AMDGPUMemoryUtils.cpp

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//===-- AMDGPUMemoryUtils.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
//
//===----------------------------------------------------------------------===//
 
#include "AMDGPUMemoryUtils.h"
#include "AMDGPU.h"
#include "AMDGPUBaseInfo.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/IntrinsicsAMDGPU.h"
#include "llvm/IR/ReplaceConstant.h"
 
#define DEBUG_TYPE "amdgpu-memory-utils"
 
using namespace llvm;
 
namespace llvm {
 
namespace AMDGPU {
 
Align getAlign(DataLayout const &DL, const GlobalVariable *GV) {
  return DL.getValueOrABITypeAlignment(GV->getPointerAlignment(DL),
                                       GV->getValueType());
}
 
static bool shouldLowerLDSToStruct(const GlobalVariable &GV,
                                   const Function *F) {
  // We are not interested in kernel LDS lowering for module LDS itself.
  if (F && GV.getName() == "llvm.amdgcn.module.lds")
    return false;
 
  bool Ret = false;
  SmallPtrSet<const User *, 8> Visited;
  SmallVector<const User *, 16> Stack(GV.users());
 
  assert(!F || isKernelCC(F));
 
  while (!Stack.empty()) {
    const User *V = Stack.pop_back_val();
    Visited.insert(V);
 
    if (isa<GlobalValue>(V)) {
      // This use of the LDS variable is the initializer of a global variable.
      // This is ill formed. The address of an LDS variable is kernel dependent
      // and unknown until runtime. It can't be written to a global variable.
      continue;
    }
 
    if (auto *I = dyn_cast<Instruction>(V)) {
      const Function *UF = I->getFunction();
      if (UF == F) {
        // Used from this kernel, we want to put it into the structure.
        Ret = true;
      } else if (!F) {
        // For module LDS lowering, lowering is required if the user instruction
        // is from non-kernel function.
        Ret |= !isKernelCC(UF);
      }
      continue;
    }
 
    // User V should be a constant, recursively visit users of V.
    assert(isa<Constant>(V) && "Expected a constant.");
    append_range(Stack, V->users());
  }
 
  return Ret;
}
 
bool isLDSVariableToLower(const GlobalVariable &GV) {
  if (GV.getType()->getPointerAddressSpace() != AMDGPUAS::LOCAL_ADDRESS) {
    return false;
  }
  if (!GV.hasInitializer()) {
    // addrspace(3) without initializer implies cuda/hip extern __shared__
    // the semantics for such a variable appears to be that all extern
    // __shared__ variables alias one another, in which case this transform
    // is not required
    return false;
  }
  if (!isa<UndefValue>(GV.getInitializer())) {
    // Initializers are unimplemented for LDS address space.
    // Leave such variables in place for consistent error reporting.
    return false;
  }
  if (GV.isConstant()) {
    // A constant undef variable can't be written to, and any load is
    // undef, so it should be eliminated by the optimizer. It could be
    // dropped by the back end if not. This pass skips over it.
    return false;
  }
  return true;
}
 
std::vector<GlobalVariable *> findLDSVariablesToLower(Module &M,
                                                      const Function *F) {
  std::vector<llvm::GlobalVariable *> LocalVars;
  for (auto &GV : M.globals()) {
    if (!isLDSVariableToLower(GV)) {
      continue;
    }
    if (!shouldLowerLDSToStruct(GV, F)) {
      continue;
    }
    LocalVars.push_back(&GV);
  }
  return LocalVars;
}
 
bool isReallyAClobber(const Value *Ptr, MemoryDef *Def, AAResults *AA) {
  Instruction *DefInst = Def->getMemoryInst();
 
  if (isa<FenceInst>(DefInst))
    return false;
 
  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(DefInst)) {
    switch (II->getIntrinsicID()) {
    case Intrinsic::amdgcn_s_barrier:
    case Intrinsic::amdgcn_wave_barrier:
    case Intrinsic::amdgcn_sched_barrier:
    case Intrinsic::amdgcn_sched_group_barrier:
      return false;
    default:
      break;
    }
  }
 
  // Ignore atomics not aliasing with the original load, any atomic is a
  // universal MemoryDef from MSSA's point of view too, just like a fence.
  const auto checkNoAlias = [AA, Ptr](auto I) -> bool {
    return I && AA->isNoAlias(I->getPointerOperand(), Ptr);
  };
 
  if (checkNoAlias(dyn_cast<AtomicCmpXchgInst>(DefInst)) ||
      checkNoAlias(dyn_cast<AtomicRMWInst>(DefInst)))
    return false;
 
  return true;
}
 
bool isClobberedInFunction(const LoadInst *Load, MemorySSA *MSSA,
                           AAResults *AA) {
  MemorySSAWalker *Walker = MSSA->getWalker();
  SmallVector<MemoryAccess *> WorkList{Walker->getClobberingMemoryAccess(Load)};
  SmallSet<MemoryAccess *, 8> Visited;
  MemoryLocation Loc(MemoryLocation::get(Load));
 
  LLVM_DEBUG(dbgs() << "Checking clobbering of: " << *Load << '\n');
 
  // Start with a nearest dominating clobbering access, it will be either
  // live on entry (nothing to do, load is not clobbered), MemoryDef, or
  // MemoryPhi if several MemoryDefs can define this memory state. In that
  // case add all Defs to WorkList and continue going up and checking all
  // the definitions of this memory location until the root. When all the
  // defs are exhausted and came to the entry state we have no clobber.
  // Along the scan ignore barriers and fences which are considered clobbers
  // by the MemorySSA, but not really writing anything into the memory.
  while (!WorkList.empty()) {
    MemoryAccess *MA = WorkList.pop_back_val();
    if (!Visited.insert(MA).second)
      continue;
 
    if (MSSA->isLiveOnEntryDef(MA))
      continue;
 
    if (MemoryDef *Def = dyn_cast<MemoryDef>(MA)) {
      LLVM_DEBUG(dbgs() << "  Def: " << *Def->getMemoryInst() << '\n');
 
      if (isReallyAClobber(Load->getPointerOperand(), Def, AA)) {
        LLVM_DEBUG(dbgs() << "      -> load is clobbered\n");
        return true;
      }
 
      WorkList.push_back(
          Walker->getClobberingMemoryAccess(Def->getDefiningAccess(), Loc));
      continue;
    }
 
    const MemoryPhi *Phi = cast<MemoryPhi>(MA);
    for (const auto &Use : Phi->incoming_values())
      WorkList.push_back(cast<MemoryAccess>(&Use));
  }
 
  LLVM_DEBUG(dbgs() << "      -> no clobber\n");
  return false;
}
 
} // end namespace AMDGPU
 
} // end namespace llvm