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| 1 | +//===------ NVPTXTagInvariantLoads.cpp - Tag invariant loads --------------===// |
| 2 | +// |
| 3 | +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| 4 | +// See https://llvm.org/LICENSE.txt for license information. |
| 5 | +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| 6 | +// |
| 7 | +//===----------------------------------------------------------------------===// |
| 8 | +// |
| 9 | +// This file implements invaraint load tagging. It traverses load instructions |
| 10 | +// in a function, and determines if each load can be tagged as invariant. |
| 11 | +// |
| 12 | +// We currently infer invariance for loads from |
| 13 | +// - constant global variables, and |
| 14 | +// - kernel function pointer params that are noalias (i.e. __restrict) and |
| 15 | +// never written to. |
| 16 | +// |
| 17 | +// TODO: Perform a more powerful invariance analysis (ideally IPO). |
| 18 | +// |
| 19 | +//===----------------------------------------------------------------------===// |
| 20 | + |
| 21 | +#include "NVPTXUtilities.h" |
| 22 | +#include "llvm/Analysis/ValueTracking.h" |
| 23 | +#include "llvm/IR/InstIterator.h" |
| 24 | +#include "llvm/IR/Instructions.h" |
| 25 | +#include "llvm/IR/Metadata.h" |
| 26 | +#include "llvm/Support/NVPTXAddrSpace.h" |
| 27 | + |
| 28 | +using namespace llvm; |
| 29 | + |
| 30 | +static bool isInvariantLoad(const LoadInst *LI, const bool IsKernelFn) { |
| 31 | + // Don't bother with non-global loads |
| 32 | + if (LI->getPointerAddressSpace() != NVPTXAS::ADDRESS_SPACE_GLOBAL) |
| 33 | + return false; |
| 34 | + |
| 35 | + // If the load is already marked as invariant, we don't need to do anything |
| 36 | + if (LI->getMetadata(LLVMContext::MD_invariant_load)) |
| 37 | + return false; |
| 38 | + |
| 39 | + // We use getUnderlyingObjects() here instead of getUnderlyingObject() |
| 40 | + // mainly because the former looks through phi nodes while the latter does |
| 41 | + // not. We need to look through phi nodes to handle pointer induction |
| 42 | + // variables. |
| 43 | + SmallVector<const Value *, 8> Objs; |
| 44 | + getUnderlyingObjects(LI->getPointerOperand(), Objs); |
| 45 | + |
| 46 | + return all_of(Objs, [&](const Value *V) { |
| 47 | + if (const auto *A = dyn_cast<const Argument>(V)) |
| 48 | + return IsKernelFn && ((A->onlyReadsMemory() && A->hasNoAliasAttr()) || |
| 49 | + isParamGridConstant(*A)); |
| 50 | + if (const auto *GV = dyn_cast<const GlobalVariable>(V)) |
| 51 | + return GV->isConstant(); |
| 52 | + return false; |
| 53 | + }); |
| 54 | +} |
| 55 | + |
| 56 | +static void markLoadsAsInvariant(LoadInst *LI) { |
| 57 | + LI->setMetadata(LLVMContext::MD_invariant_load, |
| 58 | + MDNode::get(LI->getContext(), {})); |
| 59 | +} |
| 60 | + |
| 61 | +static bool tagInvariantLoads(Function &F) { |
| 62 | + const bool IsKernelFn = isKernelFunction(F); |
| 63 | + |
| 64 | + bool Changed = false; |
| 65 | + for (auto &I : instructions(F)) { |
| 66 | + if (auto *LI = dyn_cast<LoadInst>(&I)) { |
| 67 | + if (isInvariantLoad(LI, IsKernelFn)) { |
| 68 | + markLoadsAsInvariant(LI); |
| 69 | + Changed = true; |
| 70 | + } |
| 71 | + } |
| 72 | + } |
| 73 | + return Changed; |
| 74 | +} |
| 75 | + |
| 76 | +namespace { |
| 77 | + |
| 78 | +struct NVPTXTagInvariantLoadLegacyPass : public FunctionPass { |
| 79 | + static char ID; |
| 80 | + |
| 81 | + NVPTXTagInvariantLoadLegacyPass() : FunctionPass(ID) {} |
| 82 | + bool runOnFunction(Function &F) override; |
| 83 | +}; |
| 84 | + |
| 85 | +} // namespace |
| 86 | + |
| 87 | +INITIALIZE_PASS(NVPTXTagInvariantLoadLegacyPass, "nvptx-tag-invariant-loads", |
| 88 | + "NVPTX Tag Invariant Loads", false, false) |
| 89 | + |
| 90 | +bool NVPTXTagInvariantLoadLegacyPass::runOnFunction(Function &F) { |
| 91 | + return tagInvariantLoads(F); |
| 92 | +} |
| 93 | + |
| 94 | +char NVPTXTagInvariantLoadLegacyPass::ID = 0; |
| 95 | + |
| 96 | +FunctionPass *llvm::createNVPTXTagInvariantLoadsPass() { |
| 97 | + return new NVPTXTagInvariantLoadLegacyPass(); |
| 98 | +} |
| 99 | + |
| 100 | +PreservedAnalyses NVPTXTagInvariantLoadsPass::run(Function &F, |
| 101 | + FunctionAnalysisManager &) { |
| 102 | + return tagInvariantLoads(F) ? PreservedAnalyses::none() |
| 103 | + : PreservedAnalyses::all(); |
| 104 | +} |
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