NVPTXTagInvariantLoads.cpp

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//===------ NVPTXTagInvariantLoads.cpp - Tag invariant loads --------------===//
//
// 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 file implements invaraint load tagging. It traverses load instructions
// in a function, and determines if each load can be tagged as invariant.
//
// We currently infer invariance for loads from
//  - constant global variables, and
//  - kernel function pointer params that are noalias (i.e. __restrict) and
//    never written to.
//
// TODO: Perform a more powerful invariance analysis (ideally IPO).
//
//===----------------------------------------------------------------------===//
 
#include "NVPTXUtilities.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/NVPTXAddrSpace.h"
 
using namespace llvm;
 
static bool isInvariantLoad(const LoadInst *LI, const bool IsKernelFn) {
  // Don't bother with non-global loads
  if (LI->getPointerAddressSpace() != NVPTXAS::ADDRESS_SPACE_GLOBAL)
    return false;
 
  // If the load is already marked as invariant, we don't need to do anything
  if (LI->getMetadata(LLVMContext::MD_invariant_load))
    return false;
 
  // We use getUnderlyingObjects() here instead of getUnderlyingObject()
  // mainly because the former looks through phi nodes while the latter does
  // not. We need to look through phi nodes to handle pointer induction
  // variables.
  SmallVector<const Value *, 8> Objs;
  getUnderlyingObjects(LI->getPointerOperand(), Objs);
 
  return all_of(Objs, [&](const Value *V) {
    if (const auto *A = dyn_cast<const Argument>(V))
      return IsKernelFn && ((A->onlyReadsMemory() && A->hasNoAliasAttr()) ||
                            isParamGridConstant(*A));
    if (const auto *GV = dyn_cast<const GlobalVariable>(V))
      return GV->isConstant();
    return false;
  });
}
 
static void markLoadsAsInvariant(LoadInst *LI) {
  LI->setMetadata(LLVMContext::MD_invariant_load,
                  MDNode::get(LI->getContext(), {}));
}
 
static bool tagInvariantLoads(Function &F) {
  const bool IsKernelFn = isKernelFunction(F);
 
  bool Changed = false;
  for (auto &I : instructions(F)) {
    if (auto *LI = dyn_cast<LoadInst>(&I)) {
      if (isInvariantLoad(LI, IsKernelFn)) {
        markLoadsAsInvariant(LI);
        Changed = true;
      }
    }
  }
  return Changed;
}
 
namespace {
 
struct NVPTXTagInvariantLoadLegacyPass : public FunctionPass {
  static char ID;
 
  NVPTXTagInvariantLoadLegacyPass() : FunctionPass(ID) {}
  bool runOnFunction(Function &F) override;
};
 
} // namespace
 
INITIALIZE_PASS(NVPTXTagInvariantLoadLegacyPass, "nvptx-tag-invariant-loads",
                "NVPTX Tag Invariant Loads", false, false)
 
bool NVPTXTagInvariantLoadLegacyPass::runOnFunction(Function &F) {
  return tagInvariantLoads(F);
}
 
char NVPTXTagInvariantLoadLegacyPass::ID = 0;
 
FunctionPass *llvm::createNVPTXTagInvariantLoadsPass() {
  return new NVPTXTagInvariantLoadLegacyPass();
}
 
PreservedAnalyses NVPTXTagInvariantLoadsPass::run(Function &F,
                                                  FunctionAnalysisManager &) {
  return tagInvariantLoads(F) ? PreservedAnalyses::none()
                              : PreservedAnalyses::all();
}