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//===-------- PPCELFStreamer.cpp - ELF Object Output ---------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This is a custom MCELFStreamer for PowerPC.
//
// The purpose of the custom ELF streamer is to allow us to intercept
// instructions as they are being emitted and align all 8 byte instructions
// to a 64 byte boundary if required (by adding a 4 byte nop). This is important
// because 8 byte instructions are not allowed to cross 64 byte boundaries
// and by aliging anything that is within 4 bytes of the boundary we can
// guarantee that the 8 byte instructions do not cross that boundary.
//
//===----------------------------------------------------------------------===//
#include "PPCELFStreamer.h"
#include "PPCFixupKinds.h"
#include "PPCInstrInfo.h"
#include "PPCMCCodeEmitter.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSymbolELF.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/SourceMgr.h"
using namespace llvm;
PPCELFStreamer::PPCELFStreamer(MCContext &Context,
std::unique_ptr<MCAsmBackend> MAB,
std::unique_ptr<MCObjectWriter> OW,
std::unique_ptr<MCCodeEmitter> Emitter)
: MCELFStreamer(Context, std::move(MAB), std::move(OW), std::move(Emitter)),
LastLabel(nullptr) {}
void PPCELFStreamer::emitPrefixedInstruction(const MCInst &Inst,
const MCSubtargetInfo &STI) {
// Prefixed instructions must not cross a 64-byte boundary (i.e. prefix is
// before the boundary and the remaining 4-bytes are after the boundary). In
// order to achieve this, a nop is added prior to any such boundary-crossing
// prefixed instruction. Align to 64 bytes if possible but add a maximum of 4
// bytes when trying to do that. If alignment requires adding more than 4
// bytes then the instruction won't be aligned. When emitting a code alignment
// a new fragment is created for this alignment. This fragment will contain
// all of the nops required as part of the alignment operation. In the cases
// when no nops are added then The fragment is still created but it remains
// empty.
emitCodeAlignment(Align(64), &STI, 4);
// Emit the instruction.
// Since the previous emit created a new fragment then adding this instruction
// also forces the addition of a new fragment. Inst is now the first
// instruction in that new fragment.
MCELFStreamer::emitInstruction(Inst, STI);
// The above instruction is forced to start a new fragment because it
// comes after a code alignment fragment. Get that new fragment.
MCFragment *InstructionFragment = getCurrentFragment();
SMLoc InstLoc = Inst.getLoc();
// Check if there was a last label emitted.
if (LastLabel && !LastLabel->isUnset() && LastLabelLoc.isValid() &&
InstLoc.isValid()) {
const SourceMgr *SourceManager = getContext().getSourceManager();
unsigned InstLine = SourceManager->FindLineNumber(InstLoc);
unsigned LabelLine = SourceManager->FindLineNumber(LastLabelLoc);
// If the Label and the Instruction are on the same line then move the
// label to the top of the fragment containing the aligned instruction that
// was just added.
if (InstLine == LabelLine) {
assignFragment(LastLabel, InstructionFragment);
LastLabel->setOffset(0);
}
}
}
void PPCELFStreamer::emitInstruction(const MCInst &Inst,
const MCSubtargetInfo &STI) {
PPCMCCodeEmitter *Emitter =
static_cast<PPCMCCodeEmitter*>(getAssembler().getEmitterPtr());
// If the instruction is a part of the GOT to PC-Rel link time optimization
// instruction pair, return a value, otherwise return std::nullopt. A true
// returned value means the instruction is the PLDpc and a false value means
// it is the user instruction.
std::optional<bool> IsPartOfGOTToPCRelPair =
isPartOfGOTToPCRelPair(Inst, STI);
// User of the GOT-indirect address.
// For example, the load that will get the relocation as follows:
// .reloc .Lpcrel1-8,R_PPC64_PCREL_OPT,.-(.Lpcrel1-8)
// lwa 3, 4(3)
if (IsPartOfGOTToPCRelPair && !*IsPartOfGOTToPCRelPair)
emitGOTToPCRelReloc(Inst);
// Special handling is only for prefixed instructions.
if (!Emitter->isPrefixedInstruction(Inst)) {
MCELFStreamer::emitInstruction(Inst, STI);
return;
}
emitPrefixedInstruction(Inst, STI);
// Producer of the GOT-indirect address.
