992 lines
39 KiB
C++
992 lines
39 KiB
C++
//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
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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 pass identifies expensive constants to hoist and coalesces them to
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// better prepare it for SelectionDAG-based code generation. This works around
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// the limitations of the basic-block-at-a-time approach.
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//
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// First it scans all instructions for integer constants and calculates its
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// cost. If the constant can be folded into the instruction (the cost is
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// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
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// consider it expensive and leave it alone. This is the default behavior and
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// the default implementation of getIntImmCostInst will always return TCC_Free.
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//
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// If the cost is more than TCC_BASIC, then the integer constant can't be folded
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// into the instruction and it might be beneficial to hoist the constant.
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// Similar constants are coalesced to reduce register pressure and
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// materialization code.
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//
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// When a constant is hoisted, it is also hidden behind a bitcast to force it to
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// be live-out of the basic block. Otherwise the constant would be just
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// duplicated and each basic block would have its own copy in the SelectionDAG.
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// The SelectionDAG recognizes such constants as opaque and doesn't perform
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// certain transformations on them, which would create a new expensive constant.
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//
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// This optimization is only applied to integer constants in instructions and
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// simple (this means not nested) constant cast expressions. For example:
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// %0 = load i64* inttoptr (i64 big_constant to i64*)
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/ConstantHoisting.h"
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#include "llvm/ADT/APInt.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/None.h"
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#include "llvm/ADT/Optional.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/BlockFrequencyInfo.h"
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#include "llvm/Analysis/ProfileSummaryInfo.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/Constants.h"
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#include "llvm/IR/DebugInfoMetadata.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/InstrTypes.h"
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#include "llvm/IR/Instruction.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Value.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/BlockFrequency.h"
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#include "llvm/Support/Casting.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/SizeOpts.h"
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#include <algorithm>
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#include <cassert>
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#include <cstdint>
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#include <iterator>
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#include <tuple>
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#include <utility>
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using namespace llvm;
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using namespace consthoist;
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#define DEBUG_TYPE "consthoist"
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STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
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STATISTIC(NumConstantsRebased, "Number of constants rebased");
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static cl::opt<bool> ConstHoistWithBlockFrequency(
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"consthoist-with-block-frequency", cl::init(true), cl::Hidden,
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cl::desc("Enable the use of the block frequency analysis to reduce the "
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"chance to execute const materialization more frequently than "
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"without hoisting."));
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static cl::opt<bool> ConstHoistGEP(
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"consthoist-gep", cl::init(false), cl::Hidden,
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cl::desc("Try hoisting constant gep expressions"));
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static cl::opt<unsigned>
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MinNumOfDependentToRebase("consthoist-min-num-to-rebase",
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cl::desc("Do not rebase if number of dependent constants of a Base is less "
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"than this number."),
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cl::init(0), cl::Hidden);
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namespace {
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/// The constant hoisting pass.
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class ConstantHoistingLegacyPass : public FunctionPass {
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public:
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static char ID; // Pass identification, replacement for typeid
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ConstantHoistingLegacyPass() : FunctionPass(ID) {
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initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &Fn) override;
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StringRef getPassName() const override { return "Constant Hoisting"; }
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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if (ConstHoistWithBlockFrequency)
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AU.addRequired<BlockFrequencyInfoWrapperPass>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<ProfileSummaryInfoWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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}
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private:
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ConstantHoistingPass Impl;
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};
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} // end anonymous namespace
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char ConstantHoistingLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist",
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"Constant Hoisting", false, false)
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INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist",
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"Constant Hoisting", false, false)
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FunctionPass *llvm::createConstantHoistingPass() {
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return new ConstantHoistingLegacyPass();
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}
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/// Perform the constant hoisting optimization for the given function.
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bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) {
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if (skipFunction(Fn))
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return false;
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LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
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LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
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bool MadeChange =
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Impl.runImpl(Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn),
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getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
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ConstHoistWithBlockFrequency
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? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI()
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: nullptr,
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Fn.getEntryBlock(),
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&getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI());
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if (MadeChange) {
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LLVM_DEBUG(dbgs() << "********** Function after Constant Hoisting: "
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<< Fn.getName() << '\n');
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LLVM_DEBUG(dbgs() << Fn);
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}
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LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
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return MadeChange;
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}
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/// Find the constant materialization insertion point.
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Instruction *ConstantHoistingPass::findMatInsertPt(Instruction *Inst,
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unsigned Idx) const {
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// If the operand is a cast instruction, then we have to materialize the
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// constant before the cast instruction.
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if (Idx != ~0U) {
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Value *Opnd = Inst->getOperand(Idx);
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if (auto CastInst = dyn_cast<Instruction>(Opnd))
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if (CastInst->isCast())
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return CastInst;
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}
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// The simple and common case. This also includes constant expressions.
