[RISCV] Introduce pass to promote double constants to a global array #160536
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| //==- RISCVPromoteConstant.cpp - Promote constant fp to global for RISC-V --==// | ||
| // | ||
| // 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 | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #include "RISCV.h" | ||
| #include "RISCVSubtarget.h" | ||
| #include "llvm/ADT/DenseMap.h" | ||
| #include "llvm/ADT/SmallVector.h" | ||
| #include "llvm/ADT/Statistic.h" | ||
| #include "llvm/CodeGen/TargetLowering.h" | ||
| #include "llvm/CodeGen/TargetPassConfig.h" | ||
| #include "llvm/IR/BasicBlock.h" | ||
| #include "llvm/IR/Constant.h" | ||
| #include "llvm/IR/Constants.h" | ||
| #include "llvm/IR/Function.h" | ||
| #include "llvm/IR/GlobalValue.h" | ||
| #include "llvm/IR/GlobalVariable.h" | ||
| #include "llvm/IR/IRBuilder.h" | ||
| #include "llvm/IR/InstIterator.h" | ||
| #include "llvm/IR/Instruction.h" | ||
| #include "llvm/IR/Instructions.h" | ||
| #include "llvm/IR/Module.h" | ||
| #include "llvm/IR/Type.h" | ||
| #include "llvm/InitializePasses.h" | ||
| #include "llvm/Pass.h" | ||
| #include "llvm/Support/Casting.h" | ||
| #include "llvm/Support/Debug.h" | ||
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| using namespace llvm; | ||
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| #define DEBUG_TYPE "riscv-promote-const" | ||
| #define RISCV_PROMOTE_CONSTANT_NAME "RISC-V Promote Constants" | ||
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| STATISTIC(NumPromoted, "Number of constant literals promoted to globals"); | ||
| STATISTIC(NumPromotedUses, "Number of uses of promoted literal constants"); | ||
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| namespace { | ||
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| class RISCVPromoteConstant : public ModulePass { | ||
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Contributor
There was a problem hiding this comment. Also add new pass manager version
Contributor
Author
There was a problem hiding this comment. Based on my understanding form
Collaborator
There was a problem hiding this comment. On this particular suggestion, I would actively prefer this be done in a follow on change. We do not have new pass manager versions of most RISCV codegen passes, and we should do these together. |
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| public: | ||
| static char ID; | ||
| RISCVPromoteConstant() : ModulePass(ID) {} | ||
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| StringRef getPassName() const override { return RISCV_PROMOTE_CONSTANT_NAME; } | ||
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| void getAnalysisUsage(AnalysisUsage &AU) const override { | ||
| AU.addRequired<TargetPassConfig>(); | ||
| AU.setPreservesCFG(); | ||
| } | ||
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| /// Iterate over the functions and promote the double fp constants that | ||
| /// would otherwise go into the constant pool to a constant array. | ||
| bool runOnModule(Module &M) override { | ||
| if (skipModule(M)) | ||
| return false; | ||
| // TargetMachine and Subtarget are needed to query isFPImmlegal. | ||
| const TargetPassConfig &TPC = getAnalysis<TargetPassConfig>(); | ||
| const TargetMachine &TM = TPC.getTM<TargetMachine>(); | ||
| bool Changed = false; | ||
| for (Function &F : M) { | ||
| const RISCVSubtarget &ST = TM.getSubtarget<RISCVSubtarget>(F); | ||
| const RISCVTargetLowering *TLI = ST.getTargetLowering(); | ||
| Changed |= runOnFunction(F, TLI); | ||
| } | ||
| return Changed; | ||
| } | ||
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| private: | ||
| bool runOnFunction(Function &F, const RISCVTargetLowering *TLI); | ||
| }; | ||
| } // end anonymous namespace | ||
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| char RISCVPromoteConstant::ID = 0; | ||
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| INITIALIZE_PASS(RISCVPromoteConstant, DEBUG_TYPE, RISCV_PROMOTE_CONSTANT_NAME, | ||
| false, false) | ||
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| ModulePass *llvm::createRISCVPromoteConstantPass() { | ||
| return new RISCVPromoteConstant(); | ||
| } | ||
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| bool RISCVPromoteConstant::runOnFunction(Function &F, | ||
| const RISCVTargetLowering *TLI) { | ||
| if (F.hasOptNone() || F.hasOptSize()) | ||
| return false; | ||
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| // Bail out and make no transformation if the target doesn't support | ||
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Contributor
There was a problem hiding this comment. Need to skip optnone functions / skipFunction
Contributor
Author
There was a problem hiding this comment. skipFunction isn't available in a ModulePass, but I've added a skip if OptNone (and you can see if call check skipModule in runOnModule). |
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| // doubles, or if we're not targeting RV64 as we currently see some | ||
