1 //===- WebAssemblyTargetMachine.cpp - Define TargetMachine for WebAssembly -==//
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 defines the WebAssembly-specific subclass of TargetMachine.
11 ///
12 //===----------------------------------------------------------------------===//
13
14 #include "WebAssemblyTargetMachine.h"
15 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
16 #include "TargetInfo/WebAssemblyTargetInfo.h"
17 #include "WebAssembly.h"
18 #include "WebAssemblyMachineFunctionInfo.h"
19 #include "WebAssemblyTargetObjectFile.h"
20 #include "WebAssemblyTargetTransformInfo.h"
21 #include "llvm/CodeGen/MIRParser/MIParser.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/CodeGen/RegAllocRegistry.h"
25 #include "llvm/CodeGen/TargetPassConfig.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/Support/TargetRegistry.h"
28 #include "llvm/Target/TargetOptions.h"
29 #include "llvm/Transforms/Scalar.h"
30 #include "llvm/Transforms/Scalar/LowerAtomic.h"
31 #include "llvm/Transforms/Utils.h"
32 using namespace llvm;
33
34 #define DEBUG_TYPE "wasm"
35
36 // Emscripten's asm.js-style exception handling
37 static cl::opt<bool> EnableEmException(
38 "enable-emscripten-cxx-exceptions",
39 cl::desc("WebAssembly Emscripten-style exception handling"),
40 cl::init(false));
41
42 // Emscripten's asm.js-style setjmp/longjmp handling
43 static cl::opt<bool> EnableEmSjLj(
44 "enable-emscripten-sjlj",
45 cl::desc("WebAssembly Emscripten-style setjmp/longjmp handling"),
46 cl::init(false));
47
48 // A command-line option to keep implicit locals
49 // for the purpose of testing with lit/llc ONLY.
50 // This produces output which is not valid WebAssembly, and is not supported
51 // by assemblers/disassemblers and other MC based tools.
52 static cl::opt<bool> WasmDisableExplicitLocals(
53 "wasm-disable-explicit-locals", cl::Hidden,
54 cl::desc("WebAssembly: output implicit locals in"
55 " instruction output for test purposes only."),
56 cl::init(false));
57
LLVMInitializeWebAssemblyTarget()58 extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeWebAssemblyTarget() {
59 // Register the target.
60 RegisterTargetMachine<WebAssemblyTargetMachine> X(
61 getTheWebAssemblyTarget32());
62 RegisterTargetMachine<WebAssemblyTargetMachine> Y(
63 getTheWebAssemblyTarget64());
64
65 // Register backend passes
66 auto &PR = *PassRegistry::getPassRegistry();
67 initializeWebAssemblyAddMissingPrototypesPass(PR);
68 initializeWebAssemblyLowerEmscriptenEHSjLjPass(PR);
69 initializeLowerGlobalDtorsPass(PR);
70 initializeFixFunctionBitcastsPass(PR);
71 initializeOptimizeReturnedPass(PR);
72 initializeWebAssemblyArgumentMovePass(PR);
73 initializeWebAssemblySetP2AlignOperandsPass(PR);
74 initializeWebAssemblyReplacePhysRegsPass(PR);
75 initializeWebAssemblyPrepareForLiveIntervalsPass(PR);
76 initializeWebAssemblyOptimizeLiveIntervalsPass(PR);
77 initializeWebAssemblyMemIntrinsicResultsPass(PR);
78 initializeWebAssemblyRegStackifyPass(PR);
79 initializeWebAssemblyRegColoringPass(PR);
80 initializeWebAssemblyFixIrreducibleControlFlowPass(PR);
81 initializeWebAssemblyLateEHPreparePass(PR);
82 initializeWebAssemblyExceptionInfoPass(PR);
83 initializeWebAssemblyCFGSortPass(PR);
84 initializeWebAssemblyCFGStackifyPass(PR);
85 initializeWebAssemblyExplicitLocalsPass(PR);
86 initializeWebAssemblyLowerBrUnlessPass(PR);
87 initializeWebAssemblyRegNumberingPass(PR);
88 initializeWebAssemblyDebugFixupPass(PR);
89 initializeWebAssemblyPeepholePass(PR);
90 }
91
92 //===----------------------------------------------------------------------===//
93 // WebAssembly Lowering public interface.
