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(
124     MCSymbol *Sym, int64_t Offset, bool IsFunc, unsigned TargetFlags) const {
125   MCSymbolRefExpr::VariantKind Kind = MCSymbolRefExpr::VK_None;
126   if (TargetFlags & WebAssemblyII::MO_GOT)
127     Kind = MCSymbolRefExpr::VK_GOT;
128   const MCExpr *Expr = MCSymbolRefExpr::create(Sym, Kind, Ctx);
129 
130   if (Offset != 0) {
131     if (TargetFlags & WebAssemblyII::MO_GOT)
132       report_fatal_error("GOT symbol references do not support offsets");
133     unsigned Type = TargetFlags & WebAssemblyII::MO_SYMBOL_MASK;
134     assert((Type == WebAssemblyII::MO_SYMBOL_FUNCTION) == IsFunc);
135     if (Type == WebAssemblyII::MO_SYMBOL_FUNCTION || IsFunc)
136       report_fatal_error("Function addresses with offsets not supported");
137     if (Type == WebAssemblyII::MO_SYMBOL_GLOBAL)
138       report_fatal_error("Global indexes with offsets not supported");
139     if (Type == WebAssemblyII::MO_SYMBOL_EVENT)
140       report_fatal_error("Event indexes with offsets not supported");
141     Expr =
142         MCBinaryExpr::createAdd(Expr, MCConstantExpr::create(Offset, Ctx), Ctx);
143   }
144 
145   return MCOperand::createExpr(Expr);
146 }
147 
148 // Return the WebAssembly type associated with the given register class.
149 static wasm::ValType getType(const TargetRegisterClass *RC) {
150   if (RC == &WebAssembly::I32RegClass)
151     return wasm::ValType::I32;
152   if (RC == &WebAssembly::I64RegClass)
153     return wasm::ValType::I64;
154   if (RC == &WebAssembly::F32RegClass)
155     return wasm::ValType::F32;
156   if (RC == &WebAssembly::F64RegClass)
157     return wasm::ValType::F64;
158   if (RC == &WebAssembly::V128RegClass)
159     return wasm::ValType::V128;
160   llvm_unreachable("Unexpected register class");
161 }
162 
163 void WebAssemblyMCInstLower::lower(const MachineInstr *MI,
164                                    MCInst &OutMI) const {
165   OutMI.setOpcode(MI->getOpcode());
166 
167   const MCInstrDesc &Desc = MI->getDesc();
168   for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
169     const MachineOperand &MO = MI->getOperand(I);
170 
171     MCOperand MCOp;
172     switch (MO.getType()) {
173     default:
174       MI->print(errs());
175       llvm_unreachable("unknown operand type");
176     case MachineOperand::MO_MachineBasicBlock:
177       MI->print(errs());
178       llvm_unreachable("MachineBasicBlock operand should have been rewritten");
179     case MachineOperand::MO_Register: {
180       // Ignore all implicit register operands.
181       if (MO.isImplicit())
182         continue;
183       const WebAssemblyFunctionInfo &MFI =
184           *MI->getParent()->getParent()->getInfo<WebAssemblyFunctionInfo>();
185       unsigned WAReg = MFI.getWAReg(MO.getReg());
186       MCOp = MCOperand::createReg(WAReg);
187       break;
188     }
189     case MachineOperand::MO_Immediate:
190       if (I < Desc.NumOperands) {
191         const MCOperandInfo &Info = Desc.OpInfo[I];
192         if (Info.OperandType == WebAssembly::OPERAND_TYPEINDEX) {
193           MCSymbol *Sym = Printer.createTempSymbol("typeindex");
194 
195           SmallVector<wasm::ValType, 4> Returns;
196           SmallVector<wasm::ValType, 4> Params;
197 
198           const MachineRegisterInfo &MRI =
199               MI->getParent()->getParent()->getRegInfo();
200           for (const MachineOperand &MO : MI->defs())
201             Returns.push_back(getType(MRI.getRegClass(MO.getReg())));
202           for (const MachineOperand &MO : MI->explicit_uses())
203             if (MO.isReg())
204               Params.push_back(getType(MRI.getRegClass(MO.getReg())));
205 
206           // call_indirect instructions have a callee operand at the end which
207           // doesn't count as a param.