// For example, the prefixed load from the got that will get the label as
// follows:
// pld 3, vec@got@pcrel(0), 1
// .Lpcrel1:
if (IsPartOfGOTToPCRelPair && *IsPartOfGOTToPCRelPair)
emitGOTToPCRelLabel(Inst);
}
void PPCELFStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
LastLabel = Symbol;
LastLabelLoc = Loc;
MCELFStreamer::emitLabel(Symbol);
}
// This linker time GOT PC Relative optimization relocation will look like this:
// pld <reg> symbol@got@pcrel
// <Label###>:
// .reloc Label###-8,R_PPC64_PCREL_OPT,.-(Label###-8)
// load <loadedreg>, 0(<reg>)
// The reason we place the label after the PLDpc instruction is that there
// may be an alignment nop before it since prefixed instructions must not
// cross a 64-byte boundary (please see
// PPCELFStreamer::emitPrefixedInstruction()). When referring to the
// label, we subtract the width of a prefixed instruction (8 bytes) to ensure
// we refer to the PLDpc.
void PPCELFStreamer::emitGOTToPCRelReloc(const MCInst &Inst) {
// Get the last operand which contains the symbol.
const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
assert(Operand.isExpr() && "Expecting an MCExpr.");
// Cast the last operand to MCSymbolRefExpr to get the symbol.
const MCExpr *Expr = Operand.getExpr();
const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
"Expecting a symbol of type VK_PPC_PCREL_OPT");
MCSymbol *LabelSym =
getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
const MCExpr *LabelExpr = MCSymbolRefExpr::create(LabelSym, getContext());
const MCExpr *Eight = MCConstantExpr::create(8, getContext());
// SubExpr is just Label###-8
const MCExpr *SubExpr =
MCBinaryExpr::createSub(LabelExpr, Eight, getContext());
MCSymbol *CurrentLocation = getContext().createTempSymbol();
const MCExpr *CurrentLocationExpr =
MCSymbolRefExpr::create(CurrentLocation, getContext());
// SubExpr2 is .-(Label###-8)
const MCExpr *SubExpr2 =
MCBinaryExpr::createSub(CurrentLocationExpr, SubExpr, getContext());
MCDataFragment *DF = static_cast<MCDataFragment *>(LabelSym->getFragment());
assert(DF && "Expecting a valid data fragment.");
MCFixupKind FixupKind = static_cast<MCFixupKind>(FirstLiteralRelocationKind +
ELF::R_PPC64_PCREL_OPT);
DF->getFixups().push_back(
MCFixup::create(LabelSym->getOffset() - 8, SubExpr2,
FixupKind, Inst.getLoc()));
emitLabel(CurrentLocation, Inst.getLoc());
}
// Emit the label that immediately follows the PLDpc for a link time GOT PC Rel
// optimization.
void PPCELFStreamer::emitGOTToPCRelLabel(const MCInst &Inst) {
// Get the last operand which contains the symbol.
const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
assert(Operand.isExpr() && "Expecting an MCExpr.");
// Cast the last operand to MCSymbolRefExpr to get the symbol.
const MCExpr *Expr = Operand.getExpr();
const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
"Expecting a symbol of type VK_PPC_PCREL_OPT");
MCSymbol *LabelSym =
getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
emitLabel(LabelSym, Inst.getLoc());
}
// This funciton checks if the parameter Inst is part of the setup for a link
// time GOT PC Relative optimization. For example in this situation:
// <MCInst PLDpc <MCOperand Reg:282> <MCOperand Expr:(glob_double@got@pcrel)>
// <MCOperand Imm:0> <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
// <MCInst SOME_LOAD <MCOperand Reg:22> <MCOperand Imm:0> <MCOperand Reg:282>
// <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
// The above is a pair of such instructions and this function will not return
// std::nullopt for either one of them. In both cases we are looking for the
// last operand <MCOperand Expr:(.Lpcrel@<<invalid>>)> which needs to be an
// MCExpr and has the flag MCSymbolRefExpr::VK_PPC_PCREL_OPT. After that we just
// look at the opcode and in the case of PLDpc we will return true. For the load
// (or store) this function will return false indicating it has found the second
// instruciton in the pair.
std::optional<bool> llvm::isPartOfGOTToPCRelPair(const MCInst &Inst,
const MCSubtargetInfo &STI) {
// Need at least two operands.
if (Inst.getNumOperands() < 2)
return std::nullopt;
unsigned LastOp = Inst.getNumOperands() - 1;
// The last operand needs to be an MCExpr and it needs to have a variant kind
// of VK_PPC_PCREL_OPT. If it does not satisfy these conditions it is not a
// link time GOT PC Rel opt instruction and we can ignore it and return
// std::nullopt.
const MCOperand &Operand = Inst.getOperand(LastOp);
if (!Operand.isExpr())
return std::nullopt;
// Check for the variant kind VK_PPC_PCREL_OPT in this expression.
const MCExpr *Expr = Operand.getExpr();
const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
if (!SymExpr || SymExpr->getKind() != MCSymbolRefExpr::VK_PPC_PCREL_OPT)
return std::nullopt;
return (Inst.getOpcode() == PPC::PLDpc);
}
MCELFStreamer *llvm::createPPCELFStreamer(
MCContext &Context, std::unique_ptr<MCAsmBackend> MAB,
std::unique_ptr<MCObjectWriter> OW,
std::unique_ptr<MCCodeEmitter> Emitter) {
return new PPCELFStreamer(Context, std::move(MAB), std::move(OW),
std::move(Emitter));
}
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