AMDGPULowerKernelArguments.cpp

Go to the documentation of this file.

//===-- AMDGPULowerKernelArguments.cpp ------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
/// \file This pass replaces accesses to kernel arguments with loads from
/// offsets from the kernarg base pointer.
//
//===----------------------------------------------------------------------===//
 
#include "AMDGPU.h"
#include "GCNSubtarget.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/ScopedNoAliasAA.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicsAMDGPU.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/Target/TargetMachine.h"
#include <optional>
 
#define DEBUG_TYPE "amdgpu-lower-kernel-arguments"
 
using namespace llvm;
 
namespace {
 
class AMDGPULowerKernelArguments : public FunctionPass {
public:
  static char ID;
 
  AMDGPULowerKernelArguments() : FunctionPass(ID) {}
 
  bool runOnFunction(Function &F) override;
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<TargetPassConfig>();
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.setPreservesAll();
 }
};
 
} // end anonymous namespace
 
// skip allocas
static BasicBlock::iterator getInsertPt(BasicBlock &BB) {
  BasicBlock::iterator InsPt = BB.getFirstInsertionPt();
  for (BasicBlock::iterator E = BB.end(); InsPt != E; ++InsPt) {
    AllocaInst *AI = dyn_cast<AllocaInst>(&*InsPt);
 
    // If this is a dynamic alloca, the value may depend on the loaded kernargs,
    // so loads will need to be inserted before it.
    if (!AI || !AI->isStaticAlloca())
      break;
  }
 
  return InsPt;
}
 
static void addAliasScopeMetadata(Function &F, const DataLayout &DL,
                                  DominatorTree &DT) {
  // Collect noalias arguments.
  SmallVector<const Argument *, 4u> NoAliasArgs;
 
  for (Argument &Arg : F.args())
    if (Arg.hasNoAliasAttr() && !Arg.use_empty())
      NoAliasArgs.push_back(&Arg);
 
  if (NoAliasArgs.empty())
    return;
 
  // Add alias scopes for each noalias argument.
  MDBuilder MDB(F.getContext());
  DenseMap<const Argument *, MDNode *> NewScopes;
  MDNode *NewDomain = MDB.createAnonymousAliasScopeDomain(F.getName());
 
  for (unsigned I = 0u; I < NoAliasArgs.size(); ++I) {
    const Argument *Arg = NoAliasArgs[I];
    MDNode *NewScope = MDB.createAnonymousAliasScope(NewDomain, Arg->getName());
    NewScopes.insert({Arg, NewScope});
  }
 
  // Iterate over all instructions.
  for (inst_iterator Inst = inst_begin(F), InstEnd = inst_end(F);
       Inst != InstEnd; ++Inst) {
    // If instruction accesses memory, collect its pointer arguments.
    Instruction *I = &(*Inst);
    SmallVector<const Value *, 2u> PtrArgs;
    // May reach a noalias argument via a copy captured before the call.
    bool IsUnrestrictedCall = false;
 
    if (std::optional<MemoryLocation> MO = MemoryLocation::getOrNone(I))
      PtrArgs.push_back(MO->Ptr);
    else if (const CallBase *Call = dyn_cast<CallBase>(I)) {
      MemoryEffects ME = Call->getMemoryEffects();
      if (ME.doesNotAccessMemory())
        continue;
 
      // Inaccessible memory cannot alias any IR-visible pointer.
      if (ME.onlyAccessesInaccessibleMem())
        continue;
      IsUnrestrictedCall = !ME.onlyAccessesArgPointees();
 
      for (Value *Arg : Call->args()) {
        if (!Arg->getType()->isPointerTy())
          continue;
 
        PtrArgs.push_back(Arg);
      }
    } else {
      // Not a memory access and not a call — nothing to annotate.
      continue;
    }
 