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if (!isa<PHINode>(Inst) && !Inst->isEHPad())
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return Inst;
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// We can't insert directly before a phi node or an eh pad. Insert before
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// the terminator of the incoming or dominating block.
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assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
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if (Idx != ~0U && isa<PHINode>(Inst))
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return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
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// This must be an EH pad. Iterate over immediate dominators until we find a
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// non-EH pad. We need to skip over catchswitch blocks, which are both EH pads
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// and terminators.
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auto IDom = DT->getNode(Inst->getParent())->getIDom();
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while (IDom->getBlock()->isEHPad()) {
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assert(Entry != IDom->getBlock() && "eh pad in entry block");
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IDom = IDom->getIDom();
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}
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return IDom->getBlock()->getTerminator();
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}
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/// Given \p BBs as input, find another set of BBs which collectively
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/// dominates \p BBs and have the minimal sum of frequencies. Return the BB
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/// set found in \p BBs.
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static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI,
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BasicBlock *Entry,
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SetVector<BasicBlock *> &BBs) {
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assert(!BBs.count(Entry) && "Assume Entry is not in BBs");
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// Nodes on the current path to the root.
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SmallPtrSet<BasicBlock *, 8> Path;
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// Candidates includes any block 'BB' in set 'BBs' that is not strictly
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// dominated by any other blocks in set 'BBs', and all nodes in the path
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// in the dominator tree from Entry to 'BB'.
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SmallPtrSet<BasicBlock *, 16> Candidates;
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for (auto BB : BBs) {
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// Ignore unreachable basic blocks.
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if (!DT.isReachableFromEntry(BB))
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continue;
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Path.clear();
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// Walk up the dominator tree until Entry or another BB in BBs
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// is reached. Insert the nodes on the way to the Path.
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BasicBlock *Node = BB;
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// The "Path" is a candidate path to be added into Candidates set.
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bool isCandidate = false;
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do {
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Path.insert(Node);
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if (Node == Entry || Candidates.count(Node)) {
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isCandidate = true;
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break;
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}
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assert(DT.getNode(Node)->getIDom() &&
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"Entry doens't dominate current Node");
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Node = DT.getNode(Node)->getIDom()->getBlock();
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} while (!BBs.count(Node));
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// If isCandidate is false, Node is another Block in BBs dominating
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// current 'BB'. Drop the nodes on the Path.
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if (!isCandidate)
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continue;
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// Add nodes on the Path into Candidates.
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Candidates.insert(Path.begin(), Path.end());
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}
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// Sort the nodes in Candidates in top-down order and save the nodes
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// in Orders.
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unsigned Idx = 0;
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SmallVector<BasicBlock *, 16> Orders;
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Orders.push_back(Entry);
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while (Idx != Orders.size()) {
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BasicBlock *Node = Orders[Idx++];
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for (auto ChildDomNode : DT.getNode(Node)->children()) {
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if (Candidates.count(ChildDomNode->getBlock()))
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Orders.push_back(ChildDomNode->getBlock());
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}
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}
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// Visit Orders in bottom-up order.
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using InsertPtsCostPair =
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std::pair<SetVector<BasicBlock *>, BlockFrequency>;
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// InsertPtsMap is a map from a BB to the best insertion points for the
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// subtree of BB (subtree not including the BB itself).
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DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap;
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InsertPtsMap.reserve(Orders.size() + 1);
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for (auto RIt = Orders.rbegin(); RIt != Orders.rend(); RIt++) {
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BasicBlock *Node = *RIt;
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bool NodeInBBs = BBs.count(Node);
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auto &InsertPts = InsertPtsMap[Node].first;
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BlockFrequency &InsertPtsFreq = InsertPtsMap[Node].second;
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// Return the optimal insert points in BBs.
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if (Node == Entry) {
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BBs.clear();
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if (InsertPtsFreq > BFI.getBlockFreq(Node) ||
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(InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1))
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BBs.insert(Entry);
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else
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BBs.insert(InsertPts.begin(), InsertPts.end());
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break;
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}
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BasicBlock *Parent = DT.getNode(Node)->getIDom()->getBlock();
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// Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child
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// will update its parent's ParentInsertPts and ParentPtsFreq.
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auto &ParentInsertPts = InsertPtsMap[Parent].first;
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BlockFrequency &ParentPtsFreq = InsertPtsMap[Parent].second;
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// Choose to insert in Node or in subtree of Node.
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// Don't hoist to EHPad because we may not find a proper place to insert
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// in EHPad.
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// If the total frequency of InsertPts is the same as the frequency of the
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// target Node, and InsertPts contains more than one nodes, choose hoisting
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// to reduce code size.
