AMDGPUAliasAnalysis.cpp

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//===- AMDGPUAliasAnalysis ------------------------------------------------===//
//
// 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 is the AMGPU address space based alias analysis pass.
//===----------------------------------------------------------------------===//
 
#include "AMDGPUAliasAnalysis.h"
#include "AMDGPU.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Instructions.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "amdgpu-aa"
 
AnalysisKey AMDGPUAA::Key;
 
// Register this pass...
char AMDGPUAAWrapperPass::ID = 0;
char AMDGPUExternalAAWrapper::ID = 0;
 
INITIALIZE_PASS(AMDGPUAAWrapperPass, "amdgpu-aa",
                "AMDGPU Address space based Alias Analysis", false, true)
 
INITIALIZE_PASS(AMDGPUExternalAAWrapper, "amdgpu-aa-wrapper",
                "AMDGPU Address space based Alias Analysis Wrapper", false, true)
 
ImmutablePass *llvm::createAMDGPUAAWrapperPass() {
  return new AMDGPUAAWrapperPass();
}
 
ImmutablePass *llvm::createAMDGPUExternalAAWrapperPass() {
  return new AMDGPUExternalAAWrapper();
}
 
AMDGPUAAWrapperPass::AMDGPUAAWrapperPass() : ImmutablePass(ID) {
  initializeAMDGPUAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
 
void AMDGPUAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
}
 
static AliasResult getAliasResult(unsigned AS1, unsigned AS2) {
  static_assert(AMDGPUAS::MAX_AMDGPU_ADDRESS <= 7, "Addr space out of range");
 
  if (AS1 > AMDGPUAS::MAX_AMDGPU_ADDRESS || AS2 > AMDGPUAS::MAX_AMDGPU_ADDRESS)
    return AliasResult::MayAlias;
 
#define ASMay AliasResult::MayAlias
#define ASNo AliasResult::NoAlias
  // This array is indexed by address space value enum elements 0 ... to 7
  static const AliasResult ASAliasRules[8][8] = {
    /*                    Flat    Global Region Group  Constant Private Const32 Buf Fat Ptr */
    /* Flat     */        {ASMay, ASMay, ASNo,  ASMay, ASMay,   ASMay,  ASMay,  ASMay},
    /* Global   */        {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASMay,  ASMay},
    /* Region   */        {ASNo,  ASNo,  ASMay, ASNo,  ASNo,    ASNo,   ASNo,   ASNo},
    /* Group    */        {ASMay, ASNo,  ASNo,  ASMay, ASNo,    ASNo,   ASNo,   ASNo},
    /* Constant */        {ASMay, ASMay, ASNo,  ASNo,  ASNo,    ASNo,   ASMay,  ASMay},
    /* Private  */        {ASMay, ASNo,  ASNo,  ASNo,  ASNo,    ASMay,  ASNo,   ASNo},
    /* Constant 32-bit */ {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASNo,   ASMay},
    /* Buffer Fat Ptr  */ {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASMay,  ASMay}
  };
#undef ASMay
#undef ASNo
 
  return ASAliasRules[AS1][AS2];
}
 
AliasResult AMDGPUAAResult::alias(const MemoryLocation &LocA,
                                  const MemoryLocation &LocB,
                                  AAQueryInfo &AAQI) {
  unsigned asA = LocA.Ptr->getType()->getPointerAddressSpace();
  unsigned asB = LocB.Ptr->getType()->getPointerAddressSpace();
 
  AliasResult Result = getAliasResult(asA, asB);
  if (Result == AliasResult::NoAlias)
    return Result;
 
  // In general, FLAT (generic) pointers could be aliased to LOCAL or PRIVATE
  // pointers. However, as LOCAL or PRIVATE pointers point to local objects, in
  // certain cases, it's still viable to check whether a FLAT pointer won't
  // alias to a LOCAL or PRIVATE pointer.
  MemoryLocation A = LocA;
  MemoryLocation B = LocB;
  // Canonicalize the location order to simplify the following alias check.
  if (asA != AMDGPUAS::FLAT_ADDRESS) {
    std::swap(asA, asB);
    std::swap(A, B);
  }
  if (asA == AMDGPUAS::FLAT_ADDRESS &&
      (asB == AMDGPUAS::LOCAL_ADDRESS || asB == AMDGPUAS::PRIVATE_ADDRESS)) {
    const auto *ObjA =
        getUnderlyingObject(A.Ptr->stripPointerCastsForAliasAnalysis());
    if (const LoadInst *LI = dyn_cast<LoadInst>(ObjA)) {
      // If a generic pointer is loaded from the constant address space, it
      // could only be a GLOBAL or CONSTANT one as that address space is solely
      // prepared on the host side, where only GLOBAL or CONSTANT variables are
      // visible. Note that this even holds for regular functions.
      if (LI->getPointerAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS)
        return AliasResult::NoAlias;
    } else if (const Argument *Arg = dyn_cast<Argument>(ObjA)) {
      const Function *F = Arg->getParent();
      switch (F->getCallingConv()) {
      case CallingConv::AMDGPU_KERNEL:
        // In the kernel function, kernel arguments won't alias to (local)
        // variables in shared or private address space.
        return AliasResult::NoAlias;
      default:
        // TODO: In the regular function, if that local variable in the
        // location B is not captured, that argument pointer won't alias to it
        // as well.
        break;
      }
    }
  }
 
  // Forward the query to the next alias analysis.
  return AAResultBase::alias(LocA, LocB, AAQI);
}
 
bool AMDGPUAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
                                            AAQueryInfo &AAQI, bool OrLocal) {
  unsigned AS = Loc.Ptr->getType()->getPointerAddressSpace();
  if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
      AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
    return true;
 
  const Value *Base = getUnderlyingObject(Loc.Ptr);
  AS = Base->getType()->getPointerAddressSpace();
  if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
      AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
    return true;
 
  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
    if (GV->isConstant())
      return true;
  } else if (const Argument *Arg = dyn_cast<Argument>(Base)) {
    const Function *F = Arg->getParent();
 
    // Only assume constant memory for arguments on kernels.
    switch (F->getCallingConv()) {
    default:
      return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
    case CallingConv::AMDGPU_LS:
    case CallingConv::AMDGPU_HS:
    case CallingConv::AMDGPU_ES:
    case CallingConv::AMDGPU_GS:
    case CallingConv::AMDGPU_VS:
    case CallingConv::AMDGPU_PS:
    case CallingConv::AMDGPU_CS:
    case CallingConv::AMDGPU_KERNEL:
    case CallingConv::SPIR_KERNEL:
      break;
    }
 
    unsigned ArgNo = Arg->getArgNo();
    /* On an argument, ReadOnly attribute indicates that the function does
       not write through this pointer argument, even though it may write
       to the memory that the pointer points to.
       On an argument, ReadNone attribute indicates that the function does
       not dereference that pointer argument, even though it may read or write
       the memory that the pointer points to if accessed through other pointers.
     */
    if (F->hasParamAttribute(ArgNo, Attribute::NoAlias) &&
        (F->hasParamAttribute(ArgNo, Attribute::ReadNone) ||
         F->hasParamAttribute(ArgNo, Attribute::ReadOnly))) {
      return true;
    }
  }
  return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
}