1 //===- WebAssemblyTargetMachine.cpp - Define TargetMachine for WebAssembly -==// 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 /// \brief This file defines the WebAssembly-specific subclass of TargetMachine. 12 /// 13 //===----------------------------------------------------------------------===// 14 15 #include "WebAssemblyTargetMachine.h" 16 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h" 17 #include "WebAssembly.h" 18 #include "WebAssemblyTargetObjectFile.h" 19 #include "WebAssemblyTargetTransformInfo.h" 20 #include "llvm/CodeGen/MachineFunctionPass.h" 21 #include "llvm/CodeGen/Passes.h" 22 #include "llvm/CodeGen/RegAllocRegistry.h" 23 #include "llvm/CodeGen/TargetPassConfig.h" 24 #include "llvm/IR/Function.h" 25 #include "llvm/Support/TargetRegistry.h" 26 #include "llvm/Target/TargetOptions.h" 27 #include "llvm/Transforms/Scalar.h" 28 using namespace llvm; 29 30 #define DEBUG_TYPE "wasm" 31 32 // Emscripten's asm.js-style exception handling 33 static cl::opt<bool> EnableEmException( 34 "enable-emscripten-cxx-exceptions", 35 cl::desc("WebAssembly Emscripten-style exception handling"), 36 cl::init(false)); 37 38 // Emscripten's asm.js-style setjmp/longjmp handling 39 static cl::opt<bool> EnableEmSjLj( 40 "enable-emscripten-sjlj", 41 cl::desc("WebAssembly Emscripten-style setjmp/longjmp handling"), 42 cl::init(false)); 43 44 extern "C" void LLVMInitializeWebAssemblyTarget() { 45 // Register the target. 46 RegisterTargetMachine<WebAssemblyTargetMachine> X( 47 getTheWebAssemblyTarget32()); 48 RegisterTargetMachine<WebAssemblyTargetMachine> Y( 49 getTheWebAssemblyTarget64()); 50 51 // Register exception handling pass to opt 52 initializeWebAssemblyLowerEmscriptenEHSjLjPass( 53 *PassRegistry::getPassRegistry()); 54 } 55 56 //===----------------------------------------------------------------------===// 57 // WebAssembly Lowering public interface. 58 //===----------------------------------------------------------------------===// 59 60 static Reloc::Model getEffectiveRelocModel(Optional<Reloc::Model> RM) { 61 if (!RM.hasValue()) 62 return Reloc::PIC_; 63 return *RM; 64 } 65 66 /// Create an WebAssembly architecture model. 67 /// 68 WebAssemblyTargetMachine::WebAssemblyTargetMachine( 69 const Target &T, const Triple &TT, StringRef CPU, StringRef FS, 70 const TargetOptions &Options, Optional<Reloc::Model> RM, 71 Optional<CodeModel::Model> CM, CodeGenOpt::Level OL, bool JIT) 72 : LLVMTargetMachine(T, 73 TT.isArch64Bit() ? "e-m:e-p:64:64-i64:64-n32:64-S128" 74 : "e-m:e-p:32:32-i64:64-n32:64-S128", 75 TT, CPU, FS, Options, getEffectiveRelocModel(RM), 76 CM ? *CM : CodeModel::Large, OL), 77 TLOF(TT.isOSBinFormatELF() ? 78 static_cast<TargetLoweringObjectFile*>( 79 new WebAssemblyTargetObjectFileELF()) : 80 static_cast<TargetLoweringObjectFile*>( 81 new WebAssemblyTargetObjectFile())) { 82 // WebAssembly type-checks instructions, but a noreturn function with a return 83 // type that doesn't match the context will cause a check failure. So we lower 84 // LLVM 'unreachable' to ISD::TRAP and then lower that to WebAssembly's 85 // 'unreachable' instructions which is meant for that case. 86 this->Options.TrapUnreachable = true; 87 88 // WebAssembly treats each function as an independent unit. Force 89 // -ffunction-sections, effectively, so that we can emit them independently. 90 if (!TT.isOSBinFormatELF()) { 91 this->Options.FunctionSections = true; 92 this->Options.DataSections = true; 93 this->Options.UniqueSectionNames = true; 94 } 95 96 initAsmInfo(); 97 98 // Note that we don't use setRequiresStructuredCFG(true). It disables 99 // optimizations than we're ok with, and want, such as critical edge 100 // splitting and tail merging. 