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if (NodeInBBs ||
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(!Node->isEHPad() &&
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(InsertPtsFreq > BFI.getBlockFreq(Node) ||
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(InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)))) {
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ParentInsertPts.insert(Node);
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ParentPtsFreq += BFI.getBlockFreq(Node);
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} else {
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ParentInsertPts.insert(InsertPts.begin(), InsertPts.end());
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ParentPtsFreq += InsertPtsFreq;
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}
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}
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}
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/// Find an insertion point that dominates all uses.
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SetVector<Instruction *> ConstantHoistingPass::findConstantInsertionPoint(
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const ConstantInfo &ConstInfo) const {
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assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
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// Collect all basic blocks.
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SetVector<BasicBlock *> BBs;
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SetVector<Instruction *> InsertPts;
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for (auto const &RCI : ConstInfo.RebasedConstants)
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for (auto const &U : RCI.Uses)
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BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
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if (BBs.count(Entry)) {
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InsertPts.insert(&Entry->front());
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return InsertPts;
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}
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if (BFI) {
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findBestInsertionSet(*DT, *BFI, Entry, BBs);
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for (auto BB : BBs) {
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BasicBlock::iterator InsertPt = BB->begin();
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for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
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;
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InsertPts.insert(&*InsertPt);
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}
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return InsertPts;
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}
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while (BBs.size() >= 2) {
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BasicBlock *BB, *BB1, *BB2;
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BB1 = BBs.pop_back_val();
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BB2 = BBs.pop_back_val();
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BB = DT->findNearestCommonDominator(BB1, BB2);
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if (BB == Entry) {
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InsertPts.insert(&Entry->front());
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return InsertPts;
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}
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BBs.insert(BB);
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}
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assert((BBs.size() == 1) && "Expected only one element.");
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Instruction &FirstInst = (*BBs.begin())->front();
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InsertPts.insert(findMatInsertPt(&FirstInst));
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return InsertPts;
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}
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/// Record constant integer ConstInt for instruction Inst at operand
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/// index Idx.
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///
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/// The operand at index Idx is not necessarily the constant integer itself. It
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/// could also be a cast instruction or a constant expression that uses the
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/// constant integer.
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void ConstantHoistingPass::collectConstantCandidates(
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ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
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ConstantInt *ConstInt) {
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unsigned Cost;
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// Ask the target about the cost of materializing the constant for the given
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// instruction and operand index.
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if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
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Cost = TTI->getIntImmCostIntrin(IntrInst->getIntrinsicID(), Idx,
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ConstInt->getValue(), ConstInt->getType(),
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TargetTransformInfo::TCK_SizeAndLatency);
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else
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Cost = TTI->getIntImmCostInst(
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Inst->getOpcode(), Idx, ConstInt->getValue(), ConstInt->getType(),
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TargetTransformInfo::TCK_SizeAndLatency, Inst);
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// Ignore cheap integer constants.
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if (Cost > TargetTransformInfo::TCC_Basic) {
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ConstCandMapType::iterator Itr;
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bool Inserted;
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ConstPtrUnionType Cand = ConstInt;
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std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
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if (Inserted) {
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ConstIntCandVec.push_back(ConstantCandidate(ConstInt));
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Itr->second = ConstIntCandVec.size() - 1;
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}
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ConstIntCandVec[Itr->second].addUser(Inst, Idx, Cost);
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LLVM_DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs()
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<< "Collect constant " << *ConstInt << " from " << *Inst
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<< " with cost " << Cost << '\n';
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else dbgs() << "Collect constant " << *ConstInt
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<< " indirectly from " << *Inst << " via "
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<< *Inst->getOperand(Idx) << " with cost " << Cost
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<< '\n';);
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}
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}
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/// Record constant GEP expression for instruction Inst at operand index Idx.
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void ConstantHoistingPass::collectConstantCandidates(
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ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
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ConstantExpr *ConstExpr) {
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// TODO: Handle vector GEPs
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if (ConstExpr->getType()->isVectorTy())
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return;
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GlobalVariable *BaseGV = dyn_cast<GlobalVariable>(ConstExpr->getOperand(0));
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if (!BaseGV)
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return;
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// Get offset from the base GV.
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PointerType *GVPtrTy = cast<PointerType>(BaseGV->getType());
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IntegerType *PtrIntTy = DL->getIntPtrType(*Ctx, GVPtrTy->getAddressSpace());
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APInt Offset(DL->getTypeSizeInBits(PtrIntTy), /*val*/0, /*isSigned*/true);
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auto *GEPO = cast<GEPOperator>(ConstExpr);
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if (!GEPO->accumulateConstantOffset(*DL, Offset))
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return;
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if (!Offset.isIntN(32))
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return;
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// A constant GEP expression that has a GlobalVariable as base pointer is
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// usually lowered to a load from constant pool. Such operation is unlikely
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// to be cheaper than compute it by <Base + Offset>, which can be lowered to
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// an ADD instruction or folded into Load/Store instruction.
