20531b3a6b
This code queries TTI on a single function, which is considered to be representative. This is a bit odd, but probably fine in practice. However, I think we should at least avoid querying declarations, which e.g. will generally lack target attributes, and for which we don't seem to ever query TTI in other places.
218 lines
8.0 KiB
C++
218 lines
8.0 KiB
C++
//===- RelLookupTableConverterPass - Rel Table Conv -----------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements relative lookup table converter that converts
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// lookup tables to relative lookup tables to make them PIC-friendly.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/RelLookupTableConverter.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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using namespace llvm;
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static bool shouldConvertToRelLookupTable(Module &M, GlobalVariable &GV) {
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// If lookup table has more than one user,
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// do not generate a relative lookup table.
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// This is to simplify the analysis that needs to be done for this pass.
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// TODO: Add support for lookup tables with multiple uses.
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// For ex, this can happen when a function that uses a lookup table gets
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// inlined into multiple call sites.
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if (!GV.hasInitializer() ||
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!GV.isConstant() ||
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!GV.hasOneUse())
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return false;
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GetElementPtrInst *GEP =
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dyn_cast<GetElementPtrInst>(GV.use_begin()->getUser());
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if (!GEP || !GEP->hasOneUse() ||
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GV.getValueType() != GEP->getSourceElementType())
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return false;
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LoadInst *Load = dyn_cast<LoadInst>(GEP->use_begin()->getUser());
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if (!Load || !Load->hasOneUse() ||
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Load->getType() != GEP->getResultElementType())
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return false;
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// If the original lookup table does not have local linkage and is
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// not dso_local, do not generate a relative lookup table.
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// This optimization creates a relative lookup table that consists of
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// offsets between the start of the lookup table and its elements.
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// To be able to generate these offsets, relative lookup table and
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// its elements should have internal linkage and be dso_local, which means
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// that they should resolve to symbols within the same linkage unit.
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if (!GV.hasLocalLinkage() ||
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!GV.isDSOLocal() ||
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!GV.isImplicitDSOLocal())
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return false;
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ConstantArray *Array = dyn_cast<ConstantArray>(GV.getInitializer());
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// If values are not pointers, do not generate a relative lookup table.
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if (!Array || !Array->getType()->getElementType()->isPointerTy())
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return false;
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const DataLayout &DL = M.getDataLayout();
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for (const Use &Op : Array->operands()) {
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Constant *ConstOp = cast<Constant>(&Op);
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GlobalValue *GVOp;
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APInt Offset;
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// If an operand is not a constant offset from a lookup table,
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// do not generate a relative lookup table.
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if (!IsConstantOffsetFromGlobal(ConstOp, GVOp, Offset, DL))
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return false;
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// If operand is mutable, do not generate a relative lookup table.
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auto *GlovalVarOp = dyn_cast<GlobalVariable>(GVOp);
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if (!GlovalVarOp || !GlovalVarOp->isConstant())
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return false;
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if (!GlovalVarOp->hasLocalLinkage() ||
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!GlovalVarOp->isDSOLocal() ||
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!GlovalVarOp->isImplicitDSOLocal())
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return false;
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}
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return true;
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}
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static GlobalVariable *createRelLookupTable(Function &Func,
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GlobalVariable &LookupTable) {
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Module &M = *Func.getParent();
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ConstantArray *LookupTableArr =
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cast<ConstantArray>(LookupTable.getInitializer());
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unsigned NumElts = LookupTableArr->getType()->getNumElements();
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ArrayType *IntArrayTy =
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ArrayType::get(Type::getInt32Ty(M.getContext()), NumElts);
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GlobalVariable *RelLookupTable = new GlobalVariable(
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M, IntArrayTy, LookupTable.isConstant(), LookupTable.getLinkage(),
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nullptr, "reltable." + Func.getName(), &LookupTable,
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LookupTable.getThreadLocalMode(), LookupTable.getAddressSpace(),
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LookupTable.isExternallyInitialized());
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uint64_t Idx = 0;
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SmallVector<Constant *, 64> RelLookupTableContents(NumElts);
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for (Use &Operand : LookupTableArr->operands()) {
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Constant *Element = cast<Constant>(Operand);
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Type *IntPtrTy = M.getDataLayout().getIntPtrType(M.getContext());
