1 //===-- AMDGPULowerModuleLDSPass.cpp ------------------------------*- C++ -*-=// 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 // This pass eliminates LDS uses from non-kernel functions. 10 // 11 // The strategy is to create a new struct with a field for each LDS variable 12 // and allocate that struct at the same address for every kernel. Uses of the 13 // original LDS variables are then replaced with compile time offsets from that 14 // known address. AMDGPUMachineFunction allocates the LDS global. 15 // 16 // Local variables with constant annotation or non-undef initializer are passed 17 // through unchanged for simplification or error diagnostics in later passes. 18 // 19 // To reduce the memory overhead variables that are only used by kernels are 20 // excluded from this transform. The analysis to determine whether a variable 21 // is only used by a kernel is cheap and conservative so this may allocate 22 // a variable in every kernel when it was not strictly necessary to do so. 23 // 24 // A possible future refinement is to specialise the structure per-kernel, so 25 // that fields can be elided based on more expensive analysis. 26 // 27 //===----------------------------------------------------------------------===// 28 29 #include "AMDGPU.h" 30 #include "Utils/AMDGPUBaseInfo.h" 31 #include "Utils/AMDGPUMemoryUtils.h" 32 #include "llvm/ADT/STLExtras.h" 33 #include "llvm/Analysis/CallGraph.h" 34 #include "llvm/IR/Constants.h" 35 #include "llvm/IR/DerivedTypes.h" 36 #include "llvm/IR/IRBuilder.h" 37 #include "llvm/IR/InlineAsm.h" 38 #include "llvm/IR/Instructions.h" 39 #include "llvm/IR/MDBuilder.h" 40 #include "llvm/InitializePasses.h" 41 #include "llvm/Pass.h" 42 #include "llvm/Support/CommandLine.h" 43 #include "llvm/Support/Debug.h" 44 #include "llvm/Support/OptimizedStructLayout.h" 45 #include "llvm/Transforms/Utils/ModuleUtils.h" 46 #include <vector> 47 48 #define DEBUG_TYPE "amdgpu-lower-module-lds" 49 50 using namespace llvm; 51 52 static cl::opt<bool> SuperAlignLDSGlobals( 53 "amdgpu-super-align-lds-globals", 54 cl::desc("Increase alignment of LDS if it is not on align boundary"), 55 cl::init(true), cl::Hidden); 56 57 namespace { 58 class AMDGPULowerModuleLDS : public ModulePass { 59 60 static void removeFromUsedList(Module &M, StringRef Name, 61 SmallPtrSetImpl<Constant *> &ToRemove) { 62 GlobalVariable *GV = M.getNamedGlobal(Name); 63 if (!GV || ToRemove.empty()) { 64 return; 65 } 66 67 SmallVector<Constant *, 16> Init; 68 auto *CA = cast<ConstantArray>(GV->getInitializer()); 69 for (auto &Op : CA->operands()) { 70 // ModuleUtils::appendToUsed only inserts Constants 71 Constant *C = cast<Constant>(Op); 72 if (!ToRemove.contains(C->stripPointerCasts())) { 73 Init.push_back(C); 74 } 75 } 76 77 if (Init.size() == CA->getNumOperands()) { 78 return; // none to remove 79 } 80 81 GV->eraseFromParent(); 82 83 for (Constant *C : ToRemove) { 84 C->removeDeadConstantUsers(); 85 } 86 87 if (!Init.empty()) { 88 ArrayType *ATy = 89 ArrayType::get(Type::getInt8PtrTy(M.getContext()), Init.size()); 90 GV = 91 new llvm::GlobalVariable(M, ATy, false, GlobalValue::AppendingLinkage, 92 ConstantArray::get(ATy, Init), Name); 93 GV->setSection("llvm.metadata"); 94 } 95 } 96 97 static void 98 