues();
break;
}
case MCFragment::FT_Nops: {
const auto *NF = cast<MCNopsFragment>(this);
OS << " NumBytes:" << NF->getNumBytes()
<< " ControlledNopLength:" << NF->getControlledNopLength();
break;
}
case MCFragment::FT_Relaxable: {
const auto *F = cast<MCRelaxableFragment>(this);
OS << "\n ";
OS << " Inst:";
F->getInst().dump_pretty(OS);
OS << " (" << F->getContents().size() << " bytes)";
break;
}
case MCFragment::FT_Org: {
const auto *OF = cast<MCOrgFragment>(this);
OS << "\n ";
OS << " Offset:" << OF->getOffset()
<< " Value:" << static_cast<unsigned>(OF->getValue());
break;
}
case MCFragment::FT_Dwarf: {
const auto *OF = cast<MCDwarfLineAddrFragment>(this);
OS << "\n ";
OS << " AddrDelta:" << OF->getAddrDelta()
<< " LineDelta:" << OF->getLineDelta();
break;
}
case MCFragment::FT_DwarfFrame: {
const auto *CF = cast<MCDwarfCallFrameFragment>(this);
OS << "\n ";
OS << " AddrDelta:" << CF->getAddrDelta();
break;
}
case MCFragment::FT_LEB: {
const auto *LF = cast<MCLEBFragment>(this);
OS << "\n ";
OS << " Value:" << LF->getValue() << " Signed:" << LF->isSigned();
break;
}
case MCFragment::FT_BoundaryAlign: {
const auto *BF = cast<MCBoundaryAlignFragment>(this);
OS << "\n ";
OS << " BoundarySize:" << BF->getAlignment().value()
<< " LastFragment:" << BF->getLastFragment()
<< " Size:" << BF->getSize();
break;
}
case MCFragment::FT_SymbolId: {
const auto *F = cast<MCSymbolIdFragment>(this);
OS << "\n ";
OS << " Sym:" << F->getSymbol();
break;
}
case MCFragment::FT_CVInlineLines: {
const auto *F = cast<MCCVInlineLineTableFragment>(this);
OS << "\n ";
OS << " Sym:" << *F->getFnStartSym();
break;
}
case MCFragment::FT_CVDefRange: {
const auto *F = cast<MCCVDefRangeFragment>(this);
OS << "\n ";
for (std::pair<const MCSymbol *, const MCSymbol *> RangeStartEnd :
F->getRanges()) {
OS << " RangeStart:" << RangeStartEnd.first;
OS << " RangeEnd:" << RangeStartEnd.second;
}
break;
}
case MCFragment::FT_PseudoProbe: {
const auto *OF = cast<MCPseudoProbeAddrFragment>(this);
OS << "\n ";
OS << " AddrDelta:" << OF->getAddrDelta();
break;
}
case MCFragment::FT_Dummy:
break;
}
OS << ">";
}
#endif
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