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Constant *Base = llvm::ConstantExpr::getPtrToInt(RelLookupTable, IntPtrTy);
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Constant *Target = llvm::ConstantExpr::getPtrToInt(Element, IntPtrTy);
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Constant *Sub = llvm::ConstantExpr::getSub(Target, Base);
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Constant *RelOffset =
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llvm::ConstantExpr::getTrunc(Sub, Type::getInt32Ty(M.getContext()));
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RelLookupTableContents[Idx++] = RelOffset;
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}
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Constant *Initializer =
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ConstantArray::get(IntArrayTy, RelLookupTableContents);
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RelLookupTable->setInitializer(Initializer);
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RelLookupTable->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
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RelLookupTable->setAlignment(llvm::Align(4));
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return RelLookupTable;
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}
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static void convertToRelLookupTable(GlobalVariable &LookupTable) {
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GetElementPtrInst *GEP =
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cast<GetElementPtrInst>(LookupTable.use_begin()->getUser());
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LoadInst *Load = cast<LoadInst>(GEP->use_begin()->getUser());
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Module &M = *LookupTable.getParent();
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BasicBlock *BB = GEP->getParent();
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IRBuilder<> Builder(BB);
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Function &Func = *BB->getParent();
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// Generate an array that consists of relative offsets.
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GlobalVariable *RelLookupTable = createRelLookupTable(Func, LookupTable);
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// Place new instruction sequence before GEP.
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Builder.SetInsertPoint(GEP);
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Value *Index = GEP->getOperand(2);
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IntegerType *IntTy = cast<IntegerType>(Index->getType());
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Value *Offset =
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Builder.CreateShl(Index, ConstantInt::get(IntTy, 2), "reltable.shift");
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// Insert the call to load.relative instrinsic before LOAD.
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// GEP might not be immediately followed by a LOAD, like it can be hoisted
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// outside the loop or another instruction might be inserted them in between.
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Builder.SetInsertPoint(Load);
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Function *LoadRelIntrinsic = llvm::Intrinsic::getDeclaration(
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&M, Intrinsic::load_relative, {Index->getType()});
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Value *Base = Builder.CreateBitCast(RelLookupTable, Builder.getInt8PtrTy());
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// Create a call to load.relative intrinsic that computes the target address
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// by adding base address (lookup table address) and relative offset.
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Value *Result = Builder.CreateCall(LoadRelIntrinsic, {Base, Offset},
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"reltable.intrinsic");
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// Create a bitcast instruction if necessary.
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if (Load->getType() != Builder.getInt8PtrTy())
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Result = Builder.CreateBitCast(Result, Load->getType(), "reltable.bitcast");
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// Replace load instruction with the new generated instruction sequence.
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Load->replaceAllUsesWith(Result);
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// Remove Load and GEP instructions.
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Load->eraseFromParent();
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GEP->eraseFromParent();
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}
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// Convert lookup tables to relative lookup tables in the module.
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static bool convertToRelativeLookupTables(
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Module &M, function_ref<TargetTransformInfo &(Function &)> GetTTI) {
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for (Function &F : M) {
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if (F.isDeclaration())
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continue;
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// Check if we have a target that supports relative lookup tables.
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if (!GetTTI(F).shouldBuildRelLookupTables())
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return false;
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// We assume that the result is independent of the checked function.
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break;
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}
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bool Changed = false;
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for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
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if (!shouldConvertToRelLookupTable(M, GV))
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continue;
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convertToRelLookupTable(GV);
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// Remove the original lookup table.
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GV.eraseFromParent();
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Changed = true;
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}
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return Changed;
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}
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PreservedAnalyses RelLookupTableConverterPass::run(Module &M,
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ModuleAnalysisManager &AM) {
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FunctionAnalysisManager &FAM =
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AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
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auto GetTTI = [&](Function &F) -> TargetTransformInfo & {
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return FAM.getResult<TargetIRAnalysis>(F);
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};
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if (!convertToRelativeLookupTables(M, GetTTI))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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return PA;
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}
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