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int Cost =
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TTI->getIntImmCostInst(Instruction::Add, 1, Offset, PtrIntTy,
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TargetTransformInfo::TCK_SizeAndLatency, Inst);
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ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV];
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ConstCandMapType::iterator Itr;
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bool Inserted;
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ConstPtrUnionType Cand = ConstExpr;
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std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
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if (Inserted) {
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ExprCandVec.push_back(ConstantCandidate(
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ConstantInt::get(Type::getInt32Ty(*Ctx), Offset.getLimitedValue()),
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ConstExpr));
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Itr->second = ExprCandVec.size() - 1;
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}
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ExprCandVec[Itr->second].addUser(Inst, Idx, Cost);
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}
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/// Check the operand for instruction Inst at index Idx.
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void ConstantHoistingPass::collectConstantCandidates(
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ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) {
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Value *Opnd = Inst->getOperand(Idx);
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// Visit constant integers.
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if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
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collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
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return;
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}
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// Visit cast instructions that have constant integers.
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if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
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// Only visit cast instructions, which have been skipped. All other
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// instructions should have already been visited.
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if (!CastInst->isCast())
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return;
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if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
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// Pretend the constant is directly used by the instruction and ignore
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// the cast instruction.
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collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
|
|
return;
|
|
}
|
|
}
|
|
|
|
// Visit constant expressions that have constant integers.
|
|
if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
|
|
// Handle constant gep expressions.
|
|
if (ConstHoistGEP && ConstExpr->isGEPWithNoNotionalOverIndexing())
|
|
collectConstantCandidates(ConstCandMap, Inst, Idx, ConstExpr);
|
|
|
|
// Only visit constant cast expressions.
|
|
if (!ConstExpr->isCast())
|
|
return;
|
|
|
|
if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
|
|
// Pretend the constant is directly used by the instruction and ignore
|
|
// the constant expression.
|
|
collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
|
|
return;
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Scan the instruction for expensive integer constants and record them
|
|
/// in the constant candidate vector.
|
|
void ConstantHoistingPass::collectConstantCandidates(
|
|
ConstCandMapType &ConstCandMap, Instruction *Inst) {
|
|
// Skip all cast instructions. They are visited indirectly later on.
|
|
if (Inst->isCast())
|
|
return;
|
|
|
|
// Scan all operands.
|
|
for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
|
|
// The cost of materializing the constants (defined in
|
|
// `TargetTransformInfo::getIntImmCostInst`) for instructions which only
|
|
// take constant variables is lower than `TargetTransformInfo::TCC_Basic`.
|
|
// So it's safe for us to collect constant candidates from all
|
|
// IntrinsicInsts.
|
|
if (canReplaceOperandWithVariable(Inst, Idx)) {
|
|
collectConstantCandidates(ConstCandMap, Inst, Idx);
|
|
}
|
|
} // end of for all operands
|
|
}
|
|
|
|
/// Collect all integer constants in the function that cannot be folded
|
|
/// into an instruction itself.
|
|
void ConstantHoistingPass::collectConstantCandidates(Function &Fn) {
|
|
ConstCandMapType ConstCandMap;
|
|
for (BasicBlock &BB : Fn) {
|
|
// Ignore unreachable basic blocks.
|
|
if (!DT->isReachableFromEntry(&BB))
|
|
continue;
|
|
for (Instruction &Inst : BB)
|
|
collectConstantCandidates(ConstCandMap, &Inst);
|
|
}
|
|
}
|
|
|
|
// This helper function is necessary to deal with values that have different
|
|
// bit widths (APInt Operator- does not like that). If the value cannot be
|
|
// represented in uint64 we return an "empty" APInt. This is then interpreted
|
|
// as the value is not in range.
|
|
static Optional<APInt> calculateOffsetDiff(const APInt &V1, const APInt &V2) {
|
|
Optional<APInt> Res = None;
|
|
unsigned BW = V1.getBitWidth() > V2.getBitWidth() ?
|
|
V1.getBitWidth() : V2.getBitWidth();
|
|
uint64_t LimVal1 = V1.getLimitedValue();
|
|
uint64_t LimVal2 = V2.getLimitedValue();
|
|
|
|
if (LimVal1 == ~0ULL || LimVal2 == ~0ULL)
|
|
return Res;
|
|
|
|
uint64_t Diff = LimVal1 - LimVal2;
|
|
return APInt(BW, Diff, true);
|
|
}
|
|
|
|
// From a list of constants, one needs to picked as the base and the other
|
|
// constants will be transformed into an offset from that base constant. The
|
|
// question is which we can pick best? For example, consider these constants
|
|
// and their number of uses:
|
|
//
|
|
// Constants| 2 | 4 | 12 | 42 |
|
|
// NumUses | 3 | 2 | 8 | 7 |
|
|
//
|
|
// Selecting constant 12 because it has the most uses will generate negative
|
|
// offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative
|
|
// offsets lead to less optimal code generation, then there might be better
|
|
// solutions. Suppose immediates in the range of 0..35 are most optimally
|
|
// supported by the architecture, then selecting constant 2 is most optimal
|
|
// because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in
|
|
// range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would
|
|
// have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in
|
|
// selecting the base constant the range of the offsets is a very important
|
|
// factor too that we take into account here. This algorithm calculates a total
|
|
// costs for selecting a constant as the base and substract the costs if
|
|
// immediates are out of range. It has quadratic complexity, so we call this
|
|
// function only when we're optimising for size and there are less than 100
|
|
// constants, we fall back to the straightforward algorithm otherwise
|
|
// which does not do all the offset calculations.
