1 // WebAssemblyMCInstLower.cpp - Convert WebAssembly MachineInstr to an MCInst //
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 ///
10 /// \file
11 /// This file contains code to lower WebAssembly MachineInstrs to their
12 /// corresponding MCInst records.
13 ///
14 //===----------------------------------------------------------------------===//
15 
16 #include "WebAssemblyMCInstLower.h"
17 #include "WebAssemblyAsmPrinter.h"
18 #include "WebAssemblyMachineFunctionInfo.h"
19 #include "WebAssemblyRuntimeLibcallSignatures.h"
20 #include "WebAssemblyUtilities.h"
21 #include "llvm/CodeGen/AsmPrinter.h"
22 #include "llvm/CodeGen/MachineFunction.h"
23 #include "llvm/IR/Constants.h"
24 #include "llvm/MC/MCAsmInfo.h"
25 #include "llvm/MC/MCContext.h"
26 #include "llvm/MC/MCExpr.h"
27 #include "llvm/MC/MCInst.h"
28 #include "llvm/MC/MCSymbolWasm.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/Support/raw_ostream.h"
31 using namespace llvm;
32 
33 // This disables the removal of registers when lowering into MC, as required
34 // by some current tests.
35 static cl::opt<bool>
36     WasmKeepRegisters("wasm-keep-registers", cl::Hidden,
37                       cl::desc("WebAssembly: output stack registers in"
38                                " instruction output for test purposes only."),
39                       cl::init(false));
40 
41 static unsigned regInstructionToStackInstruction(unsigned OpCode);
42 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI);
43 
44 MCSymbol *
45 WebAssemblyMCInstLower::GetGlobalAddressSymbol(const MachineOperand &MO) const {
46   const GlobalValue *Global = MO.getGlobal();
47   MCSymbolWasm *WasmSym = cast<MCSymbolWasm>(Printer.getSymbol(Global));
48 
49   if (const auto *FuncTy = dyn_cast<FunctionType>(Global->getValueType())) {
50     const MachineFunction &MF = *MO.getParent()->getParent()->getParent();
51     const TargetMachine &TM = MF.getTarget();
52     const Function &CurrentFunc = MF.getFunction();
53 
54     SmallVector<wasm::ValType, 4> Returns;
55     SmallVector<wasm::ValType, 4> Params;
56 
57     wasm::ValType iPTR = MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
58                              ? wasm::ValType::I64
59                              : wasm::ValType::I32;
60 
61     SmallVector<MVT, 4> ResultMVTs;
62     ComputeLegalValueVTs(CurrentFunc, TM, FuncTy->getReturnType(), ResultMVTs);
63     // WebAssembly can't currently handle returning tuples.
64     if (ResultMVTs.size() <= 1)
65       for (MVT ResultMVT : ResultMVTs)
66         Returns.push_back(WebAssembly::toValType(ResultMVT));
67     else
68       Params.push_back(iPTR);
69 
70     for (Type *Ty : FuncTy->params()) {
71       SmallVector<MVT, 4> ParamMVTs;
72       ComputeLegalValueVTs(CurrentFunc, TM, Ty, ParamMVTs);
73       for (MVT ParamMVT : ParamMVTs)
74         Params.push_back(WebAssembly::toValType(ParamMVT));
75     }
76 
77     if (FuncTy->isVarArg())
78       Params.push_back(iPTR);
79 
80     WasmSym->setReturns(std::move(Returns));
81     WasmSym->setParams(std::move(Params));
82     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
83   }
84 
85   return WasmSym;
86 }
87 
88 MCSymbol *WebAssemblyMCInstLower::GetExternalSymbolSymbol(
89     const MachineOperand &MO) const {
90   const char *Name = MO.getSymbolName();
91   MCSymbolWasm *WasmSym =
92       cast<MCSymbolWasm>(Printer.GetExternalSymbolSymbol(Name));
93   const WebAssemblySubtarget &Subtarget = Printer.getSubtarget();
94 
95   // __stack_pointer is a global variable; all other external symbols used by
96   // CodeGen are functions.  It's OK to hardcode knowledge of specific symbols
97   // here; this method is precisely there for fetching the signatures of known
98   // Clang-provided symbols.