removeFromUsedLists(Module &M, 99 const std::vector<GlobalVariable *> &LocalVars) { 100 SmallPtrSet<Constant *, 32> LocalVarsSet; 101 for (GlobalVariable *LocalVar : LocalVars) 102 if (Constant *C = dyn_cast<Constant>(LocalVar->stripPointerCasts())) 103 LocalVarsSet.insert(C); 104 removeFromUsedList(M, "llvm.used", LocalVarsSet); 105 removeFromUsedList(M, "llvm.compiler.used", LocalVarsSet); 106 } 107 108 static void markUsedByKernel(IRBuilder<> &Builder, Function *Func, 109 GlobalVariable *SGV) { 110 // The llvm.amdgcn.module.lds instance is implicitly used by all kernels 111 // that might call a function which accesses a field within it. This is 112 // presently approximated to 'all kernels' if there are any such functions 113 // in the module. This implicit use is redefined as an explicit use here so 114 // that later passes, specifically PromoteAlloca, account for the required 115 // memory without any knowledge of this transform. 116 117 // An operand bundle on llvm.donothing works because the call instruction 118 // survives until after the last pass that needs to account for LDS. It is 119 // better than inline asm as the latter survives until the end of codegen. A 120 // totally robust solution would be a function with the same semantics as 121 // llvm.donothing that takes a pointer to the instance and is lowered to a 122 // no-op after LDS is allocated, but that is not presently necessary. 123 124 LLVMContext &Ctx = Func->getContext(); 125 126 Builder.SetInsertPoint(Func->getEntryBlock().getFirstNonPHI()); 127 128 FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx), {}); 129 130 Function *Decl = 131 Intrinsic::getDeclaration(Func->getParent(), Intrinsic::donothing, {}); 132 133 Value *UseInstance[1] = {Builder.CreateInBoundsGEP( 134 SGV->getValueType(), SGV, ConstantInt::get(Type::getInt32Ty(Ctx), 0))}; 135 136 Builder.CreateCall(FTy, Decl, {}, 137 {OperandBundleDefT<Value *>("ExplicitUse", UseInstance)}, 138 ""); 139 } 140 141 public: 142 static char ID; 143 144 AMDGPULowerModuleLDS() : ModulePass(ID) { 145 initializeAMDGPULowerModuleLDSPass(*PassRegistry::getPassRegistry()); 146 } 147 148 bool runOnModule(Module &M) override { 149 CallGraph CG = CallGraph(M); 150 bool Changed = superAlignLDSGlobals(M); 151 std::vector<GlobalVariable *> ModuleScopeVariables = 152 AMDGPU::findVariablesToLower(M, nullptr); 153 Changed |= processUsedLDS(CG, M, ModuleScopeVariables); 154 155 for (Function &F : M.functions()) { 156 if (F.isDeclaration()) 157 continue; 158 159 // Only lower compute kernels' LDS. 160 if (!AMDGPU::isKernel(F.getCallingConv())) 161 continue; 162 std::vector<GlobalVariable *> KernelUsedVariables = 163 AMDGPU::findVariablesToLower(M, &F); 164 Changed |= processUsedLDS(CG, M, KernelUsedVariables, &F); 165 } 166 167 return Changed; 168 } 169 170 private: 171 // Increase the alignment of LDS globals if necessary to maximise the chance 172 // that we can use aligned LDS instructions to access them. 173 static bool superAlignLDSGlobals(Module &M) { 174 const DataLayout &DL = M.getDataLayout(); 175 bool Changed = false; 176 if (!SuperAlignLDSGlobals) { 177 return Changed; 178 } 179 180 for (auto &GV : M.globals()) { 181 if (GV.getType()->getPointerAddressSpace() != AMDGPUAS::LOCAL_ADDRESS) { 