94 //===----------------------------------------------------------------------===//
95
getEffectiveRelocModel(Optional<Reloc::Model> RM,const Triple & TT)96 static Reloc::Model getEffectiveRelocModel(Optional<Reloc::Model> RM,
97 const Triple &TT) {
98 if (!RM.hasValue()) {
99 // Default to static relocation model. This should always be more optimial
100 // than PIC since the static linker can determine all global addresses and
101 // assume direct function calls.
102 return Reloc::Static;
103 }
104
105 if (!TT.isOSEmscripten()) {
106 // Relocation modes other than static are currently implemented in a way
107 // that only works for Emscripten, so disable them if we aren't targeting
108 // Emscripten.
109 return Reloc::Static;
110 }
111
112 return *RM;
113 }
114
115 /// Create an WebAssembly architecture model.
116 ///
WebAssemblyTargetMachine(const Target & T,const Triple & TT,StringRef CPU,StringRef FS,const TargetOptions & Options,Optional<Reloc::Model> RM,Optional<CodeModel::Model> CM,CodeGenOpt::Level OL,bool JIT)117 WebAssemblyTargetMachine::WebAssemblyTargetMachine(
118 const Target &T, const Triple &TT, StringRef CPU, StringRef FS,
119 const TargetOptions &Options, Optional<Reloc::Model> RM,
120 Optional<CodeModel::Model> CM, CodeGenOpt::Level OL, bool JIT)
121 : LLVMTargetMachine(T,
122 TT.isArch64Bit() ? "e-m:e-p:64:64-i64:64-n32:64-S128"
123 : "e-m:e-p:32:32-i64:64-n32:64-S128",
124 TT, CPU, FS, Options, getEffectiveRelocModel(RM, TT),
125 getEffectiveCodeModel(CM, CodeModel::Large), OL),
126 TLOF(new WebAssemblyTargetObjectFile()) {
127 // WebAssembly type-checks instructions, but a noreturn function with a return
128 // type that doesn't match the context will cause a check failure. So we lower
129 // LLVM 'unreachable' to ISD::TRAP and then lower that to WebAssembly's
130 // 'unreachable' instructions which is meant for that case.
131 this->Options.TrapUnreachable = true;
132
133 // WebAssembly treats each function as an independent unit. Force
134 // -ffunction-sections, effectively, so that we can emit them independently.
135 this->Options.FunctionSections = true;
136 this->Options.DataSections = true;
137 this->Options.UniqueSectionNames = true;
138
139 initAsmInfo();
140
141 // Note that we don't use setRequiresStructuredCFG(true). It disables
142 // optimizations than we're ok with, and want, such as critical edge
143 // splitting and tail merging.
144 }
145
146 WebAssemblyTargetMachine::~WebAssemblyTargetMachine() = default; // anchor.
147
148 const WebAssemblySubtarget *
getSubtargetImpl(std::string CPU,std::string FS) const149 WebAssemblyTargetMachine::getSubtargetImpl(std::string CPU,
150 std::string FS) const {
151 auto &I = SubtargetMap[CPU + FS];
152 if (!I) {
153 I = std::make_unique<WebAssemblySubtarget>(TargetTriple, CPU, FS, *this);
154 }
155 return I.get();
156 }
157
158 const WebAssemblySubtarget *
getSubtargetImpl(const Function & F) const159 WebAssemblyTargetMachine::getSubtargetImpl(const Function &F) const {
160 Attribute CPUAttr = F.getFnAttribute("target-cpu");
161 Attribute FSAttr = F.getFnAttribute("target-features");
162
163 std::string CPU = !CPUAttr.hasAttribute(Attribute::None)
164 ? CPUAttr.getValueAsString().str()
165 : TargetCPU;
166 std::string FS = !FSAttr.hasAttribute(Attribute::None)
167 ? FSAttr.getValueAsString().str()
168 : TargetFS;
169
170 // This needs to be done before we create a new subtarget since any
171 // creation will depend on the TM and the code generation flags on the
172 // function that reside in TargetOptions.