101 } 102 103 WebAssemblyTargetMachine::~WebAssemblyTargetMachine() {} 104 105 const WebAssemblySubtarget * 106 WebAssemblyTargetMachine::getSubtargetImpl(const Function &F) const { 107 Attribute CPUAttr = F.getFnAttribute("target-cpu"); 108 Attribute FSAttr = F.getFnAttribute("target-features"); 109 110 std::string CPU = !CPUAttr.hasAttribute(Attribute::None) 111 ? CPUAttr.getValueAsString().str() 112 : TargetCPU; 113 std::string FS = !FSAttr.hasAttribute(Attribute::None) 114 ? FSAttr.getValueAsString().str() 115 : TargetFS; 116 117 auto &I = SubtargetMap[CPU + FS]; 118 if (!I) { 119 // This needs to be done before we create a new subtarget since any 120 // creation will depend on the TM and the code generation flags on the 121 // function that reside in TargetOptions. 122 resetTargetOptions(F); 123 I = llvm::make_unique<WebAssemblySubtarget>(TargetTriple, CPU, FS, *this); 124 } 125 return I.get(); 126 } 127 128 namespace { 129 /// WebAssembly Code Generator Pass Configuration Options. 130 class WebAssemblyPassConfig final : public TargetPassConfig { 131 public: 132 WebAssemblyPassConfig(WebAssemblyTargetMachine &TM, PassManagerBase &PM) 133 : TargetPassConfig(TM, PM) {} 134 135 WebAssemblyTargetMachine &getWebAssemblyTargetMachine() const { 136 return getTM<WebAssemblyTargetMachine>(); 137 } 138 139 FunctionPass *createTargetRegisterAllocator(bool) override; 140 141 void addIRPasses() override; 142 bool addInstSelector() override; 143 void addPostRegAlloc() override; 144 bool addGCPasses() override { return false; } 145 void addPreEmitPass() override; 146 }; 147 } // end anonymous namespace 148 149 TargetTransformInfo 150 WebAssemblyTargetMachine::getTargetTransformInfo(const Function &F) { 151 return TargetTransformInfo(WebAssemblyTTIImpl(this, F)); 152 } 153 154 TargetPassConfig * 155 WebAssemblyTargetMachine::createPassConfig(PassManagerBase &PM) { 156 return new WebAssemblyPassConfig(*this, PM); 157 } 158 159 FunctionPass *WebAssemblyPassConfig::createTargetRegisterAllocator(bool) { 160 return nullptr; // No reg alloc 161 } 162 163 //===----------------------------------------------------------------------===// 164 // The following functions are called from lib/CodeGen/Passes.cpp to modify 165 // the CodeGen pass sequence. 166 //===----------------------------------------------------------------------===// 167 168 void WebAssemblyPassConfig::addIRPasses() { 169 if (TM->Options.ThreadModel == ThreadModel::Single) 170 // In "single" mode, atomics get lowered to non-atomics. 171 addPass(createLowerAtomicPass()); 172 else 173 // Expand some atomic operations. WebAssemblyTargetLowering has hooks which 174 // control specifically what gets lowered. 175 addPass(createAtomicExpandPass()); 176 177 // Lower .llvm.global_dtors into .llvm_global_ctors with __cxa_atexit calls. 178 addPass(createWebAssemblyLowerGlobalDtors()); 179 180 // Fix function bitcasts, as WebAssembly requires caller and callee signatures 181 // to match. 182 addPass(createWebAssemblyFixFunctionBitcasts()); 183 184 // Optimize "returned" function attributes. 185 if (getOptLevel() != CodeGenOpt::None) 186 addPass(createWebAssemblyOptimizeReturned()); 187 188 // If exception handling is not enabled and setjmp/longjmp handling is 189 // enabled, we lower invokes into calls and delete unreachable landingpad 190 // blocks. Lowering invokes when there is no EH support is done in 191 // TargetPassConfig::addPassesToHandleExceptions, but this runs after this 192 // function and SjLj handling expects all invokes to be lowered before. 193 if (!EnableEmException && 194 TM->Options.ExceptionModel == ExceptionHandling::None) { 195 addPass(createLowerInvokePass()); 196 // The lower invoke pass may create unreachable code. Remove it in order not 197 // to process dead blocks in setjmp/longjmp handling. 198 addPass(createUnreachableBlockEliminationPass()); 199 } 200 201 // Handle exceptions and setjmp/longjmp if enabled. 