182 // Only changing alignment of LDS variables 183 continue; 184 } 185 if (!GV.hasInitializer()) { 186 // cuda/hip extern __shared__ variable, leave alignment alone 187 continue; 188 } 189 190 Align Alignment = AMDGPU::getAlign(DL, &GV); 191 TypeSize GVSize = DL.getTypeAllocSize(GV.getValueType()); 192 193 if (GVSize > 8) { 194 // We might want to use a b96 or b128 load/store 195 Alignment = std::max(Alignment, Align(16)); 196 } else if (GVSize > 4) { 197 // We might want to use a b64 load/store 198 Alignment = std::max(Alignment, Align(8)); 199 } else if (GVSize > 2) { 200 // We might want to use a b32 load/store 201 Alignment = std::max(Alignment, Align(4)); 202 } else if (GVSize > 1) { 203 // We might want to use a b16 load/store 204 Alignment = std::max(Alignment, Align(2)); 205 } 206 207 if (Alignment != AMDGPU::getAlign(DL, &GV)) { 208 Changed = true; 209 GV.setAlignment(Alignment); 210 } 211 } 212 return Changed; 213 } 214 215 bool processUsedLDS(CallGraph const &CG, Module &M, 216 std::vector<GlobalVariable *> const &LDSVarsToTransform, 217 Function *F = nullptr) { 218 LLVMContext &Ctx = M.getContext(); 219 const DataLayout &DL = M.getDataLayout(); 220 221 if (LDSVarsToTransform.empty()) { 222 // No variables to rewrite, no changes made. 223 return false; 224 } 225 226 SmallVector<OptimizedStructLayoutField, 8> LayoutFields; 227 LayoutFields.reserve(LDSVarsToTransform.size()); 228 for (GlobalVariable *GV : LDSVarsToTransform) { 229 OptimizedStructLayoutField F(GV, DL.getTypeAllocSize(GV->getValueType()), 230 AMDGPU::getAlign(DL, GV)); 231 LayoutFields.emplace_back(F); 232 } 233 234 performOptimizedStructLayout(LayoutFields); 235 236 std::vector<GlobalVariable *> LocalVars; 237 LocalVars.reserve(LDSVarsToTransform.size()); // will be at least this large 238 { 239 // This usually won't need to insert any padding, perhaps avoid the alloc 240 uint64_t CurrentOffset = 0; 241 for (size_t I = 0; I < LayoutFields.size(); I++) { 242 GlobalVariable *FGV = static_cast<GlobalVariable *>( 243 const_cast<void *>(LayoutFields[I].Id)); 244 Align DataAlign = LayoutFields[I].Alignment; 245 246 uint64_t DataAlignV = DataAlign.value(); 247 if (uint64_t Rem = CurrentOffset % DataAlignV) { 248 uint64_t Padding = DataAlignV - Rem; 249 250 // Append an array of padding bytes to meet alignment requested 251 // Note (o + (a - (o % a)) ) % a == 0 252 // (offset + Padding ) % align == 0 253 254 Type *ATy = ArrayType::get(Type::getInt8Ty(Ctx), Padding); 255 LocalVars.push_back(new GlobalVariable( 256 M, ATy, false, GlobalValue::InternalLinkage, UndefValue::get(ATy), 257 "", nullptr, GlobalValue::NotThreadLocal, AMDGPUAS::LOCAL_ADDRESS, 258 false)); 259 CurrentOffset += Padding; 260 } 261 262 LocalVars.push_back(FGV); 263 CurrentOffset += LayoutFields[I].Size; 264 } 265 } 266 267 std::vector<Type *> LocalVarTypes; 268 LocalVarTypes.reserve(LocalVars.size()); 269 std::transform( 270 LocalVars.cbegin(), LocalVars.cend(), std::back_inserter(LocalVarTypes), 271 [](const GlobalVariable *V) -> Type * { return V->getValueType(); }); 272 273 std::string VarName( 274 F ? (Twine("llvm.amdgcn.kernel.") + F->getName() + ".lds").str() 275 : "llvm.amdgcn.module.lds"); 276 StructType *LDSTy = StructType::create(Ctx, LocalVarTypes, VarName + ".t"); 277 278 Align StructAlign = 279 AMDGPU::getAlign(DL, LocalVars[0]); 280 281 GlobalVariable *SGV = new GlobalVariable( 282 M, LDSTy, false, GlobalValue::InternalLinkage, UndefValue::get(LDSTy), 283 VarName, nullptr, GlobalValue::NotThreadLocal, AMDGPUAS::LOCAL_ADDRESS, 284 false); 285 SGV->setAlignment(StructAlign); 286 if (!F) { 287 appendToCompilerUsed( 288 M, {static_cast<GlobalValue *>( 289 ConstantExpr::getPointerBitCastOrAddrSpaceCast( 290 cast<Constant>(SGV), Type::getInt8PtrTy(Ctx)))}); 291 } 292 293 // The verifier rejects used lists containing an inttoptr of a constant 294 // so remove the variables from these lists before replaceAllUsesWith 295 removeFromUsedLists(M, LocalVars); 296 297 // Create alias.scope and their lists. Each field in the new structure 298 // does not alias with all other fields. 299 SmallVector<MDNode *> AliasScopes; 300 SmallVector<Metadata *> NoAliasList; 301 if (LocalVars.size() > 1) { 302 MDBuilder MDB(Ctx); 303 AliasScopes.reserve(LocalVars.size()); 304 MDNode *Domain = MDB.createAnonymousAliasScopeDomain(); 305 for (size_t I = 0; I < LocalVars.size(); I++) { 306 MDNode *Scope = MDB.createAnonymousAliasScope(Domain); 307 AliasScopes.push_back(Scope); 308 } 309 NoAliasList.append(&AliasScopes[1], AliasScopes.end()); 310 } 311 312 // Replace uses of ith variable with a constantexpr to the ith field of the 313 // instance that will be allocated by AMDGPUMachineFunction 314 Type *I32 = Type::getInt32Ty(Ctx); 315 for (size_t I = 0; I < LocalVars.size(); I++) { 316 GlobalVariable *GV = LocalVars[I]; 317 Constant *GEPIdx[] = {ConstantInt::get(I32, 0), ConstantInt::get(I32, I)}; 318 Constant *GEP = ConstantExpr::getGetElementPtr(LDSTy, SGV, GEPIdx); 319 if (F) { 320 // Replace all constant uses with instructions if they belong to the 321 // current kernel. 322 for (User *U : make_early_inc_range(GV->users())) { 323 if (ConstantExpr *C = dyn_cast<ConstantExpr>(U)) 324 AMDGPU::replaceConstantUsesInFunction(C, F); 325 } 326 327 GV->removeDeadConstantUsers(); 328 329 GV->replaceUsesWithIf(GEP, [F](Use &U) { 330 Instruction *I = dyn_cast<Instruction>(U.getUser()); 331 return I && I->getFunction() == F; 332 }); 333 } else { 334 GV->replaceAllUsesWith(GEP); 335 } 336 if (GV->use_empty()) { 337 GV->eraseFromParent(); 338 } 339 340 uint64_t Off = DL.getStructLayout(LDSTy)->getElementOffset(I); 341 Align A = commonAlignment(StructAlign, Off); 342 343 if (I) 344 NoAliasList[I - 1] = AliasScopes[I - 1]; 345 MDNode *NoAlias = 346 NoAliasList.empty() ? nullptr : MDNode::get(Ctx, NoAliasList); 347 MDNode *AliasScope = 348 AliasScopes.empty() ? nullptr : MDNode::get(Ctx, {AliasScopes[I]}); 349 350 refineUsesAlignmentAndAA(GEP, A, DL, AliasScope, NoAlias); 351 } 352 353 // This ensures the variable is allocated when called functions access it. 354 // It also lets other passes, specifically PromoteAlloca, accurately 355 // calculate how much LDS will be used by the kernel after lowering. 