    // Collect underlying objects of pointer arguments.
    SmallVector<Metadata *, 4u> Scopes;
    SmallPtrSet<const Value *, 4u> ObjSet;
    SmallVector<Metadata *, 4u> NoAliases;
 
    for (const Value *Val : PtrArgs) {
      SmallVector<const Value *, 4u> Objects;
      getUnderlyingObjects(Val, Objects);
      ObjSet.insert_range(Objects);
    }
 
    bool RequiresNoCaptureBefore = false;
    bool UsesUnknownObject = false;
    bool UsesAliasingPtr = false;
 
    for (const Value *Val : ObjSet) {
      if (isa<ConstantData>(Val))
        continue;
 
      if (const Argument *Arg = dyn_cast<Argument>(Val)) {
        if (!Arg->hasAttribute(Attribute::NoAlias))
          UsesAliasingPtr = true;
      } else
        UsesAliasingPtr = true;
 
      if (isEscapeSource(Val)) {
        // Can only alias a noalias argument if captured beforehand.
        RequiresNoCaptureBefore = true;
      } else if (!isa<Argument>(Val) && !isIdentifiedObject(Val)) {
        // Unknown provenance: assume nothing.
        UsesUnknownObject = true;
      }
    }
 
    if (UsesUnknownObject)
      continue;
 
    if (IsUnrestrictedCall)
      RequiresNoCaptureBefore = true;
 
    // Collect noalias scopes for instruction.
    for (const Argument *Arg : NoAliasArgs) {
      if (ObjSet.contains(Arg))
        continue;
 
      if (!RequiresNoCaptureBefore ||
          !capturesAnything(PointerMayBeCapturedBefore(
              Arg, false, I, &DT, false, CaptureComponents::Provenance)))
        NoAliases.push_back(NewScopes[Arg]);
    }
 
    // Collect scopes for alias.scope metadata. Skip unrestricted calls: they
    // may touch memory beyond their pointer arguments' pointees.
    if (!UsesAliasingPtr && !IsUnrestrictedCall)
      for (const Argument *Arg : NoAliasArgs) {
        if (ObjSet.count(Arg))
          Scopes.push_back(NewScopes[Arg]);
      }
 
    // Add noalias metadata to instruction.
    if (!NoAliases.empty()) {
      MDNode *NewMD =
          MDNode::concatenate(Inst->getMetadata(LLVMContext::MD_noalias),
                              MDNode::get(F.getContext(), NoAliases));
      Inst->setMetadata(LLVMContext::MD_noalias, NewMD);
    }
 
    // Add alias.scope metadata to instruction.
    if (!Scopes.empty()) {
      MDNode *NewMD =
          MDNode::concatenate(Inst->getMetadata(LLVMContext::MD_alias_scope),
                              MDNode::get(F.getContext(), Scopes));
      Inst->setMetadata(LLVMContext::MD_alias_scope, NewMD);
    }
  }
}
 
static bool lowerKernelArguments(Function &F, const TargetMachine &TM,
                                 DominatorTree &DT) {
  CallingConv::ID CC = F.getCallingConv();
  if (CC != CallingConv::AMDGPU_KERNEL || F.arg_empty())
    return false;
 
  const GCNSubtarget &ST = TM.getSubtarget<GCNSubtarget>(F);
  LLVMContext &Ctx = F.getContext();
  const DataLayout &DL = F.getDataLayout();
  BasicBlock &EntryBlock = *F.begin();
  IRBuilder<> Builder(&EntryBlock, getInsertPt(EntryBlock));
 
  const Align KernArgBaseAlign(16); // FIXME: Increase if necessary
  const uint64_t BaseOffset = ST.getExplicitKernelArgOffset();
 
  Align MaxAlign;
  // FIXME: Alignment is broken with explicit arg offset.;
  const uint64_t TotalKernArgSize = ST.getKernArgSegmentSize(F, MaxAlign);
  if (TotalKernArgSize == 0)
    return false;
 
  CallInst *KernArgSegment = Builder.CreateIntrinsicWithoutFolding(
      Intrinsic::amdgcn_kernarg_segment_ptr, {}, nullptr,
      F.getName() + ".kernarg.segment");
  KernArgSegment->addRetAttr(Attribute::NonNull);
  KernArgSegment->addRetAttr(
      Attribute::getWithDereferenceableBytes(Ctx, TotalKernArgSize));
 
  uint64_t ExplicitArgOffset = 0;
 
  addAliasScopeMetadata(F, F.getParent()->getDataLayout(), DT);
 
  for (Argument &Arg : F.args()) {
    const bool IsByRef = Arg.hasByRefAttr();
    Type *ArgTy = IsByRef ? Arg.getParamByRefType() : Arg.getType();
    MaybeAlign ParamAlign = IsByRef ? Arg.getParamAlign() : std::nullopt;
    Align ABITypeAlign = DL.getValueOrABITypeAlignment(ParamAlign, ArgTy);
 
    uint64_t Size = DL.getTypeSizeInBits(ArgTy);
    uint64_t AllocSize = DL.getTypeAllocSize(ArgTy);
 
    uint64_t EltOffset = alignTo(ExplicitArgOffset, ABITypeAlign) + BaseOffset;
    ExplicitArgOffset = alignTo(ExplicitArgOffset, ABITypeAlign) + AllocSize;
 
