1 //===-------- PPCELFStreamer.cpp - ELF Object Output ---------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This is a custom MCELFStreamer for PowerPC.
10 //
11 // The purpose of the custom ELF streamer is to allow us to intercept
12 // instructions as they are being emitted and align all 8 byte instructions
13 // to a 64 byte boundary if required (by adding a 4 byte nop). This is important
14 // because 8 byte instructions are not allowed to cross 64 byte boundaries
15 // and by aliging anything that is within 4 bytes of the boundary we can
16 // guarantee that the 8 byte instructions do not cross that boundary.
17 //
18 //===----------------------------------------------------------------------===//
19 
20 #include "PPCELFStreamer.h"
21 #include "PPCFixupKinds.h"
22 #include "PPCMCCodeEmitter.h"
23 #include "PPCMCTargetDesc.h"
24 #include "llvm/BinaryFormat/ELF.h"
25 #include "llvm/MC/MCAsmBackend.h"
26 #include "llvm/MC/MCAssembler.h"
27 #include "llvm/MC/MCCodeEmitter.h"
28 #include "llvm/MC/MCContext.h"
29 #include "llvm/MC/MCInst.h"
30 #include "llvm/MC/MCInstrDesc.h"
31 #include "llvm/MC/MCObjectWriter.h"
32 #include "llvm/MC/MCSymbolELF.h"
33 #include "llvm/Support/Casting.h"
34 #include "llvm/Support/SourceMgr.h"
35 
36 using namespace llvm;
37 
38 PPCELFStreamer::PPCELFStreamer(MCContext &Context,
39                                std::unique_ptr<MCAsmBackend> MAB,
40                                std::unique_ptr<MCObjectWriter> OW,
41                                std::unique_ptr<MCCodeEmitter> Emitter)
42     : MCELFStreamer(Context, std::move(MAB), std::move(OW), std::move(Emitter)),
43       LastLabel(nullptr) {}
44 
45 void PPCELFStreamer::emitPrefixedInstruction(const MCInst &Inst,
46                                              const MCSubtargetInfo &STI) {
47   // Prefixed instructions must not cross a 64-byte boundary (i.e. prefix is
48   // before the boundary and the remaining 4-bytes are after the boundary). In
49   // order to achieve this, a nop is added prior to any such boundary-crossing
50   // prefixed instruction. Align to 64 bytes if possible but add a maximum of 4
51   // bytes when trying to do that. If alignment requires adding more than 4
52   // bytes then the instruction won't be aligned. When emitting a code alignment
53   // a new fragment is created for this alignment. This fragment will contain
54   // all of the nops required as part of the alignment operation. In the cases
55   // when no nops are added then The fragment is still created but it remains
56   // empty.
57   emitCodeAlignment(Align(64), &STI, 4);
58 
59   // Emit the instruction.
60   // Since the previous emit created a new fragment then adding this instruction
61   // also forces the addition of a new fragment. Inst is now the first
62   // instruction in that new fragment.
63   MCELFStreamer::emitInstruction(Inst, STI);
64 
65   // The above instruction is forced to start a new fragment because it
66   // comes after a code alignment fragment. Get that new fragment.
67   MCFragment *InstructionFragment = getCurrentFragment();
68   SMLoc InstLoc = Inst.getLoc();
69   // Check if there was a last label emitted.
70   if (LastLabel && !LastLabel->isUnset() && LastLabelLoc.isValid() &&
71       InstLoc.isValid()) {
72     const SourceMgr *SourceManager = getContext().getSourceManager();
73     unsigned InstLine = SourceManager->FindLineNumber(InstLoc);
74     unsigned LabelLine = SourceManager->FindLineNumber(LastLabelLoc);
75     // If the Label and the Instruction are on the same line then move the
76     // label to the top of the fragment containing the aligned instruction that
77     // was just added.