| // regressions for those targets. | ||
| if (!TLI->isTypeLegal(MVT::f64) || !TLI->isTypeLegal(MVT::i64)) | ||
| return false; | ||
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| // Collect all unique double constants and their uses in the function. Use | ||
| // MapVector to preserve insertion order. | ||
| MapVector<ConstantFP *, SmallVector<Use *, 8>> ConstUsesMap; | ||
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| for (Instruction &I : instructions(F)) { | ||
| for (Use &U : I.operands()) { | ||
| auto *C = dyn_cast<ConstantFP>(U.get()); | ||
| if (!C || !C->getType()->isDoubleTy()) | ||
| continue; | ||
| if (TLI->isFPImmLegal(C->getValueAPF(), MVT::f64, | ||
| /*ForCodeSize=*/false)) | ||
| continue; | ||
| ConstUsesMap[C].push_back(&U); | ||
| } | ||
| } | ||
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| int PromotableConstants = ConstUsesMap.size(); | ||
| LLVM_DEBUG(dbgs() << "Found " << PromotableConstants | ||
| << " promotable constants in " << F.getName() << "\n"); | ||
| // Bail out if no promotable constants found, or if only one is found. | ||
| if (PromotableConstants < 2) { | ||
| LLVM_DEBUG(dbgs() << "Performing no promotions as insufficient promotable " | ||
| "constants found\n"); | ||
| return false; | ||
| } | ||
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| NumPromoted += PromotableConstants; | ||
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| // Create a global array containing the promoted constants. | ||
| Module *M = F.getParent(); | ||
| Type *DoubleTy = Type::getDoubleTy(M->getContext()); | ||
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| SmallVector<Constant *, 16> ConstantVector; | ||
| for (auto const &Pair : ConstUsesMap) | ||
| ConstantVector.push_back(Pair.first); | ||
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| ArrayType *ArrayTy = ArrayType::get(DoubleTy, ConstantVector.size()); | ||
| Constant *GlobalArrayInitializer = | ||
| ConstantArray::get(ArrayTy, ConstantVector); | ||
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| auto *GlobalArray = new GlobalVariable( | ||
| *M, ArrayTy, | ||
| /*isConstant=*/true, GlobalValue::InternalLinkage, GlobalArrayInitializer, | ||
| ".promoted_doubles." + F.getName()); | ||
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| // A cache to hold the loaded value for a given constant within a basic block. | ||
| DenseMap<std::pair<ConstantFP *, BasicBlock *>, Value *> LocalLoads; | ||
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| // Replace all uses with the loaded value. | ||
| unsigned Idx = 0; | ||
| for (auto const &Pair : ConstUsesMap) { | ||
| ConstantFP *Const = Pair.first; | ||
| const SmallVector<Use *, 8> &Uses = Pair.second; | ||
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| for (Use *U : Uses) { | ||
| Instruction *UserInst = cast<Instruction>(U->getUser()); | ||
| BasicBlock *InsertionBB; | ||
| BasicBlock::iterator InsertionPt; | ||
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| // If the user is a PHI node, we must insert the load in the | ||
| // corresponding predecessor basic block. Otherwise, it's inserted into | ||
| // the same block as the use. | ||
| if (auto *PN = dyn_cast<PHINode>(UserInst)) | ||
| InsertionBB = PN->getIncomingBlock(*U); | ||
| else | ||
| InsertionBB = UserInst->getParent(); | ||
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| // It is always safe to insert in the first insertion point in the BB, | ||
| // so do that and let other passes reorder. | ||
| InsertionPt = InsertionBB->getFirstInsertionPt(); | ||
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| auto CacheKey = std::make_pair(Const, InsertionBB); | ||
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| Value *LoadedVal = nullptr; | ||
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| // Re-use a load if it exists in the insertion block. | ||
| if (LocalLoads.count(CacheKey)) { | ||
| LoadedVal = LocalLoads.at(CacheKey); | ||
| } else { | ||
| // Otherwise, create a new GEP and Load at the correct insertion point. | ||
| IRBuilder<> Builder(InsertionBB, InsertionPt); | ||
| Value *ElementPtr = Builder.CreateConstInBoundsGEP2_64( | ||
| GlobalArray->getValueType(), GlobalArray, 0, Idx, "double.addr"); | ||
| LoadedVal = Builder.CreateLoad(DoubleTy, ElementPtr, "double.val"); | ||
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Contributor
Author
There was a problem hiding this comment. The loads will be created with appropraite alignment - but of course the lack of an IR->IR test means that wasn't obvious! I've added tests now, which confirms this. |
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| // Cache the newly created load for this block. | ||
| LocalLoads[CacheKey] = LoadedVal; | ||
| } | ||
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| U->set(LoadedVal); | ||
| ++NumPromotedUses; | ||
| } | ||
| ++Idx; | ||
| } | ||
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| return true; | ||
| } | ||
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