    // Skip inreg arguments which should be preloaded.
    if (Arg.use_empty() || Arg.hasInRegAttr())
      continue;
 
    // If this is byval, the loads are already explicit in the function. We just
    // need to rewrite the pointer values.
    if (IsByRef) {
      Value *ArgOffsetPtr = Builder.CreateConstInBoundsGEP1_64(
          Builder.getInt8Ty(), KernArgSegment, EltOffset,
          Arg.getName() + ".byval.kernarg.offset");
 
      Value *CastOffsetPtr =
          Builder.CreateAddrSpaceCast(ArgOffsetPtr, Arg.getType());
      Arg.replaceAllUsesWith(CastOffsetPtr);
      continue;
    }
 
    if (PointerType *PT = dyn_cast<PointerType>(ArgTy)) {
      // FIXME: Hack. We rely on AssertZext to be able to fold DS addressing
      // modes on SI to know the high bits are 0 so pointer adds don't wrap. We
      // can't represent this with range metadata because it's only allowed for
      // integer types.
      if ((PT->getAddressSpace() == AMDGPUAS::LOCAL_ADDRESS ||
           PT->getAddressSpace() == AMDGPUAS::REGION_ADDRESS) &&
          !ST.hasUsableDSOffset())
        continue;
    }
 
    auto *VT = dyn_cast<FixedVectorType>(ArgTy);
    bool IsV3 = VT && VT->getNumElements() == 3;
    bool DoShiftOpt = Size < 32 && !ArgTy->isAggregateType();
 
    VectorType *V4Ty = nullptr;
 
    int64_t AlignDownOffset = alignDown(EltOffset, 4);
    int64_t OffsetDiff = EltOffset - AlignDownOffset;
    Align AdjustedAlign = commonAlignment(
        KernArgBaseAlign, DoShiftOpt ? AlignDownOffset : EltOffset);
 
    Value *ArgPtr;
    Type *AdjustedArgTy;
    if (DoShiftOpt) { // FIXME: Handle aggregate types
      // Since we don't have sub-dword scalar loads, avoid doing an extload by
      // loading earlier than the argument address, and extracting the relevant
      // bits.
      // TODO: Update this for GFX12 which does have scalar sub-dword loads.
      //
      // Additionally widen any sub-dword load to i32 even if suitably aligned,
      // so that CSE between different argument loads works easily.
      ArgPtr = Builder.CreateConstInBoundsGEP1_64(
          Builder.getInt8Ty(), KernArgSegment, AlignDownOffset,
          Arg.getName() + ".kernarg.offset.align.down");
      AdjustedArgTy = Builder.getInt32Ty();
    } else {
      ArgPtr = Builder.CreateConstInBoundsGEP1_64(
          Builder.getInt8Ty(), KernArgSegment, EltOffset,
          Arg.getName() + ".kernarg.offset");
      AdjustedArgTy = ArgTy;
    }
 
    if (IsV3 && Size >= 32) {
      V4Ty = FixedVectorType::get(VT->getElementType(), 4);
      // Use the hack that clang uses to avoid SelectionDAG ruining v3 loads
      AdjustedArgTy = V4Ty;
    }
 
    LoadInst *Load =
        Builder.CreateAlignedLoad(AdjustedArgTy, ArgPtr, AdjustedAlign);
    Load->setMetadata(LLVMContext::MD_invariant_load, MDNode::get(Ctx, {}));
 