173 resetTargetOptions(F);
174
175 return getSubtargetImpl(CPU, FS);
176 }
177
178 namespace {
179
180 class CoalesceFeaturesAndStripAtomics final : public ModulePass {
181 // Take the union of all features used in the module and use it for each
182 // function individually, since having multiple feature sets in one module
183 // currently does not make sense for WebAssembly. If atomics are not enabled,
184 // also strip atomic operations and thread local storage.
185 static char ID;
186 WebAssemblyTargetMachine *WasmTM;
187
188 public:
CoalesceFeaturesAndStripAtomics(WebAssemblyTargetMachine * WasmTM)189 CoalesceFeaturesAndStripAtomics(WebAssemblyTargetMachine *WasmTM)
190 : ModulePass(ID), WasmTM(WasmTM) {}
191
runOnModule(Module & M)192 bool runOnModule(Module &M) override {
193 FeatureBitset Features = coalesceFeatures(M);
194
195 std::string FeatureStr = getFeatureString(Features);
196 for (auto &F : M)
197 replaceFeatures(F, FeatureStr);
198
199 bool StrippedAtomics = false;
200 bool StrippedTLS = false;
201
202 if (!Features[WebAssembly::FeatureAtomics])
203 StrippedAtomics = stripAtomics(M);
204
205 if (!Features[WebAssembly::FeatureBulkMemory])
206 StrippedTLS = stripThreadLocals(M);
207
208 if (StrippedAtomics && !StrippedTLS)
209 stripThreadLocals(M);
210 else if (StrippedTLS && !StrippedAtomics)
211 stripAtomics(M);
212
213 recordFeatures(M, Features, StrippedAtomics || StrippedTLS);
214
215 // Conservatively assume we have made some change
216 return true;
217 }
218
219 private:
coalesceFeatures(const Module & M)220 FeatureBitset coalesceFeatures(const Module &M) {
221 FeatureBitset Features =
222 WasmTM
223 ->getSubtargetImpl(std::string(WasmTM->getTargetCPU()),
224 std::string(WasmTM->getTargetFeatureString()))
225 ->getFeatureBits();
226 for (auto &F : M)
227 Features |= WasmTM->getSubtargetImpl(F)->getFeatureBits();
228 return Features;
229 }
230
getFeatureString(const FeatureBitset & Features)231 std::string getFeatureString(const FeatureBitset &Features) {
232 std::string Ret;
233 for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
234 if (Features[KV.Value])
235 Ret += (StringRef("+") + KV.Key + ",").str();
236 }
237 return Ret;
238 }
239
replaceFeatures(Function & F,const std::string & Features)240 void replaceFeatures(Function &F, const std::string &Features) {
241 F.removeFnAttr("target-features");
242 F.removeFnAttr("target-cpu");
243 F.addFnAttr("target-features", Features);
244 }
245
stripAtomics(Module & M)246 bool stripAtomics(Module &M) {
247 // Detect whether any atomics will be lowered, since there is no way to tell
248 // whether the LowerAtomic pass lowers e.g. stores.