78     if (InstLine == LabelLine) {
79       assignFragment(LastLabel, InstructionFragment);
80       LastLabel->setOffset(0);
81     }
82   }
83 }
84 
85 void PPCELFStreamer::emitInstruction(const MCInst &Inst,
86                                      const MCSubtargetInfo &STI) {
87   PPCMCCodeEmitter *Emitter =
88       static_cast<PPCMCCodeEmitter*>(getAssembler().getEmitterPtr());
89 
90   // If the instruction is a part of the GOT to PC-Rel link time optimization
91   // instruction pair, return a value, otherwise return std::nullopt. A true
92   // returned value means the instruction is the PLDpc and a false value means
93   // it is the user instruction.
94   std::optional<bool> IsPartOfGOTToPCRelPair =
95       isPartOfGOTToPCRelPair(Inst, STI);
96 
97   // User of the GOT-indirect address.
98   // For example, the load that will get the relocation as follows:
99   // .reloc .Lpcrel1-8,R_PPC64_PCREL_OPT,.-(.Lpcrel1-8)
100   //  lwa 3, 4(3)
101   if (IsPartOfGOTToPCRelPair && !*IsPartOfGOTToPCRelPair)
102     emitGOTToPCRelReloc(Inst);
103 
104   // Special handling is only for prefixed instructions.
105   if (!Emitter->isPrefixedInstruction(Inst)) {
106     MCELFStreamer::emitInstruction(Inst, STI);
107     return;
108   }
109   emitPrefixedInstruction(Inst, STI);
110 
111   // Producer of the GOT-indirect address.
112   // For example, the prefixed load from the got that will get the label as
113   // follows:
114   //  pld 3, vec@got@pcrel(0), 1
115   // .Lpcrel1:
116   if (IsPartOfGOTToPCRelPair && *IsPartOfGOTToPCRelPair)
117     emitGOTToPCRelLabel(Inst);
118 }
119 
120 void PPCELFStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
121   LastLabel = Symbol;
122   LastLabelLoc = Loc;
123   MCELFStreamer::emitLabel(Symbol);
124 }
125 
126 // This linker time GOT PC Relative optimization relocation will look like this:
127 //   pld <reg> symbol@got@pcrel
128 // <Label###>:
129 //   .reloc Label###-8,R_PPC64_PCREL_OPT,.-(Label###-8)
130 //   load <loadedreg>, 0(<reg>)
131 // The reason we place the label after the PLDpc instruction is that there
132 // may be an alignment nop before it since prefixed instructions must not
133 // cross a 64-byte boundary (please see
134 // PPCELFStreamer::emitPrefixedInstruction()). When referring to the
135 // label, we subtract the width of a prefixed instruction (8 bytes) to ensure
136 // we refer to the PLDpc.
137 void PPCELFStreamer::emitGOTToPCRelReloc(const MCInst &Inst) {
138   // Get the last operand which contains the symbol.
139   const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
140   assert(Operand.isExpr() && "Expecting an MCExpr.");
141   // Cast the last operand to MCSymbolRefExpr to get the symbol.
142   const MCExpr *Expr = Operand.getExpr();
143   const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
144   assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
145          "Expecting a symbol of type VK_PPC_PCREL_OPT");
146   MCSymbol *LabelSym =
147       getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
148   const MCExpr *LabelExpr = MCSymbolRefExpr::create(LabelSym, getContext());
149   const MCExpr *Eight = MCConstantExpr::create(8, getContext());
150   // SubExpr is just Label###-8
151   const MCExpr *SubExpr =
152       MCBinaryExpr::createSub(LabelExpr, Eight, getContext());
153   MCSymbol *CurrentLocation = getContext().createTempSymbol();
154   const MCExpr *CurrentLocationExpr =
155       MCSymbolRefExpr::create(CurrentLocation, getContext());
156   // SubExpr2 is .-(Label###-8)
157   const MCExpr *SubExpr2 =
158       MCBinaryExpr::createSub(CurrentLocationExpr, SubExpr, getContext());
159 
160   MCDataFragment *DF = static_cast<MCDataFragment *>(LabelSym->getFragment());
161   assert(DF && "Expecting a valid data fragment.");
162   MCFixupKind FixupKind = static_cast<MCFixupKind>(FirstLiteralRelocationKind +
163                                                    ELF::R_PPC64_PCREL_OPT);
164   DF->getFixups().push_back(
165       MCFixup::create(LabelSym->getOffset() - 8, SubExpr2,
166                       FixupKind, Inst.getLoc()));
167   emitLabel(CurrentLocation, Inst.getLoc());
168 }
169 
170 // Emit the label that immediately follows the PLDpc for a link time GOT PC Rel
171 // optimization.
