1 // WebAssemblyMCInstLower.cpp - Convert WebAssembly MachineInstr to an MCInst //
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 /// \file
10 /// This file contains code to lower WebAssembly MachineInstrs to their
11 /// corresponding MCInst records.
12 ///
13 //===----------------------------------------------------------------------===//
14 
15 #include "WebAssemblyMCInstLower.h"
16 #include "WebAssemblyAsmPrinter.h"
17 #include "WebAssemblyMachineFunctionInfo.h"
18 #include "WebAssemblyRuntimeLibcallSignatures.h"
19 #include "WebAssemblyUtilities.h"
20 #include "llvm/CodeGen/AsmPrinter.h"
21 #include "llvm/CodeGen/MachineFunction.h"
22 #include "llvm/IR/Constants.h"
23 #include "llvm/MC/MCAsmInfo.h"
24 #include "llvm/MC/MCContext.h"
25 #include "llvm/MC/MCExpr.h"
26 #include "llvm/MC/MCInst.h"
27 #include "llvm/MC/MCSymbolWasm.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Support/raw_ostream.h"
30 using namespace llvm;
31 
32 // Defines llvm::WebAssembly::getStackOpcode to convert register instructions to
33 // stack instructions
34 #define GET_INSTRMAP_INFO 1
35 #include "WebAssemblyGenInstrInfo.inc"
36 
37 // This disables the removal of registers when lowering into MC, as required
38 // by some current tests.
39 cl::opt<bool>
40     WasmKeepRegisters("wasm-keep-registers", cl::Hidden,
41                       cl::desc("WebAssembly: output stack registers in"
42                                " instruction output for test purposes only."),
43                       cl::init(false));
44 
45 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI);
46 
47 MCSymbol *
48 WebAssemblyMCInstLower::GetGlobalAddressSymbol(const MachineOperand &MO) const {
49   const GlobalValue *Global = MO.getGlobal();
50   auto *WasmSym = cast<MCSymbolWasm>(Printer.getSymbol(Global));
51 
52   if (const auto *FuncTy = dyn_cast<FunctionType>(Global->getValueType())) {
53     const MachineFunction &MF = *MO.getParent()->getParent()->getParent();
54     const TargetMachine &TM = MF.getTarget();
55     const Function &CurrentFunc = MF.getFunction();
56 
57     SmallVector<MVT, 1> ResultMVTs;
58     SmallVector<MVT, 4> ParamMVTs;
59     computeSignatureVTs(FuncTy, CurrentFunc, TM, ParamMVTs, ResultMVTs);
60 
61     auto Signature = signatureFromMVTs(ResultMVTs, ParamMVTs);
62     WasmSym->setSignature(Signature.get());
63     Printer.addSignature(std::move(Signature));
64     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
65   }
66 
67   return WasmSym;
68 }
69 
70 MCSymbol *WebAssemblyMCInstLower::GetExternalSymbolSymbol(
71     const MachineOperand &MO) const {
72   const char *Name = MO.getSymbolName();
73   auto *WasmSym = cast<MCSymbolWasm>(Printer.GetExternalSymbolSymbol(Name));
74   const WebAssemblySubtarget &Subtarget = Printer.getSubtarget();
75 
76   // Except for certain known symbols, all symbols used by CodeGen are
77   // functions. It's OK to hardcode knowledge of specific symbols here; this
78   // method is precisely there for fetching the signatures of known
79   // Clang-provided symbols.
80   if (strcmp(Name, "__stack_pointer") == 0 ||
81       strcmp(Name, "__memory_base") == 0 || strcmp(Name, "__table_base") == 0) {
82     bool Mutable = strcmp(Name, "__stack_pointer") == 0;
83     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_GLOBAL);
84     WasmSym->setGlobalType(wasm::WasmGlobalType{
85         uint8_t(Subtarget.hasAddr64() ? wasm::WASM_TYPE_I64
86                                       : wasm::WASM_TYPE_I32),
87         Mutable});
88     return WasmSym;
89   }
90 
91   SmallVector<wasm::ValType, 4> Returns;
92   SmallVector<wasm::ValType, 4> Params;
93   if (strcmp(Name, "__cpp_exception") == 0) {
94     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_EVENT);
95     // We can't confirm its signature index for now because there can be
96     // imported exceptions. Set it to be 0 for now.
97     WasmSym->setEventType(
98         {wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION, /* SigIndex */ 0});
99     // We may have multiple C++ compilation units to be linked together, each of
100     // which defines the exception symbol. To resolve them, we declare them as
101     // weak.