208           if (WebAssembly::isCallIndirect(*MI))
209             Params.pop_back();
210 
211           auto *WasmSym = cast<MCSymbolWasm>(Sym);
212           auto Signature = make_unique<wasm::WasmSignature>(std::move(Returns),
213                                                             std::move(Params));
214           WasmSym->setSignature(Signature.get());
215           Printer.addSignature(std::move(Signature));
216           WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
217 
218           const MCExpr *Expr = MCSymbolRefExpr::create(
219               WasmSym, MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX, Ctx);
220           MCOp = MCOperand::createExpr(Expr);
221           break;
222         }
223       }
224       MCOp = MCOperand::createImm(MO.getImm());
225       break;
226     case MachineOperand::MO_FPImmediate: {
227       // TODO: MC converts all floating point immediate operands to double.
228       // This is fine for numeric values, but may cause NaNs to change bits.
229       const ConstantFP *Imm = MO.getFPImm();
230       if (Imm->getType()->isFloatTy())
231         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToFloat());
232       else if (Imm->getType()->isDoubleTy())
233         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToDouble());
234       else
235         llvm_unreachable("unknown floating point immediate type");
236       break;
237     }
238     case MachineOperand::MO_GlobalAddress:
239       MCOp = lowerSymbolOperand(GetGlobalAddressSymbol(MO), MO.getOffset(),
240                                 MO.getGlobal()->getValueType()->isFunctionTy(),
241                                 MO.getTargetFlags());
242       break;
243     case MachineOperand::MO_ExternalSymbol:
244       // The target flag indicates whether this is a symbol for a
245       // variable or a function.
246       assert((MO.getTargetFlags() & ~WebAssemblyII::MO_SYMBOL_MASK) == 0 &&
247              "WebAssembly uses only symbol flags on ExternalSymbols");
248       MCOp = lowerSymbolOperand(
249           GetExternalSymbolSymbol(MO), /*Offset=*/0, false, MO.getTargetFlags());
250       break;
251     case MachineOperand::MO_MCSymbol:
252       // This is currently used only for LSDA symbols (GCC_except_table),
253       // because global addresses or other external symbols are handled above.
254       assert(MO.getTargetFlags() == 0 &&
255              "WebAssembly does not use target flags on MCSymbol");
256       MCOp = lowerSymbolOperand(MO.getMCSymbol(), /*Offset=*/0, false, MO.getTargetFlags());
257       break;
258     }
259 
260     OutMI.addOperand(MCOp);
261   }
262 
263   if (!WasmKeepRegisters)
264     removeRegisterOperands(MI, OutMI);
265 }
266 
267 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI) {
268   // Remove all uses of stackified registers to bring the instruction format
269   // into its final stack form used thruout MC, and transition opcodes to
270   // their _S variant.
271   // We do this seperate from the above code that still may need these
272   // registers for e.g. call_indirect signatures.
273   // See comments in lib/Target/WebAssembly/WebAssemblyInstrFormats.td for
274   // details.
275   // TODO: the code above creates new registers which are then removed here.
276   // That code could be slightly simplified by not doing that, though maybe
277   // it is simpler conceptually to keep the code above in "register mode"
278   // until this transition point.
279   // FIXME: we are not processing inline assembly, which contains register
280   // operands, because it is used by later target generic code.
281   if (MI->isDebugInstr() || MI->isLabel() || MI->isInlineAsm())
282     return;
283 
284   // Transform to _S instruction.
285   auto RegOpcode = OutMI.getOpcode();
286   auto StackOpcode = WebAssembly::getStackOpcode(RegOpcode);
287   assert(StackOpcode != -1 && "Failed to stackify instruction");
288   OutMI.setOpcode(StackOpcode);
289 
290   // Remove register operands.
291   for (auto I = OutMI.getNumOperands(); I; --I) {
292     auto &MO = OutMI.getOperand(I - 1);
293     if (MO.isReg()) {
294       OutMI.erase(&MO);
295     }
296   }
297 }
298