249 bool Stripped = false;
250 for (auto &F : M) {
251 for (auto &B : F) {
252 for (auto &I : B) {
253 if (I.isAtomic()) {
254 Stripped = true;
255 goto done;
256 }
257 }
258 }
259 }
260
261 done:
262 if (!Stripped)
263 return false;
264
265 LowerAtomicPass Lowerer;
266 FunctionAnalysisManager FAM;
267 for (auto &F : M)
268 Lowerer.run(F, FAM);
269
270 return true;
271 }
272
stripThreadLocals(Module & M)273 bool stripThreadLocals(Module &M) {
274 bool Stripped = false;
275 for (auto &GV : M.globals()) {
276 if (GV.getThreadLocalMode() !=
277 GlobalValue::ThreadLocalMode::NotThreadLocal) {
278 Stripped = true;
279 GV.setThreadLocalMode(GlobalValue::ThreadLocalMode::NotThreadLocal);
280 }
281 }
282 return Stripped;
283 }
284
recordFeatures(Module & M,const FeatureBitset & Features,bool Stripped)285 void recordFeatures(Module &M, const FeatureBitset &Features, bool Stripped) {
286 for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
287 if (Features[KV.Value]) {
288 // Mark features as used
289 std::string MDKey = (StringRef("wasm-feature-") + KV.Key).str();
290 M.addModuleFlag(Module::ModFlagBehavior::Error, MDKey,
291 wasm::WASM_FEATURE_PREFIX_USED);
292 }
293 }
294 // Code compiled without atomics or bulk-memory may have had its atomics or
295 // thread-local data lowered to nonatomic operations or non-thread-local
296 // data. In that case, we mark the pseudo-feature "shared-mem" as disallowed
297 // to tell the linker that it would be unsafe to allow this code ot be used
298 // in a module with shared memory.
299 if (Stripped) {
300 M.addModuleFlag(Module::ModFlagBehavior::Error, "wasm-feature-shared-mem",
301 wasm::WASM_FEATURE_PREFIX_DISALLOWED);
302 }
303 }
304 };
305 char CoalesceFeaturesAndStripAtomics::ID = 0;
306
307 /// WebAssembly Code Generator Pass Configuration Options.
308 class WebAssemblyPassConfig final : public TargetPassConfig {
309 public:
WebAssemblyPassConfig(WebAssemblyTargetMachine & TM,PassManagerBase & PM)310 WebAssemblyPassConfig(WebAssemblyTargetMachine &TM, PassManagerBase &PM)
311 : TargetPassConfig(TM, PM) {}
312
getWebAssemblyTargetMachine() const313 WebAssemblyTargetMachine &getWebAssemblyTargetMachine() const {
314 return getTM<WebAssemblyTargetMachine>();
315 }
316
317 FunctionPass *createTargetRegisterAllocator(bool) override;
318
319 void addIRPasses() override;
320 bool addInstSelector() override;
321 void addPostRegAlloc() override;
addGCPasses()322 bool addGCPasses() override { return false; }
323 void addPreEmitPass() override;
324
325 // No reg alloc
addRegAssignmentFast()326 bool addRegAssignmentFast() override { return false; }
327
328 // No reg alloc
addRegAssignmentOptimized()329 bool addRegAssignmentOptimized() override { return false; }
330 };
331 } // end anonymous namespace
332
333 TargetTransformInfo
getTargetTransformInfo(const Function & F)334 WebAssemblyTargetMachine::getTargetTransformInfo(const Function &F) {
335 return TargetTransformInfo(WebAssemblyTTIImpl(this, F));
336 }
337
338 TargetPassConfig *
createPassConfig(PassManagerBase & PM)339 WebAssemblyTargetMachine::createPassConfig(PassManagerBase &PM) {
340 return new WebAssemblyPassConfig(*this, PM);
341 }
342
createTargetRegisterAllocator(bool)343 FunctionPass *WebAssemblyPassConfig::createTargetRegisterAllocator(bool) {
344 return nullptr; // No reg alloc
345 }
346
347 //===----------------------------------------------------------------------===//
348 // The following functions are called from lib/CodeGen/Passes.cpp to modify
349 // the CodeGen pass sequence.
