AMDGPULowerModuleLDSPass.cpp

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//===-- AMDGPULowerModuleLDSPass.cpp ------------------------------*- C++ -*-=//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass eliminates LDS uses from non-kernel functions.
//
// The strategy is to create a new struct with a field for each LDS variable
// and allocate that struct at the same address for every kernel. Uses of the
// original LDS variables are then replaced with compile time offsets from that
// known address. AMDGPUMachineFunction allocates the LDS global.
//
// Local variables with constant annotation or non-undef initializer are passed
// through unchanged for simplification or error diagnostics in later passes.
//
// To reduce the memory overhead variables that are only used by kernels are
// excluded from this transform. The analysis to determine whether a variable
// is only used by a kernel is cheap and conservative so this may allocate
// a variable in every kernel when it was not strictly necessary to do so.
//
// A possible future refinement is to specialise the structure per-kernel, so
// that fields can be elided based on more expensive analysis.
//
//===----------------------------------------------------------------------===//
 
#include "AMDGPU.h"
#include "Utils/AMDGPUBaseInfo.h"
#include "Utils/AMDGPUMemoryUtils.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/OptimizedStructLayout.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include <vector>
 
#define DEBUG_TYPE "amdgpu-lower-module-lds"
 
using namespace llvm;
 
static cl::opt<bool> SuperAlignLDSGlobals(
    "amdgpu-super-align-lds-globals",
    cl::desc("Increase alignment of LDS if it is not on align boundary"),
    cl::init(true), cl::Hidden);
 
namespace {
 
SmallPtrSet<GlobalValue *, 32> getUsedList(Module &M) {
  SmallPtrSet<GlobalValue *, 32> UsedList;
 
  SmallVector<GlobalValue *, 32> TmpVec;
  collectUsedGlobalVariables(M, TmpVec, true);
  UsedList.insert(TmpVec.begin(), TmpVec.end());
 
  TmpVec.clear();
  collectUsedGlobalVariables(M, TmpVec, false);
  UsedList.insert(TmpVec.begin(), TmpVec.end());
 
  return UsedList;
}
 
class AMDGPULowerModuleLDS : public ModulePass {
 
  static void removeFromUsedList(Module &M, StringRef Name,
                                 SmallPtrSetImpl<Constant *> &ToRemove) {
    GlobalVariable *GV = M.getNamedGlobal(Name);
    if (!GV || ToRemove.empty()) {
      return;
    }
 
    SmallVector<Constant *, 16> Init;
    auto *CA = cast<ConstantArray>(GV->getInitializer());
    for (auto &Op : CA->operands()) {
      // ModuleUtils::appendToUsed only inserts Constants
      Constant *C = cast<Constant>(Op);
      if (!ToRemove.contains(C->stripPointerCasts())) {
        Init.push_back(C);
      }
    }
 
    if (Init.size() == CA->getNumOperands()) {
      return; // none to remove
    }
 
    GV->eraseFromParent();
 
    for (Constant *C : ToRemove) {
      C->removeDeadConstantUsers();
    }
 
    if (!Init.empty()) {
      ArrayType *ATy =
          ArrayType::get(Type::getInt8PtrTy(M.getContext()), Init.size());
      GV =
          new llvm::GlobalVariable(M, ATy, false, GlobalValue::AppendingLinkage,
                                   ConstantArray::get(ATy, Init), Name);
      GV->setSection("llvm.metadata");
    }
  }
 
  static void
  removeFromUsedLists(Module &M,
                      const std::vector<GlobalVariable *> &LocalVars) {
    SmallPtrSet<Constant *, 32> LocalVarsSet;
    for (GlobalVariable *LocalVar : LocalVars)
      if (Constant *C = dyn_cast<Constant>(LocalVar->stripPointerCasts()))
        LocalVarsSet.insert(C);
    removeFromUsedList(M, "llvm.used", LocalVarsSet);
    removeFromUsedList(M, "llvm.compiler.used", LocalVarsSet);
  }
 
  static void markUsedByKernel(IRBuilder<> &Builder, Function *Func,
                               GlobalVariable *SGV) {
    // The llvm.amdgcn.module.lds instance is implicitly used by all kernels
    // that might call a function which accesses a field within it. This is
    // presently approximated to 'all kernels' if there are any such functions
    // in the module. This implicit use is redefined as an explicit use here so
    // that later passes, specifically PromoteAlloca, account for the required
    // memory without any knowledge of this transform.
 
    // An operand bundle on llvm.donothing works because the call instruction
    // survives until after the last pass that needs to account for LDS. It is
    // better than inline asm as the latter survives until the end of codegen. A
    // totally robust solution would be a function with the same semantics as
    // llvm.donothing that takes a pointer to the instance and is lowered to a
    // no-op after LDS is allocated, but that is not presently necessary.
 