99   if (strcmp(Name, "__stack_pointer") == 0) {
100     WasmSym->setType(wasm::WASM_SYMBOL_TYPE_GLOBAL);
101     WasmSym->setGlobalType(wasm::WasmGlobalType{
102         uint8_t(Subtarget.hasAddr64() ? wasm::WASM_TYPE_I64
103                                       : wasm::WASM_TYPE_I32),
104         true});
105     return WasmSym;
106   }
107 
108   SmallVector<wasm::ValType, 4> Returns;
109   SmallVector<wasm::ValType, 4> Params;
110   GetSignature(Subtarget, Name, Returns, Params);
111 
112   WasmSym->setReturns(std::move(Returns));
113   WasmSym->setParams(std::move(Params));
114   WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
115 
116   return WasmSym;
117 }
118 
119 MCOperand WebAssemblyMCInstLower::LowerSymbolOperand(MCSymbol *Sym,
120                                                      int64_t Offset,
121                                                      bool IsFunc,
122                                                      bool IsGlob) const {
123   MCSymbolRefExpr::VariantKind VK =
124       IsFunc ? MCSymbolRefExpr::VK_WebAssembly_FUNCTION
125              : IsGlob ? MCSymbolRefExpr::VK_WebAssembly_GLOBAL
126                       : MCSymbolRefExpr::VK_None;
127 
128   const MCExpr *Expr = MCSymbolRefExpr::create(Sym, VK, Ctx);
129 
130   if (Offset != 0) {
131     if (IsFunc)
132       report_fatal_error("Function addresses with offsets not supported");
133     if (IsGlob)
134       report_fatal_error("Global indexes with offsets not supported");
135     Expr =
136         MCBinaryExpr::createAdd(Expr, MCConstantExpr::create(Offset, Ctx), Ctx);
137   }
138 
139   return MCOperand::createExpr(Expr);
140 }
141 
142 // Return the WebAssembly type associated with the given register class.
143 static wasm::ValType getType(const TargetRegisterClass *RC) {
144   if (RC == &WebAssembly::I32RegClass)
145     return wasm::ValType::I32;
146   if (RC == &WebAssembly::I64RegClass)
147     return wasm::ValType::I64;
148   if (RC == &WebAssembly::F32RegClass)
149     return wasm::ValType::F32;
150   if (RC == &WebAssembly::F64RegClass)
151     return wasm::ValType::F64;
152   if (RC == &WebAssembly::V128RegClass)
153     return wasm::ValType::V128;
154   llvm_unreachable("Unexpected register class");
155 }
156 
157 void WebAssemblyMCInstLower::Lower(const MachineInstr *MI,
158                                    MCInst &OutMI) const {
159   OutMI.setOpcode(MI->getOpcode());
160 
161   const MCInstrDesc &Desc = MI->getDesc();
162   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
163     const MachineOperand &MO = MI->getOperand(i);
164 
165     MCOperand MCOp;
166     switch (MO.getType()) {
167     default:
168       MI->print(errs());
169       llvm_unreachable("unknown operand type");
170     case MachineOperand::MO_MachineBasicBlock:
171       MI->print(errs());
172       llvm_unreachable("MachineBasicBlock operand should have been rewritten");
173     case MachineOperand::MO_Register: {
174       // Ignore all implicit register operands.
175       if (MO.isImplicit())
176         continue;
177       const WebAssemblyFunctionInfo &MFI =
178           *MI->getParent()->getParent()->getInfo<WebAssemblyFunctionInfo>();
179       unsigned WAReg = MFI.getWAReg(MO.getReg());
180       MCOp = MCOperand::createReg(WAReg);
181       break;
182     }
183     case MachineOperand::MO_Immediate:
184       if (i < Desc.NumOperands) {
185         const MCOperandInfo &Info = Desc.OpInfo[i];
186         if (Info.OperandType == WebAssembly::OPERAND_TYPEINDEX) {
187           MCSymbol *Sym = Printer.createTempSymbol("typeindex");
188 
189           SmallVector<wasm::ValType, 4> Returns;
190           SmallVector<wasm::ValType, 4> Params;
191 
192           const MachineRegisterInfo &MRI =
193               MI->getParent()->getParent()->getRegInfo();
194           for (const MachineOperand &MO : MI->defs())
195             Returns.push_back(getType(MRI.getRegClass(MO.getReg())));
196           for (const MachineOperand &MO : MI->explicit_uses())
197             if (MO.isReg())
198               Params.push_back(getType(MRI.getRegClass(MO.getReg())));
199 
200           // call_indirect instructions have a callee operand at the end which
201           // doesn't count as a param.