356 if (!F) { 357 IRBuilder<> Builder(Ctx); 358 for (Function &Func : M.functions()) { 359 if (!Func.isDeclaration() && AMDGPU::isKernelCC(&Func)) { 360 const CallGraphNode *N = CG[&Func]; 361 const bool CalleesRequireModuleLDS = N->size() > 0; 362 363 if (CalleesRequireModuleLDS) { 364 // If a function this kernel might call requires module LDS, 365 // annotate the kernel to let later passes know it will allocate 366 // this structure, even if not apparent from the IR. 367 markUsedByKernel(Builder, &Func, SGV); 368 } else { 369 // However if we are certain this kernel cannot call a function that 370 // requires module LDS, annotate the kernel so the backend can elide 371 // the allocation without repeating callgraph walks. 372 Func.addFnAttr("amdgpu-elide-module-lds"); 373 } 374 } 375 } 376 } 377 return true; 378 } 379 380 void refineUsesAlignmentAndAA(Value *Ptr, Align A, const DataLayout &DL, 381 MDNode *AliasScope, MDNode *NoAlias, 382 unsigned MaxDepth = 5) { 383 if (!MaxDepth || (A == 1 && !AliasScope)) 384 return; 385 386 for (User *U : Ptr->users()) { 387 if (auto *I = dyn_cast<Instruction>(U)) { 388 if (AliasScope && I->mayReadOrWriteMemory()) { 389 MDNode *AS = I->getMetadata(LLVMContext::MD_alias_scope); 390 AS = (AS ? MDNode::getMostGenericAliasScope(AS, AliasScope) 391 : AliasScope); 392 I->setMetadata(LLVMContext::MD_alias_scope, AS); 393 394 MDNode *NA = I->getMetadata(LLVMContext::MD_noalias); 395 NA = (NA ? MDNode::intersect(NA, NoAlias) : NoAlias); 396 I->setMetadata(LLVMContext::MD_noalias, NA); 397 } 398 } 399 400 if (auto *LI = dyn_cast<LoadInst>(U)) { 401 LI->setAlignment(std::max(A, LI->getAlign())); 402 continue; 403 } 404 if (auto *SI = dyn_cast<StoreInst>(U)) { 405 if (SI->getPointerOperand() == Ptr) 406 SI->setAlignment(std::max(A, SI->getAlign())); 407 continue; 408 } 409 if (auto *AI = dyn_cast<AtomicRMWInst>(U)) { 410 // None of atomicrmw operations can work on pointers, but let's 411 // check it anyway in case it will or we will process ConstantExpr. 412 if (AI->getPointerOperand() == Ptr) 413 AI->setAlignment(std::max(A, AI->getAlign())); 414 continue; 415 } 416 if (auto *AI = dyn_cast<AtomicCmpXchgInst>(U)) { 417 if (AI->getPointerOperand() == Ptr) 418 AI->setAlignment(std::max(A, AI->getAlign())); 419 continue; 420 } 421 if (auto *GEP = dyn_cast<GetElementPtrInst>(U)) { 422 unsigned BitWidth = DL.getIndexTypeSizeInBits(GEP->getType()); 423 APInt Off(BitWidth, 0); 424 if (GEP->getPointerOperand() == Ptr) { 425 Align GA; 426 if (GEP->accumulateConstantOffset(DL, Off)) 427 GA = commonAlignment(A, Off.getLimitedValue()); 428 refineUsesAlignmentAndAA(GEP, GA, DL, AliasScope, NoAlias, 429 MaxDepth - 1); 430 } 431 continue; 432 } 433 if (auto *I = dyn_cast<Instruction>(U)) { 434 if (I->getOpcode() == Instruction::BitCast || 435 I->getOpcode() == Instruction::AddrSpaceCast) 436 refineUsesAlignmentAndAA(I, A, DL, AliasScope, NoAlias, MaxDepth - 1); 437 } 438 } 439 } 440 }; 441 442 } // namespace 443 char AMDGPULowerModuleLDS::ID = 0; 444 445 char &llvm::AMDGPULowerModuleLDSID = AMDGPULowerModuleLDS::ID; 446 447 INITIALIZE_PASS(AMDGPULowerModuleLDS, DEBUG_TYPE, 448 "Lower uses of LDS variables from non-kernel functions", false, 449 false) 450 451 ModulePass *llvm::createAMDGPULowerModuleLDSPass() { 452 return new AMDGPULowerModuleLDS(); 453 } 454 455 PreservedAnalyses AMDGPULowerModuleLDSPass::run(Module &M, 456 ModuleAnalysisManager &) { 457 return AMDGPULowerModuleLDS().runOnModule(M) ? PreservedAnalyses::none() 458 : PreservedAnalyses::all(); 459 } 460