    LLVMContext &Ctx = Func->getContext();
 
    Builder.SetInsertPoint(Func->getEntryBlock().getFirstNonPHI());
 
    FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx), {});
 
    Function *Decl =
        Intrinsic::getDeclaration(Func->getParent(), Intrinsic::donothing, {});
 
    Value *UseInstance[1] = {Builder.CreateInBoundsGEP(
        SGV->getValueType(), SGV, ConstantInt::get(Type::getInt32Ty(Ctx), 0))};
 
    Builder.CreateCall(FTy, Decl, {},
                       {OperandBundleDefT<Value *>("ExplicitUse", UseInstance)},
                       "");
  }
 
private:
  SmallPtrSet<GlobalValue *, 32> UsedList;
 
public:
  static char ID;
 
  AMDGPULowerModuleLDS() : ModulePass(ID) {
    initializeAMDGPULowerModuleLDSPass(*PassRegistry::getPassRegistry());
  }
 
  bool runOnModule(Module &M) override {
    CallGraph CG = CallGraph(M);
    UsedList = getUsedList(M);
    bool Changed = superAlignLDSGlobals(M);
    Changed |= processUsedLDS(CG, M);
 
    for (Function &F : M.functions()) {
      if (F.isDeclaration())
        continue;
 
      // Only lower compute kernels' LDS.
      if (!AMDGPU::isKernel(F.getCallingConv()))
        continue;
      Changed |= processUsedLDS(CG, M, &F);
    }
 
    UsedList.clear();
    return Changed;
  }
 
private:
  // Increase the alignment of LDS globals if necessary to maximise the chance
  // that we can use aligned LDS instructions to access them.
  static bool superAlignLDSGlobals(Module &M) {
    const DataLayout &DL = M.getDataLayout();
    bool Changed = false;
    if (!SuperAlignLDSGlobals) {
      return Changed;
    }
 
    for (auto &GV : M.globals()) {
      if (GV.getType()->getPointerAddressSpace() != AMDGPUAS::LOCAL_ADDRESS) {
        // Only changing alignment of LDS variables
        continue;
      }
      if (!GV.hasInitializer()) {
        // cuda/hip extern __shared__ variable, leave alignment alone
        continue;
      }
 
      Align Alignment = AMDGPU::getAlign(DL, &GV);
      TypeSize GVSize = DL.getTypeAllocSize(GV.getValueType());
 
      if (GVSize > 8) {
        // We might want to use a b96 or b128 load/store
        Alignment = std::max(Alignment, Align(16));
      } else if (GVSize > 4) {
        // We might want to use a b64 load/store
        Alignment = std::max(Alignment, Align(8));
      } else if (GVSize > 2) {
        // We might want to use a b32 load/store
        Alignment = std::max(Alignment, Align(4));
      } else if (GVSize > 1) {
        // We might want to use a b16 load/store
        Alignment = std::max(Alignment, Align(2));
      }
 
      if (Alignment != AMDGPU::getAlign(DL, &GV)) {
        Changed = true;
        GV.setAlignment(Alignment);
      }
    }
    return Changed;
  }
 
  bool processUsedLDS(CallGraph const &CG, Module &M, Function *F = nullptr) {
    LLVMContext &Ctx = M.getContext();
    const DataLayout &DL = M.getDataLayout();
 
    // Find variables to move into new struct instance
    std::vector<GlobalVariable *> FoundLocalVars =
        AMDGPU::findVariablesToLower(M, F);
 
    if (FoundLocalVars.empty()) {
      // No variables to rewrite, no changes made.
      return false;
    }
 
    SmallVector<OptimizedStructLayoutField, 8> LayoutFields;
    LayoutFields.reserve(FoundLocalVars.size());
    for (GlobalVariable *GV : FoundLocalVars) {
      OptimizedStructLayoutField F(GV, DL.getTypeAllocSize(GV->getValueType()),
                                   AMDGPU::getAlign(DL, GV));
      LayoutFields.emplace_back(F);
    }
 
    performOptimizedStructLayout(LayoutFields);
 
    std::vector<GlobalVariable *> LocalVars;
    LocalVars.reserve(FoundLocalVars.size()); // will be at least this large
    {
      // This usually won't need to insert any padding, perhaps avoid the alloc
      uint64_t CurrentOffset = 0;
      for (size_t I = 0; I < LayoutFields.size(); I++) {
        GlobalVariable *FGV = static_cast<GlobalVariable *>(
            const_cast<void *>(LayoutFields[I].Id));
        Align DataAlign = LayoutFields[I].Alignment;
 
        uint64_t DataAlignV = DataAlign.value();
        if (uint64_t Rem = CurrentOffset % DataAlignV) {
          uint64_t Padding = DataAlignV - Rem;
 