172 void PPCELFStreamer::emitGOTToPCRelLabel(const MCInst &Inst) {
173   // Get the last operand which contains the symbol.
174   const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
175   assert(Operand.isExpr() && "Expecting an MCExpr.");
176   // Cast the last operand to MCSymbolRefExpr to get the symbol.
177   const MCExpr *Expr = Operand.getExpr();
178   const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
179   assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
180          "Expecting a symbol of type VK_PPC_PCREL_OPT");
181   MCSymbol *LabelSym =
182       getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
183   emitLabel(LabelSym, Inst.getLoc());
184 }
185 
186 // This function checks if the parameter Inst is part of the setup for a link
187 // time GOT PC Relative optimization. For example in this situation:
188 // <MCInst PLDpc <MCOperand Reg:282> <MCOperand Expr:(glob_double@got@pcrel)>
189 //   <MCOperand Imm:0> <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
190 // <MCInst SOME_LOAD <MCOperand Reg:22> <MCOperand Imm:0> <MCOperand Reg:282>
191 //   <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
192 // The above is a pair of such instructions and this function will not return
193 // std::nullopt for either one of them. In both cases we are looking for the
194 // last operand <MCOperand Expr:(.Lpcrel@<<invalid>>)> which needs to be an
195 // MCExpr and has the flag MCSymbolRefExpr::VK_PPC_PCREL_OPT. After that we just
196 // look at the opcode and in the case of PLDpc we will return true. For the load
197 // (or store) this function will return false indicating it has found the second
198 // instruciton in the pair.
199 std::optional<bool> llvm::isPartOfGOTToPCRelPair(const MCInst &Inst,
200                                                  const MCSubtargetInfo &STI) {
201   // Need at least two operands.
202   if (Inst.getNumOperands() < 2)
203     return std::nullopt;
204 
205   unsigned LastOp = Inst.getNumOperands() - 1;
206   // The last operand needs to be an MCExpr and it needs to have a variant kind
207   // of VK_PPC_PCREL_OPT. If it does not satisfy these conditions it is not a
208   // link time GOT PC Rel opt instruction and we can ignore it and return
209   // std::nullopt.
210   const MCOperand &Operand = Inst.getOperand(LastOp);
211   if (!Operand.isExpr())
212     return std::nullopt;
213 
214   // Check for the variant kind VK_PPC_PCREL_OPT in this expression.
215   const MCExpr *Expr = Operand.getExpr();
216   const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
217   if (!SymExpr || SymExpr->getKind() != MCSymbolRefExpr::VK_PPC_PCREL_OPT)
218     return std::nullopt;
219 
220   return (Inst.getOpcode() == PPC::PLDpc);
221 }
222 
223 MCELFStreamer *llvm::createPPCELFStreamer(
224     MCContext &Context, std::unique_ptr<MCAsmBackend> MAB,
225     std::unique_ptr<MCObjectWriter> OW,
226     std::unique_ptr<MCCodeEmitter> Emitter) {
227   return new PPCELFStreamer(Context, std::move(MAB), std::move(OW),
228                             std::move(Emitter));
229 }
230