    MDBuilder MDB(Ctx);
 
    if (Arg.hasAttribute(Attribute::NoUndef) && AdjustedArgTy == ArgTy)
      Load->setMetadata(LLVMContext::MD_noundef, MDNode::get(Ctx, {}));
 
    if (Arg.hasAttribute(Attribute::Range) && AdjustedArgTy == ArgTy) {
      const ConstantRange &Range =
          Arg.getAttribute(Attribute::Range).getValueAsConstantRange();
      Load->setMetadata(LLVMContext::MD_range,
                        MDB.createRange(Range.getLower(), Range.getUpper()));
    }
 
    if (Arg.hasAttribute(Attribute::NoFPClass) && AdjustedArgTy == ArgTy) {
      FPClassTest Mask = Arg.getNoFPClass();
      Load->setMetadata(
          LLVMContext::MD_nofpclass,
          MDNode::get(Ctx, ConstantAsMetadata::get(
                               ConstantInt::get(Type::getInt32Ty(Ctx), Mask))));
    }
 
    if (isa<PointerType>(ArgTy)) {
      if (Arg.hasNonNullAttr())
        Load->setMetadata(LLVMContext::MD_nonnull, MDNode::get(Ctx, {}));
 
      uint64_t DerefBytes = Arg.getDereferenceableBytes();
      if (DerefBytes != 0) {
        Load->setMetadata(
          LLVMContext::MD_dereferenceable,
          MDNode::get(Ctx,
                      MDB.createConstant(
                        ConstantInt::get(Builder.getInt64Ty(), DerefBytes))));
      }
 
      uint64_t DerefOrNullBytes = Arg.getDereferenceableOrNullBytes();
      if (DerefOrNullBytes != 0) {
        Load->setMetadata(
          LLVMContext::MD_dereferenceable_or_null,
          MDNode::get(Ctx,
                      MDB.createConstant(ConstantInt::get(Builder.getInt64Ty(),
                                                          DerefOrNullBytes))));
      }
 
      if (MaybeAlign ParamAlign = Arg.getParamAlign()) {
        Load->setMetadata(
            LLVMContext::MD_align,
            MDNode::get(Ctx, MDB.createConstant(ConstantInt::get(
                                 Builder.getInt64Ty(), ParamAlign->value()))));
      }
    }
 
    if (DoShiftOpt) {
      Value *ExtractBits = OffsetDiff == 0 ?
        Load : Builder.CreateLShr(Load, OffsetDiff * 8);
 
      IntegerType *ArgIntTy = Builder.getIntNTy(Size);
      Value *Trunc = Builder.CreateTrunc(ExtractBits, ArgIntTy);
      Value *NewVal = Builder.CreateBitCast(Trunc, ArgTy,
                                            Arg.getName() + ".load");
      Arg.replaceAllUsesWith(NewVal);
    } else if (IsV3) {
      Value *Shuf = Builder.CreateShuffleVector(Load, ArrayRef<int>{0, 1, 2},
                                                Arg.getName() + ".load");
      Arg.replaceAllUsesWith(Shuf);
    } else {
      Load->setName(Arg.getName() + ".load");
      Arg.replaceAllUsesWith(Load);
    }
  }
 
  KernArgSegment->addRetAttr(
      Attribute::getWithAlignment(Ctx, std::max(KernArgBaseAlign, MaxAlign)));
 
  return true;
}
 
bool AMDGPULowerKernelArguments::runOnFunction(Function &F) {
  auto &TPC = getAnalysis<TargetPassConfig>();
  const TargetMachine &TM = TPC.getTM<TargetMachine>();
  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  return lowerKernelArguments(F, TM, DT);
}
 
INITIALIZE_PASS_BEGIN(AMDGPULowerKernelArguments, DEBUG_TYPE,
                      "AMDGPU Lower Kernel Arguments", false, false)
INITIALIZE_PASS_END(AMDGPULowerKernelArguments, DEBUG_TYPE, "AMDGPU Lower Kernel Arguments",
                    false, false)
 
char AMDGPULowerKernelArguments::ID = 0;
 
FunctionPass *llvm::createAMDGPULowerKernelArgumentsPass() {
  return new AMDGPULowerKernelArguments();
}
 
PreservedAnalyses
AMDGPULowerKernelArgumentsPass::run(Function &F, FunctionAnalysisManager &AM) {
  DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
  bool Changed = lowerKernelArguments(F, TM, DT);
  if (Changed) {
    // TODO: Preserves a lot more.
    PreservedAnalyses PA;
    PA.preserveSet<CFGAnalyses>();
    return PA;
  }
 
  return PreservedAnalyses::all();
}