          // Append an array of padding bytes to meet alignment requested
          // Note (o +      (a - (o % a)) ) % a == 0
          //      (offset + Padding       ) % align == 0
 
          Type *ATy = ArrayType::get(Type::getInt8Ty(Ctx), Padding);
          LocalVars.push_back(new GlobalVariable(
              M, ATy, false, GlobalValue::InternalLinkage, UndefValue::get(ATy),
              "", nullptr, GlobalValue::NotThreadLocal, AMDGPUAS::LOCAL_ADDRESS,
              false));
          CurrentOffset += Padding;
        }
 
        LocalVars.push_back(FGV);
        CurrentOffset += LayoutFields[I].Size;
      }
    }
 
    std::vector<Type *> LocalVarTypes;
    LocalVarTypes.reserve(LocalVars.size());
    std::transform(
        LocalVars.cbegin(), LocalVars.cend(), std::back_inserter(LocalVarTypes),
        [](const GlobalVariable *V) -> Type * { return V->getValueType(); });
 
    std::string VarName(
        F ? (Twine("llvm.amdgcn.kernel.") + F->getName() + ".lds").str()
          : "llvm.amdgcn.module.lds");
    StructType *LDSTy = StructType::create(Ctx, LocalVarTypes, VarName + ".t");
 
    Align StructAlign =
        AMDGPU::getAlign(DL, LocalVars[0]);
 
    GlobalVariable *SGV = new GlobalVariable(
        M, LDSTy, false, GlobalValue::InternalLinkage, UndefValue::get(LDSTy),
        VarName, nullptr, GlobalValue::NotThreadLocal, AMDGPUAS::LOCAL_ADDRESS,
        false);
    SGV->setAlignment(StructAlign);
    if (!F) {
      appendToCompilerUsed(
          M, {static_cast<GlobalValue *>(
                 ConstantExpr::getPointerBitCastOrAddrSpaceCast(
                     cast<Constant>(SGV), Type::getInt8PtrTy(Ctx)))});
    }
 
    // The verifier rejects used lists containing an inttoptr of a constant
    // so remove the variables from these lists before replaceAllUsesWith
    removeFromUsedLists(M, LocalVars);
 
    // Create alias.scope and their lists. Each field in the new structure
    // does not alias with all other fields.
    SmallVector<MDNode *> AliasScopes;
    SmallVector<Metadata *> NoAliasList;
    if (LocalVars.size() > 1) {
      MDBuilder MDB(Ctx);
      AliasScopes.reserve(LocalVars.size());
      MDNode *Domain = MDB.createAnonymousAliasScopeDomain();
      for (size_t I = 0; I < LocalVars.size(); I++) {
        MDNode *Scope = MDB.createAnonymousAliasScope(Domain);
        AliasScopes.push_back(Scope);
      }
      NoAliasList.append(&AliasScopes[1], AliasScopes.end());
    }
 
    // Replace uses of ith variable with a constantexpr to the ith field of the
    // instance that will be allocated by AMDGPUMachineFunction
    Type *I32 = Type::getInt32Ty(Ctx);
    for (size_t I = 0; I < LocalVars.size(); I++) {
      GlobalVariable *GV = LocalVars[I];
      Constant *GEPIdx[] = {ConstantInt::get(I32, 0), ConstantInt::get(I32, I)};
      Constant *GEP = ConstantExpr::getGetElementPtr(LDSTy, SGV, GEPIdx);
      if (F) {
        // Replace all constant uses with instructions if they belong to the
        // current kernel.
        for (User *U : make_early_inc_range(GV->users())) {
          if (ConstantExpr *C = dyn_cast<ConstantExpr>(U))
            AMDGPU::replaceConstantUsesInFunction(C, F);
        }
 
        GV->removeDeadConstantUsers();
 
        GV->replaceUsesWithIf(GEP, [F](Use &U) {
          Instruction *I = dyn_cast<Instruction>(U.getUser());
          return I && I->getFunction() == F;
        });
      } else {
        GV->replaceAllUsesWith(GEP);
      }
      if (GV->use_empty()) {
        UsedList.erase(GV);
        GV->eraseFromParent();
      }
 
      uint64_t Off = DL.getStructLayout(LDSTy)->getElementOffset(I);
      Align A = commonAlignment(StructAlign, Off);
 
      if (I)
        NoAliasList[I - 1] = AliasScopes[I - 1];
      MDNode *NoAlias =
          NoAliasList.empty() ? nullptr : MDNode::get(Ctx, NoAliasList);
      MDNode *AliasScope =
          AliasScopes.empty() ? nullptr : MDNode::get(Ctx, {AliasScopes[I]});
 