|
|
unsigned
|
|
ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S,
|
|
ConstCandVecType::iterator E,
|
|
ConstCandVecType::iterator &MaxCostItr) {
|
|
unsigned NumUses = 0;
|
|
|
|
bool OptForSize = Entry->getParent()->hasOptSize() ||
|
|
llvm::shouldOptimizeForSize(Entry->getParent(), PSI, BFI,
|
|
PGSOQueryType::IRPass);
|
|
if (!OptForSize || std::distance(S,E) > 100) {
|
|
for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
|
|
NumUses += ConstCand->Uses.size();
|
|
if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
|
|
MaxCostItr = ConstCand;
|
|
}
|
|
return NumUses;
|
|
}
|
|
|
|
LLVM_DEBUG(dbgs() << "== Maximize constants in range ==\n");
|
|
int MaxCost = -1;
|
|
for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
|
|
auto Value = ConstCand->ConstInt->getValue();
|
|
Type *Ty = ConstCand->ConstInt->getType();
|
|
int Cost = 0;
|
|
NumUses += ConstCand->Uses.size();
|
|
LLVM_DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue()
|
|
<< "\n");
|
|
|
|
for (auto User : ConstCand->Uses) {
|
|
unsigned Opcode = User.Inst->getOpcode();
|
|
unsigned OpndIdx = User.OpndIdx;
|
|
Cost += TTI->getIntImmCostInst(Opcode, OpndIdx, Value, Ty,
|
|
TargetTransformInfo::TCK_SizeAndLatency);
|
|
LLVM_DEBUG(dbgs() << "Cost: " << Cost << "\n");
|
|
|
|
for (auto C2 = S; C2 != E; ++C2) {
|
|
Optional<APInt> Diff = calculateOffsetDiff(
|
|
C2->ConstInt->getValue(),
|
|
ConstCand->ConstInt->getValue());
|
|
if (Diff) {
|
|
const int ImmCosts =
|
|
TTI->getIntImmCodeSizeCost(Opcode, OpndIdx, Diff.getValue(), Ty);
|
|
Cost -= ImmCosts;
|
|
LLVM_DEBUG(dbgs() << "Offset " << Diff.getValue() << " "
|
|
<< "has penalty: " << ImmCosts << "\n"
|
|
<< "Adjusted cost: " << Cost << "\n");
|
|
}
|
|
}
|
|
}
|
|
LLVM_DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n");
|
|
if (Cost > MaxCost) {
|
|
MaxCost = Cost;
|
|
MaxCostItr = ConstCand;
|
|
LLVM_DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue()
|
|
<< "\n");
|
|
}
|
|
}
|
|
return NumUses;
|
|
}
|
|
|
|
/// Find the base constant within the given range and rebase all other
|
|
/// constants with respect to the base constant.
|
|
void ConstantHoistingPass::findAndMakeBaseConstant(
|
|
ConstCandVecType::iterator S, ConstCandVecType::iterator E,
|
|
SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec) {
|
|
auto MaxCostItr = S;
|
|
unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr);
|
|
|
|
// Don't hoist constants that have only one use.
|
|
if (NumUses <= 1)
|
|
return;
|
|
|
|
ConstantInt *ConstInt = MaxCostItr->ConstInt;
|
|
ConstantExpr *ConstExpr = MaxCostItr->ConstExpr;
|
|
ConstantInfo ConstInfo;
|
|
ConstInfo.BaseInt = ConstInt;
|
|
ConstInfo.BaseExpr = ConstExpr;
|
|
Type *Ty = ConstInt->getType();
|
|
|
|
// Rebase the constants with respect to the base constant.
|
|
for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
|
|
APInt Diff = ConstCand->ConstInt->getValue() - ConstInt->getValue();
|
|
Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
|
|
Type *ConstTy =
|
|
ConstCand->ConstExpr ? ConstCand->ConstExpr->getType() : nullptr;
|
|
ConstInfo.RebasedConstants.push_back(
|
|
RebasedConstantInfo(std::move(ConstCand->Uses), Offset, ConstTy));
|
|
}
|
|
ConstInfoVec.push_back(std::move(ConstInfo));
|
|
}
|
|
|
|
/// Finds and combines constant candidates that can be easily
|
|
/// rematerialized with an add from a common base constant.