102     WasmSym->setWeak(true);
103     WasmSym->setExternal(true);
104 
105     // All C++ exceptions are assumed to have a single i32 (for wasm32) or i64
106     // (for wasm64) param type and void return type. The reaon is, all C++
107     // exception values are pointers, and to share the type section with
108     // functions, exceptions are assumed to have void return type.
109     Params.push_back(Subtarget.hasAddr64() ? wasm::ValType::I64
110                                            : wasm::ValType::I32);
111   } else { // Function symbols
112     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
113     getLibcallSignature(Subtarget, Name, Returns, Params);
114   }
115   auto Signature =
116       make_unique<wasm::WasmSignature>(std::move(Returns), std::move(Params));
117   WasmSym->setSignature(Signature.get());
118   Printer.addSignature(std::move(Signature));
119 
120   return WasmSym;
121 }
122 
123 MCOperand WebAssemblyMCInstLower::lowerSymbolOperand(const MachineOperand &MO,
124                                                      MCSymbol *Sym) const {
125   MCSymbolRefExpr::VariantKind Kind = MCSymbolRefExpr::VK_None;
126   unsigned TargetFlags = MO.getTargetFlags();
127 
128   switch (TargetFlags) {
129     case WebAssemblyII::MO_NO_FLAG:
130       break;
131     case WebAssemblyII::MO_GOT:
132       Kind = MCSymbolRefExpr::VK_GOT;
133       break;
134     case WebAssemblyII::MO_MEMORY_BASE_REL:
135       Kind = MCSymbolRefExpr::VK_WASM_MBREL;
136       break;
137     case WebAssemblyII::MO_TABLE_BASE_REL:
138       Kind = MCSymbolRefExpr::VK_WASM_TBREL;
139       break;
140     default:
141       llvm_unreachable("Unknown target flag on GV operand");
142   }
143 
144   const MCExpr *Expr = MCSymbolRefExpr::create(Sym, Kind, Ctx);
145 
146   if (MO.getOffset() != 0) {
147     const auto *WasmSym = cast<MCSymbolWasm>(Sym);
148     if (TargetFlags == WebAssemblyII::MO_GOT)
149       report_fatal_error("GOT symbol references do not support offsets");
150     if (WasmSym->isFunction())
151       report_fatal_error("Function addresses with offsets not supported");
152     if (WasmSym->isGlobal())
153       report_fatal_error("Global indexes with offsets not supported");
154     if (WasmSym->isEvent())
155       report_fatal_error("Event indexes with offsets not supported");
156 
157     Expr = MCBinaryExpr::createAdd(
158         Expr, MCConstantExpr::create(MO.getOffset(), Ctx), Ctx);
159   }
160 
161   return MCOperand::createExpr(Expr);
162 }
163 
164 // Return the WebAssembly type associated with the given register class.
165 static wasm::ValType getType(const TargetRegisterClass *RC) {
166   if (RC == &WebAssembly::I32RegClass)
167     return wasm::ValType::I32;
168   if (RC == &WebAssembly::I64RegClass)
169     return wasm::ValType::I64;
170   if (RC == &WebAssembly::F32RegClass)
171     return wasm::ValType::F32;
172   if (RC == &WebAssembly::F64RegClass)
173     return wasm::ValType::F64;
174   if (RC == &WebAssembly::V128RegClass)
175     return wasm::ValType::V128;
176   llvm_unreachable("Unexpected register class");
177 }
178 
179 void WebAssemblyMCInstLower::lower(const MachineInstr *MI,
180                                    MCInst &OutMI) const {
181   OutMI.setOpcode(MI->getOpcode());
182 
183   const MCInstrDesc &Desc = MI->getDesc();
184   for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
185     const MachineOperand &MO = MI->getOperand(I);
186 
187     MCOperand MCOp;
188     switch (MO.getType()) {
189     default:
190       MI->print(errs());
191       llvm_unreachable("unknown operand type");
192     case MachineOperand::MO_MachineBasicBlock:
193       MI->print(errs());
194       llvm_unreachable("MachineBasicBlock operand should have been rewritten");
195     case MachineOperand::MO_Register: {
196       // Ignore all implicit register operands.