350 //===----------------------------------------------------------------------===//
351
addIRPasses()352 void WebAssemblyPassConfig::addIRPasses() {
353 // Runs LowerAtomicPass if necessary
354 addPass(new CoalesceFeaturesAndStripAtomics(&getWebAssemblyTargetMachine()));
355
356 // This is a no-op if atomics are not used in the module
357 addPass(createAtomicExpandPass());
358
359 // Add signatures to prototype-less function declarations
360 addPass(createWebAssemblyAddMissingPrototypes());
361
362 // Lower .llvm.global_dtors into .llvm_global_ctors with __cxa_atexit calls.
363 addPass(createWebAssemblyLowerGlobalDtors());
364
365 // Fix function bitcasts, as WebAssembly requires caller and callee signatures
366 // to match.
367 addPass(createWebAssemblyFixFunctionBitcasts());
368
369 // Optimize "returned" function attributes.
370 if (getOptLevel() != CodeGenOpt::None)
371 addPass(createWebAssemblyOptimizeReturned());
372
373 // If exception handling is not enabled and setjmp/longjmp handling is
374 // enabled, we lower invokes into calls and delete unreachable landingpad
375 // blocks. Lowering invokes when there is no EH support is done in
376 // TargetPassConfig::addPassesToHandleExceptions, but this runs after this
377 // function and SjLj handling expects all invokes to be lowered before.
378 if (!EnableEmException &&
379 TM->Options.ExceptionModel == ExceptionHandling::None) {
380 addPass(createLowerInvokePass());
381 // The lower invoke pass may create unreachable code. Remove it in order not
382 // to process dead blocks in setjmp/longjmp handling.
383 addPass(createUnreachableBlockEliminationPass());
384 }
385
386 // Handle exceptions and setjmp/longjmp if enabled.
387 if (EnableEmException || EnableEmSjLj)
388 addPass(createWebAssemblyLowerEmscriptenEHSjLj(EnableEmException,
389 EnableEmSjLj));
390
391 // Expand indirectbr instructions to switches.
392 addPass(createIndirectBrExpandPass());
393
394 TargetPassConfig::addIRPasses();
395 }
396
addInstSelector()397 bool WebAssemblyPassConfig::addInstSelector() {
398 (void)TargetPassConfig::addInstSelector();
399 addPass(
400 createWebAssemblyISelDag(getWebAssemblyTargetMachine(), getOptLevel()));
401 // Run the argument-move pass immediately after the ScheduleDAG scheduler
402 // so that we can fix up the ARGUMENT instructions before anything else
403 // sees them in the wrong place.
404 addPass(createWebAssemblyArgumentMove());
405 // Set the p2align operands. This information is present during ISel, however
406 // it's inconvenient to collect. Collect it now, and update the immediate
407 // operands.
408 addPass(createWebAssemblySetP2AlignOperands());
409
410 // Eliminate range checks and add default targets to br_table instructions.
411 addPass(createWebAssemblyFixBrTableDefaults());
412
413 return false;
414 }
415
addPostRegAlloc()416 void WebAssemblyPassConfig::addPostRegAlloc() {
417 // TODO: The following CodeGen passes don't currently support code containing
418 // virtual registers. Consider removing their restrictions and re-enabling
419 // them.
420
421 // These functions all require the NoVRegs property.
422 disablePass(&MachineCopyPropagationID);
423 disablePass(&PostRAMachineSinkingID);
424 disablePass(&PostRASchedulerID);
425 disablePass(&FuncletLayoutID);
426 disablePass(&StackMapLivenessID);
427 disablePass(&LiveDebugValuesID);
428 disablePass(&PatchableFunctionID);
429 disablePass(&ShrinkWrapID);
430
431 // This pass hurts code size for wasm because it can generate irreducible
432 // control flow.
433 disablePass(&MachineBlockPlacementID);
434
435 TargetPassConfig::addPostRegAlloc();
436 }
437
addPreEmitPass()438 void WebAssemblyPassConfig::addPreEmitPass() {
439 TargetPassConfig::addPreEmitPass();
440
441 // Eliminate multiple-entry loops.
442 addPass(createWebAssemblyFixIrreducibleControlFlow());
443
444 // Do various transformations for exception handling.