|
|
void ConstantHoistingPass::findBaseConstants(GlobalVariable *BaseGV) {
|
|
// If BaseGV is nullptr, find base among candidate constant integers;
|
|
// Otherwise find base among constant GEPs that share the same BaseGV.
|
|
ConstCandVecType &ConstCandVec = BaseGV ?
|
|
ConstGEPCandMap[BaseGV] : ConstIntCandVec;
|
|
ConstInfoVecType &ConstInfoVec = BaseGV ?
|
|
ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
|
|
|
|
// Sort the constants by value and type. This invalidates the mapping!
|
|
llvm::stable_sort(ConstCandVec, [](const ConstantCandidate &LHS,
|
|
const ConstantCandidate &RHS) {
|
|
if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
|
|
return LHS.ConstInt->getType()->getBitWidth() <
|
|
RHS.ConstInt->getType()->getBitWidth();
|
|
return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
|
|
});
|
|
|
|
// Simple linear scan through the sorted constant candidate vector for viable
|
|
// merge candidates.
|
|
auto MinValItr = ConstCandVec.begin();
|
|
for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
|
|
CC != E; ++CC) {
|
|
if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
|
|
Type *MemUseValTy = nullptr;
|
|
for (auto &U : CC->Uses) {
|
|
auto *UI = U.Inst;
|
|
if (LoadInst *LI = dyn_cast<LoadInst>(UI)) {
|
|
MemUseValTy = LI->getType();
|
|
break;
|
|
} else if (StoreInst *SI = dyn_cast<StoreInst>(UI)) {
|
|
// Make sure the constant is used as pointer operand of the StoreInst.
|
|
if (SI->getPointerOperand() == SI->getOperand(U.OpndIdx)) {
|
|
MemUseValTy = SI->getValueOperand()->getType();
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
|
|
// Check if the constant is in range of an add with immediate.
|
|
APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
|
|
if ((Diff.getBitWidth() <= 64) &&
|
|
TTI->isLegalAddImmediate(Diff.getSExtValue()) &&
|
|
// Check if Diff can be used as offset in addressing mode of the user
|
|
// memory instruction.
|
|
(!MemUseValTy || TTI->isLegalAddressingMode(MemUseValTy,
|
|
/*BaseGV*/nullptr, /*BaseOffset*/Diff.getSExtValue(),
|
|
/*HasBaseReg*/true, /*Scale*/0)))
|
|
continue;
|
|
}
|
|
// We either have now a different constant type or the constant is not in
|
|
// range of an add with immediate anymore.
|
|
findAndMakeBaseConstant(MinValItr, CC, ConstInfoVec);
|
|
// Start a new base constant search.
|
|
MinValItr = CC;
|
|
}
|
|
// Finalize the last base constant search.
|
|
findAndMakeBaseConstant(MinValItr, ConstCandVec.end(), ConstInfoVec);
|
|
}
|
|
|
|
/// Updates the operand at Idx in instruction Inst with the result of
|
|
/// instruction Mat. If the instruction is a PHI node then special
|
|
/// handling for duplicate values form the same incoming basic block is
|
|
/// required.
|
|
/// \return The update will always succeed, but the return value indicated if
|
|
/// Mat was used for the update or not.
|
|
static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
|
|
if (auto PHI = dyn_cast<PHINode>(Inst)) {
|
|
// Check if any previous operand of the PHI node has the same incoming basic
|
|
// block. This is a very odd case that happens when the incoming basic block
|
|
// has a switch statement. In this case use the same value as the previous
|
|
// operand(s), otherwise we will fail verification due to different values.
|
|
// The values are actually the same, but the variable names are different
|
|
// and the verifier doesn't like that.
|
|
BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
|
|
for (unsigned i = 0; i < Idx; ++i) {
|
|
if (PHI->getIncomingBlock(i) == IncomingBB) {
|
|
Value *IncomingVal = PHI->getIncomingValue(i);
|
|
Inst->setOperand(Idx, IncomingVal);
|
|
return false;
|
|
}
|
|
}
|
|
}
|
|
|
|
Inst->setOperand(Idx, Mat);
|
|
return true;
|
|
}
|
|
|
|
/// Emit materialization code for all rebased constants and update their
|
|
/// users.
|
|
void ConstantHoistingPass::emitBaseConstants(Instruction *Base,
|
|
Constant *Offset,
|
|
Type *Ty,
|
|
const ConstantUser &ConstUser) {
|
|
Instruction *Mat = Base;
|
|
|
|
// The same offset can be dereferenced to different types in nested struct.
|
|
if (!Offset && Ty && Ty != Base->getType())
|
|
Offset = ConstantInt::get(Type::getInt32Ty(*Ctx), 0);
|
|
|
|
if (Offset) {
|
|
Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
|
|
ConstUser.OpndIdx);
|
|
if (Ty) {
|
|
// Constant being rebased is a ConstantExpr.