202 if (EnableEmException || EnableEmSjLj) 203 addPass(createWebAssemblyLowerEmscriptenEHSjLj(EnableEmException, 204 EnableEmSjLj)); 205 206 TargetPassConfig::addIRPasses(); 207 } 208 209 bool WebAssemblyPassConfig::addInstSelector() { 210 (void)TargetPassConfig::addInstSelector(); 211 addPass( 212 createWebAssemblyISelDag(getWebAssemblyTargetMachine(), getOptLevel())); 213 // Run the argument-move pass immediately after the ScheduleDAG scheduler 214 // so that we can fix up the ARGUMENT instructions before anything else 215 // sees them in the wrong place. 216 addPass(createWebAssemblyArgumentMove()); 217 // Set the p2align operands. This information is present during ISel, however 218 // it's inconvenient to collect. Collect it now, and update the immediate 219 // operands. 220 addPass(createWebAssemblySetP2AlignOperands()); 221 return false; 222 } 223 224 void WebAssemblyPassConfig::addPostRegAlloc() { 225 // TODO: The following CodeGen passes don't currently support code containing 226 // virtual registers. Consider removing their restrictions and re-enabling 227 // them. 228 229 // Has no asserts of its own, but was not written to handle virtual regs. 230 disablePass(&ShrinkWrapID); 231 232 // These functions all require the NoVRegs property. 233 disablePass(&MachineCopyPropagationID); 234 disablePass(&PostRASchedulerID); 235 disablePass(&FuncletLayoutID); 236 disablePass(&StackMapLivenessID); 237 disablePass(&LiveDebugValuesID); 238 disablePass(&PatchableFunctionID); 239 240 TargetPassConfig::addPostRegAlloc(); 241 } 242 243 void WebAssemblyPassConfig::addPreEmitPass() { 244 TargetPassConfig::addPreEmitPass(); 245 246 // Now that we have a prologue and epilogue and all frame indices are 247 // rewritten, eliminate SP and FP. This allows them to be stackified, 248 // colored, and numbered with the rest of the registers. 249 addPass(createWebAssemblyReplacePhysRegs()); 250 251 // Rewrite pseudo call_indirect instructions as real instructions. 252 // This needs to run before register stackification, because we change the 253 // order of the arguments. 254 addPass(createWebAssemblyCallIndirectFixup()); 255 256 if (getOptLevel() != CodeGenOpt::None) { 257 // LiveIntervals isn't commonly run this late. Re-establish preconditions. 258 addPass(createWebAssemblyPrepareForLiveIntervals()); 259 260 // Depend on LiveIntervals and perform some optimizations on it. 261 addPass(createWebAssemblyOptimizeLiveIntervals()); 262 263 // Prepare store instructions for register stackifying. 264 addPass(createWebAssemblyStoreResults()); 265 266 // Mark registers as representing wasm's value stack. This is a key 267 // code-compression technique in WebAssembly. We run this pass (and 268 // StoreResults above) very late, so that it sees as much code as possible, 269 // including code emitted by PEI and expanded by late tail duplication. 270 addPass(createWebAssemblyRegStackify()); 271 272 // Run the register coloring pass to reduce the total number of registers. 273 // This runs after stackification so that it doesn't consider registers 274 // that become stackified. 275 addPass(createWebAssemblyRegColoring()); 276 } 277 278 // Eliminate multiple-entry loops. Do this before inserting explicit get_local 279 // and set_local operators because we create a new variable that we want 280 // converted into a local. 281 addPass(createWebAssemblyFixIrreducibleControlFlow()); 282 283 // Insert explicit get_local and set_local operators. 284 addPass(createWebAssemblyExplicitLocals()); 285 286 // Sort the blocks of the CFG into topological order, a prerequisite for 287 // BLOCK and LOOP markers. 288 addPass(createWebAssemblyCFGSort()); 289 290 // Insert BLOCK and LOOP markers. 291 addPass(createWebAssemblyCFGStackify()); 292 293 // Lower br_unless into br_if. 294 addPass(createWebAssemblyLowerBrUnless()); 295 296 // Perform the very last peephole optimizations on the code. 297 if (getOptLevel() != CodeGenOpt::None) 298 addPass(createWebAssemblyPeephole()); 299 300 // Create a mapping from LLVM CodeGen virtual registers to wasm registers. 301 addPass(createWebAssemblyRegNumbering()); 302 } 303