202           if (WebAssembly::isCallIndirect(*MI))
203             Params.pop_back();
204 
205           MCSymbolWasm *WasmSym = cast<MCSymbolWasm>(Sym);
206           WasmSym->setReturns(std::move(Returns));
207           WasmSym->setParams(std::move(Params));
208           WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
209 
210           const MCExpr *Expr = MCSymbolRefExpr::create(
211               WasmSym, MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX, Ctx);
212           MCOp = MCOperand::createExpr(Expr);
213           break;
214         }
215       }
216       MCOp = MCOperand::createImm(MO.getImm());
217       break;
218     case MachineOperand::MO_FPImmediate: {
219       // TODO: MC converts all floating point immediate operands to double.
220       // This is fine for numeric values, but may cause NaNs to change bits.
221       const ConstantFP *Imm = MO.getFPImm();
222       if (Imm->getType()->isFloatTy())
223         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToFloat());
224       else if (Imm->getType()->isDoubleTy())
225         MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToDouble());
226       else
227         llvm_unreachable("unknown floating point immediate type");
228       break;
229     }
230     case MachineOperand::MO_GlobalAddress:
231       assert(MO.getTargetFlags() == WebAssemblyII::MO_NO_FLAG &&
232              "WebAssembly does not use target flags on GlobalAddresses");
233       MCOp = LowerSymbolOperand(GetGlobalAddressSymbol(MO), MO.getOffset(),
234                                 MO.getGlobal()->getValueType()->isFunctionTy(),
235                                 false);
236       break;
237     case MachineOperand::MO_ExternalSymbol:
238       // The target flag indicates whether this is a symbol for a
239       // variable or a function.
240       assert((MO.getTargetFlags() & ~WebAssemblyII::MO_SYMBOL_MASK) == 0 &&
241              "WebAssembly uses only symbol flags on ExternalSymbols");
242       MCOp = LowerSymbolOperand(
243           GetExternalSymbolSymbol(MO), /*Offset=*/0,
244           (MO.getTargetFlags() & WebAssemblyII::MO_SYMBOL_FUNCTION) != 0,
245           (MO.getTargetFlags() & WebAssemblyII::MO_SYMBOL_GLOBAL) != 0);
246       break;
247     }
248 
249     OutMI.addOperand(MCOp);
250   }
251 
252   if (!WasmKeepRegisters)
253     removeRegisterOperands(MI, OutMI);
254 }
255 
256 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI) {
257   // Remove all uses of stackified registers to bring the instruction format
258   // into its final stack form used thruout MC, and transition opcodes to
259   // their _S variant.
260   // We do this seperate from the above code that still may need these
261   // registers for e.g. call_indirect signatures.
262   // See comments in lib/Target/WebAssembly/WebAssemblyInstrFormats.td for
263   // details.
264   // TODO: the code above creates new registers which are then removed here.
265   // That code could be slightly simplified by not doing that, though maybe
266   // it is simpler conceptually to keep the code above in "register mode"
267   // until this transition point.
268   // FIXME: we are not processing inline assembly, which contains register
269   // operands, because it is used by later target generic code.
270   if (MI->isDebugInstr() || MI->isLabel() || MI->isInlineAsm())
271     return;
272 
273   // Transform to _S instruction.
274   auto RegOpcode = OutMI.getOpcode();
275   auto StackOpcode = regInstructionToStackInstruction(RegOpcode);
276   OutMI.setOpcode(StackOpcode);
277 
278   // Remove register operands.
279   for (auto I = OutMI.getNumOperands(); I; --I) {
280     auto &MO = OutMI.getOperand(I - 1);
281     if (MO.isReg()) {
282       OutMI.erase(&MO);
283     }
284   }
285 }
286 
287 static unsigned regInstructionToStackInstruction(unsigned OpCode) {
288   // For most opcodes, this function could have been implemented as "return
289   // OpCode + 1", but since table-gen alphabetically sorts them, this cannot be
290   // guaranteed (see e.g. BR and BR_IF). Instead we use a giant switch statement
291   // generated by a custom TableGen backend (WebAssemblyStackifierEmitter.cpp)
292   // that emits switch cases of the form
293   //
294   //   case WebAssembly::RegisterInstr: return WebAssembly::StackInstr;
295   //
296   // for every pair of equivalent register and stack instructions.
297   switch (OpCode) {
298   default:
299     llvm_unreachable(
300         "unknown WebAssembly instruction in WebAssemblyMCInstLower pass");
301 #include "WebAssemblyGenStackifier.inc"
302   }
303 }
304