|
|
PointerType *Int8PtrTy = Type::getInt8PtrTy(*Ctx,
|
|
cast<PointerType>(Ty)->getAddressSpace());
|
|
Base = new BitCastInst(Base, Int8PtrTy, "base_bitcast", InsertionPt);
|
|
Mat = GetElementPtrInst::Create(Int8PtrTy->getElementType(), Base,
|
|
Offset, "mat_gep", InsertionPt);
|
|
Mat = new BitCastInst(Mat, Ty, "mat_bitcast", InsertionPt);
|
|
} else
|
|
// Constant being rebased is a ConstantInt.
|
|
Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
|
|
"const_mat", InsertionPt);
|
|
|
|
LLVM_DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
|
|
<< " + " << *Offset << ") in BB "
|
|
<< Mat->getParent()->getName() << '\n'
|
|
<< *Mat << '\n');
|
|
Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
|
|
}
|
|
Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
|
|
|
|
// Visit constant integer.
|
|
if (isa<ConstantInt>(Opnd)) {
|
|
LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
|
|
if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
|
|
Mat->eraseFromParent();
|
|
LLVM_DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
|
|
return;
|
|
}
|
|
|
|
// Visit cast instruction.
|
|
if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
|
|
assert(CastInst->isCast() && "Expected an cast instruction!");
|
|
// Check if we already have visited this cast instruction before to avoid
|
|
// unnecessary cloning.
|
|
Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
|
|
if (!ClonedCastInst) {
|
|
ClonedCastInst = CastInst->clone();
|
|
ClonedCastInst->setOperand(0, Mat);
|
|
ClonedCastInst->insertAfter(CastInst);
|
|
// Use the same debug location as the original cast instruction.
|
|
ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
|
|
LLVM_DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
|
|
<< "To : " << *ClonedCastInst << '\n');
|
|
}
|
|
|
|
LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
|
|
updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
|
|
LLVM_DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
|
|
return;
|
|
}
|
|
|
|
// Visit constant expression.
|
|
if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
|
|
if (ConstExpr->isGEPWithNoNotionalOverIndexing()) {
|
|
// Operand is a ConstantGEP, replace it.
|
|
updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat);
|
|
return;
|
|
}
|
|
|
|
// Aside from constant GEPs, only constant cast expressions are collected.
|
|
assert(ConstExpr->isCast() && "ConstExpr should be a cast");
|
|
Instruction *ConstExprInst = ConstExpr->getAsInstruction();
|
|
ConstExprInst->setOperand(0, Mat);
|
|
ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
|
|
ConstUser.OpndIdx));
|
|
|
|
// Use the same debug location as the instruction we are about to update.
|
|
ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
|
|
|
|
LLVM_DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
|
|
<< "From : " << *ConstExpr << '\n');
|
|
LLVM_DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
|
|
if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
|
|
ConstExprInst->eraseFromParent();
|
|
if (Offset)
|
|
Mat->eraseFromParent();
|
|
}
|
|
LLVM_DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
|
|
return;
|
|
}
|
|
}
|
|
|
|
/// Hoist and hide the base constant behind a bitcast and emit
|
|
/// materialization code for derived constants.
|
|
bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {
|
|
bool MadeChange = false;
|
|
SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec =
|
|
BaseGV ? ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
|
|
for (auto const &ConstInfo : ConstInfoVec) {
|
|
SetVector<Instruction *> IPSet = findConstantInsertionPoint(ConstInfo);
|
|
// We can have an empty set if the function contains unreachable blocks.
|
|
if (IPSet.empty())
|
|
continue;
|
|
|
|
unsigned UsesNum = 0;
|
|
unsigned ReBasesNum = 0;
|
|
unsigned NotRebasedNum = 0;
|
|
for (Instruction *IP : IPSet) {
|
|
// First, collect constants depending on this IP of the base.
|
|
unsigned Uses = 0;
|
|
using RebasedUse = std::tuple<Constant *, Type *, ConstantUser>;
|
|
SmallVector<RebasedUse, 4> ToBeRebased;
|
|
for (auto const &RCI : ConstInfo.RebasedConstants) {
|
|
for (auto const &U : RCI.Uses) {
|
|
Uses++;
|
|
BasicBlock *OrigMatInsertBB =
|
|
findMatInsertPt(U.Inst, U.OpndIdx)->getParent();
|
|
// If Base constant is to be inserted in multiple places,
|
|
// generate rebase for U using the Base dominating U.
|
|
if (IPSet.size() == 1 ||
|
|
DT->dominates(IP->getParent(), OrigMatInsertBB))
|
|
ToBeRebased.push_back(RebasedUse(RCI.Offset, RCI.Ty, U));
|
|
}
|
|
}
|
|
UsesNum = Uses;
|
|
|
|
// If only few constants depend on this IP of base, skip rebasing,
|
|
// assuming the base and the rebased have the same materialization cost.