197       if (MO.isImplicit())
198         continue;
199       const WebAssemblyFunctionInfo &MFI =
200           *MI->getParent()->getParent()->getInfo<WebAssemblyFunctionInfo>();
201       unsigned WAReg = MFI.getWAReg(MO.getReg());
202       MCOp = MCOperand::createReg(WAReg);
203       break;
204     }
205     case MachineOperand::MO_Immediate:
206       if (I < Desc.NumOperands) {
207         const MCOperandInfo &Info = Desc.OpInfo[I];
208         if (Info.OperandType == WebAssembly::OPERAND_TYPEINDEX) {
209           MCSymbol *Sym = Printer.createTempSymbol("typeindex");
210 
211           SmallVector<wasm::ValType, 4> Returns;
212           SmallVector<wasm::ValType, 4> Params;
213 
214           const MachineRegisterInfo &MRI =
215               MI->getParent()->getParent()->getRegInfo();
216           for (const MachineOperand &MO : MI->defs())
217             Returns.push_back(getType(MRI.getRegClass(MO.getReg())));
218           for (const MachineOperand &MO : MI->explicit_uses())
219             if (MO.isReg())
220               Params.push_back(getType(MRI.getRegClass(MO.getReg())));
221 
222           // call_indirect instructions have a callee operand at the end which
223           // doesn't count as a param.
224           if (WebAssembly::isCallIndirect(*MI))
225             Params.pop_back();
226 
227           auto *WasmSym = cast<MCSymbolWasm>(Sym);
228           auto Signature = make_unique<wasm::WasmSignature>(std::move(Returns),
229                                                             std::move(Params));
230           WasmSym->setSignature(Signature.get());
231           Printer.addSignature(std::move(Signature));
232           WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
233 
234           const MCExpr *Expr = MCSymbolRefExpr::create(
235               WasmSym, MCSymbolRefExpr::VK_WASM_TYPEINDEX, Ctx);
236           MCOp = MCOperand::createExpr(Expr);
237           break;
238         }
239       }
240       MCOp = MCOperand::createImm(MO.getImm());
241       break;
242     case MachineOperand::MO_FPImmediate: {
243       // TODO: MC converts all floating point immediate operands to double.
244       // This is fine for numeric values, but may cause NaNs to change bits.
245       const ConstantFP *Imm = MO.getFPImm();
246       if (Imm->getType()->isFloatTy())
247         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToFloat());
248       else if (Imm->getType()->isDoubleTy())
249         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToDouble());
250       else
251         llvm_unreachable("unknown floating point immediate type");
252       break;
253     }
254     case MachineOperand::MO_GlobalAddress:
255       MCOp = lowerSymbolOperand(MO, GetGlobalAddressSymbol(MO));
256       break;
257     case MachineOperand::MO_ExternalSymbol:
258       // The target flag indicates whether this is a symbol for a
259       // variable or a function.
260       assert(MO.getTargetFlags() == 0 &&
261              "WebAssembly uses only symbol flags on ExternalSymbols");
262       MCOp = lowerSymbolOperand(MO, GetExternalSymbolSymbol(MO));
263       break;
264     case MachineOperand::MO_MCSymbol:
265       // This is currently used only for LSDA symbols (GCC_except_table),
266       // because global addresses or other external symbols are handled above.
267       assert(MO.getTargetFlags() == 0 &&
268              "WebAssembly does not use target flags on MCSymbol");
269       MCOp = lowerSymbolOperand(MO, MO.getMCSymbol());
270       break;
271     }
272 
273     OutMI.addOperand(MCOp);
274   }
275 
276   if (!WasmKeepRegisters)
277     removeRegisterOperands(MI, OutMI);
278 }
279 
280 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI) {
281   // Remove all uses of stackified registers to bring the instruction format
282   // into its final stack form used thruout MC, and transition opcodes to
283   // their _S variant.
284   // We do this seperate from the above code that still may need these
285   // registers for e.g. call_indirect signatures.
286   // See comments in lib/Target/WebAssembly/WebAssemblyInstrFormats.td for
287   // details.
288   // TODO: the code above creates new registers which are then removed here.
289   // That code could be slightly simplified by not doing that, though maybe
290   // it is simpler conceptually to keep the code above in "register mode"
291   // until this transition point.
292   // FIXME: we are not processing inline assembly, which contains register
293   // operands, because it is used by later target generic code.
294   if (MI->isDebugInstr() || MI->isLabel() || MI->isInlineAsm())
295     return;
296 
297   // Transform to _S instruction.
298   auto RegOpcode = OutMI.getOpcode();
299   auto StackOpcode = WebAssembly::getStackOpcode(RegOpcode);
300   assert(StackOpcode != -1 && "Failed to stackify instruction");
301   OutMI.setOpcode(StackOpcode);
302 
303   // Remove register operands.
304   for (auto I = OutMI.getNumOperands(); I; --I) {
305     auto &MO = OutMI.getOperand(I - 1);
306     if (MO.isReg()) {
307       OutMI.erase(&MO);
308     }
309   }
310 }
311