445 // Every CFG-changing optimizations should come before this.
446 addPass(createWebAssemblyLateEHPrepare());
447
448 // Now that we have a prologue and epilogue and all frame indices are
449 // rewritten, eliminate SP and FP. This allows them to be stackified,
450 // colored, and numbered with the rest of the registers.
451 addPass(createWebAssemblyReplacePhysRegs());
452
453 // Preparations and optimizations related to register stackification.
454 if (getOptLevel() != CodeGenOpt::None) {
455 // LiveIntervals isn't commonly run this late. Re-establish preconditions.
456 addPass(createWebAssemblyPrepareForLiveIntervals());
457
458 // Depend on LiveIntervals and perform some optimizations on it.
459 addPass(createWebAssemblyOptimizeLiveIntervals());
460
461 // Prepare memory intrinsic calls for register stackifying.
462 addPass(createWebAssemblyMemIntrinsicResults());
463
464 // Mark registers as representing wasm's value stack. This is a key
465 // code-compression technique in WebAssembly. We run this pass (and
466 // MemIntrinsicResults above) very late, so that it sees as much code as
467 // possible, including code emitted by PEI and expanded by late tail
468 // duplication.
469 addPass(createWebAssemblyRegStackify());
470
471 // Run the register coloring pass to reduce the total number of registers.
472 // This runs after stackification so that it doesn't consider registers
473 // that become stackified.
474 addPass(createWebAssemblyRegColoring());
475 }
476
477 // Sort the blocks of the CFG into topological order, a prerequisite for
478 // BLOCK and LOOP markers.
479 addPass(createWebAssemblyCFGSort());
480
481 // Insert BLOCK and LOOP markers.
482 addPass(createWebAssemblyCFGStackify());
483
484 // Insert explicit local.get and local.set operators.
485 if (!WasmDisableExplicitLocals)
486 addPass(createWebAssemblyExplicitLocals());
487
488 // Lower br_unless into br_if.
489 addPass(createWebAssemblyLowerBrUnless());
490
491 // Perform the very last peephole optimizations on the code.
492 if (getOptLevel() != CodeGenOpt::None)
493 addPass(createWebAssemblyPeephole());
494
495 // Create a mapping from LLVM CodeGen virtual registers to wasm registers.
496 addPass(createWebAssemblyRegNumbering());
497
498 // Fix debug_values whose defs have been stackified.
499 if (!WasmDisableExplicitLocals)
500 addPass(createWebAssemblyDebugFixup());
501 }
502
503 yaml::MachineFunctionInfo *
createDefaultFuncInfoYAML() const504 WebAssemblyTargetMachine::createDefaultFuncInfoYAML() const {
505 return new yaml::WebAssemblyFunctionInfo();
506 }
507
convertFuncInfoToYAML(const MachineFunction & MF) const508 yaml::MachineFunctionInfo *WebAssemblyTargetMachine::convertFuncInfoToYAML(
509 const MachineFunction &MF) const {
510 const auto *MFI = MF.getInfo<WebAssemblyFunctionInfo>();
511 return new yaml::WebAssemblyFunctionInfo(*MFI);
512 }
513
parseMachineFunctionInfo(const yaml::MachineFunctionInfo & MFI,PerFunctionMIParsingState & PFS,SMDiagnostic & Error,SMRange & SourceRange) const514 bool WebAssemblyTargetMachine::parseMachineFunctionInfo(
515 const yaml::MachineFunctionInfo &MFI, PerFunctionMIParsingState &PFS,
516 SMDiagnostic &Error, SMRange &SourceRange) const {
517 const auto &YamlMFI =
518 reinterpret_cast<const yaml::WebAssemblyFunctionInfo &>(MFI);
519 MachineFunction &MF = PFS.MF;
520 MF.getInfo<WebAssemblyFunctionInfo>()->initializeBaseYamlFields(YamlMFI);
521 return false;
522 }
523