      refineUsesAlignmentAndAA(GEP, A, DL, AliasScope, NoAlias);
    }
 
    // This ensures the variable is allocated when called functions access it.
    // It also lets other passes, specifically PromoteAlloca, accurately
    // calculate how much LDS will be used by the kernel after lowering.
    if (!F) {
      IRBuilder<> Builder(Ctx);
      for (Function &Func : M.functions()) {
        if (!Func.isDeclaration() && AMDGPU::isKernelCC(&Func)) {
          const CallGraphNode *N = CG[&Func];
          const bool CalleesRequireModuleLDS = N->size() > 0;
 
          if (CalleesRequireModuleLDS) {
            // If a function this kernel might call requires module LDS,
            // annotate the kernel to let later passes know it will allocate
            // this structure, even if not apparent from the IR.
            markUsedByKernel(Builder, &Func, SGV);
          } else {
            // However if we are certain this kernel cannot call a function that
            // requires module LDS, annotate the kernel so the backend can elide
            // the allocation without repeating callgraph walks.
            Func.addFnAttr("amdgpu-elide-module-lds");
          }
        }
      }
    }
    return true;
  }
 
  void refineUsesAlignmentAndAA(Value *Ptr, Align A, const DataLayout &DL,
                                MDNode *AliasScope, MDNode *NoAlias,
                                unsigned MaxDepth = 5) {
    if (!MaxDepth || (A == 1 && !AliasScope))
      return;
 
    for (User *U : Ptr->users()) {
      if (auto *I = dyn_cast<Instruction>(U)) {
        if (AliasScope && I->mayReadOrWriteMemory()) {
          MDNode *AS = I->getMetadata(LLVMContext::MD_alias_scope);
          AS = (AS ? MDNode::getMostGenericAliasScope(AS, AliasScope)
                   : AliasScope);
          I->setMetadata(LLVMContext::MD_alias_scope, AS);
 
          MDNode *NA = I->getMetadata(LLVMContext::MD_noalias);
          NA = (NA ? MDNode::intersect(NA, NoAlias) : NoAlias);
          I->setMetadata(LLVMContext::MD_noalias, NA);
        }
      }
 
      if (auto *LI = dyn_cast<LoadInst>(U)) {
        LI->setAlignment(std::max(A, LI->getAlign()));
        continue;
      }
      if (auto *SI = dyn_cast<StoreInst>(U)) {
        if (SI->getPointerOperand() == Ptr)
          SI->setAlignment(std::max(A, SI->getAlign()));
        continue;
      }
      if (auto *AI = dyn_cast<AtomicRMWInst>(U)) {
        // None of atomicrmw operations can work on pointers, but let's
        // check it anyway in case it will or we will process ConstantExpr.
        if (AI->getPointerOperand() == Ptr)
          AI->setAlignment(std::max(A, AI->getAlign()));
        continue;
      }
      if (auto *AI = dyn_cast<AtomicCmpXchgInst>(U)) {
        if (AI->getPointerOperand() == Ptr)
          AI->setAlignment(std::max(A, AI->getAlign()));
        continue;
      }
      if (auto *GEP = dyn_cast<GetElementPtrInst>(U)) {
        unsigned BitWidth = DL.getIndexTypeSizeInBits(GEP->getType());
        APInt Off(BitWidth, 0);
        if (GEP->getPointerOperand() == Ptr) {
          Align GA;
          if (GEP->accumulateConstantOffset(DL, Off))
            GA = commonAlignment(A, Off.getLimitedValue());
          refineUsesAlignmentAndAA(GEP, GA, DL, AliasScope, NoAlias,
                                   MaxDepth - 1);
        }
        continue;
      }
      if (auto *I = dyn_cast<Instruction>(U)) {
        if (I->getOpcode() == Instruction::BitCast ||
            I->getOpcode() == Instruction::AddrSpaceCast)
          refineUsesAlignmentAndAA(I, A, DL, AliasScope, NoAlias, MaxDepth - 1);
      }
    }
  }
};
 
} // namespace
char AMDGPULowerModuleLDS::ID = 0;
 
char &llvm::AMDGPULowerModuleLDSID = AMDGPULowerModuleLDS::ID;
 
INITIALIZE_PASS(AMDGPULowerModuleLDS, DEBUG_TYPE,
                "Lower uses of LDS variables from non-kernel functions", false,
                false)
 
ModulePass *llvm::createAMDGPULowerModuleLDSPass() {
  return new AMDGPULowerModuleLDS();
}
 
PreservedAnalyses AMDGPULowerModuleLDSPass::run(Module &M,
                                                ModuleAnalysisManager &) {
  return AMDGPULowerModuleLDS().runOnModule(M) ? PreservedAnalyses::none()
                                               : PreservedAnalyses::all();
}