|
|
if (ToBeRebased.size() < MinNumOfDependentToRebase) {
|
|
NotRebasedNum += ToBeRebased.size();
|
|
continue;
|
|
}
|
|
|
|
// Emit an instance of the base at this IP.
|
|
Instruction *Base = nullptr;
|
|
// Hoist and hide the base constant behind a bitcast.
|
|
if (ConstInfo.BaseExpr) {
|
|
assert(BaseGV && "A base constant expression must have an base GV");
|
|
Type *Ty = ConstInfo.BaseExpr->getType();
|
|
Base = new BitCastInst(ConstInfo.BaseExpr, Ty, "const", IP);
|
|
} else {
|
|
IntegerType *Ty = ConstInfo.BaseInt->getType();
|
|
Base = new BitCastInst(ConstInfo.BaseInt, Ty, "const", IP);
|
|
}
|
|
|
|
Base->setDebugLoc(IP->getDebugLoc());
|
|
|
|
LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseInt
|
|
<< ") to BB " << IP->getParent()->getName() << '\n'
|
|
<< *Base << '\n');
|
|
|
|
// Emit materialization code for rebased constants depending on this IP.
|
|
for (auto const &R : ToBeRebased) {
|
|
Constant *Off = std::get<0>(R);
|
|
Type *Ty = std::get<1>(R);
|
|
ConstantUser U = std::get<2>(R);
|
|
emitBaseConstants(Base, Off, Ty, U);
|
|
ReBasesNum++;
|
|
// Use the same debug location as the last user of the constant.
|
|
Base->setDebugLoc(DILocation::getMergedLocation(
|
|
Base->getDebugLoc(), U.Inst->getDebugLoc()));
|
|
}
|
|
assert(!Base->use_empty() && "The use list is empty!?");
|
|
assert(isa<Instruction>(Base->user_back()) &&
|
|
"All uses should be instructions.");
|
|
}
|
|
(void)UsesNum;
|
|
(void)ReBasesNum;
|
|
(void)NotRebasedNum;
|
|
// Expect all uses are rebased after rebase is done.
|
|
assert(UsesNum == (ReBasesNum + NotRebasedNum) &&
|
|
"Not all uses are rebased");
|
|
|
|
NumConstantsHoisted++;
|
|
|
|
// Base constant is also included in ConstInfo.RebasedConstants, so
|
|
// deduct 1 from ConstInfo.RebasedConstants.size().
|
|
NumConstantsRebased += ConstInfo.RebasedConstants.size() - 1;
|
|
|
|
MadeChange = true;
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
/// Check all cast instructions we made a copy of and remove them if they
|
|
/// have no more users.
|
|
void ConstantHoistingPass::deleteDeadCastInst() const {
|
|
for (auto const &I : ClonedCastMap)
|
|
if (I.first->use_empty())
|
|
I.first->eraseFromParent();
|
|
}
|
|
|
|
/// Optimize expensive integer constants in the given function.
|
|
bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI,
|
|
DominatorTree &DT, BlockFrequencyInfo *BFI,
|
|
BasicBlock &Entry, ProfileSummaryInfo *PSI) {
|
|
this->TTI = &TTI;
|
|
this->DT = &DT;
|
|
this->BFI = BFI;
|
|
this->DL = &Fn.getParent()->getDataLayout();
|
|
this->Ctx = &Fn.getContext();
|
|
this->Entry = &Entry;
|
|
this->PSI = PSI;
|
|
// Collect all constant candidates.
|
|
collectConstantCandidates(Fn);
|
|
|
|
// Combine constants that can be easily materialized with an add from a common
|
|
// base constant.
|
|
if (!ConstIntCandVec.empty())
|
|
findBaseConstants(nullptr);
|
|
for (const auto &MapEntry : ConstGEPCandMap)
|
|
if (!MapEntry.second.empty())
|
|
findBaseConstants(MapEntry.first);
|
|
|
|
// Finally hoist the base constant and emit materialization code for dependent
|
|
// constants.
|
|
bool MadeChange = false;
|
|
if (!ConstIntInfoVec.empty())
|
|
MadeChange = emitBaseConstants(nullptr);
|
|
for (const auto &MapEntry : ConstGEPInfoMap)
|
|
if (!MapEntry.second.empty())
|
|
MadeChange |= emitBaseConstants(MapEntry.first);
|
|
|
|
|
|
// Cleanup dead instructions.
|
|
deleteDeadCastInst();
|
|
|
|
cleanup();
|
|
|
|
return MadeChange;
|
|
}
|
|
|
|
PreservedAnalyses ConstantHoistingPass::run(Function &F,
|
|
FunctionAnalysisManager &AM) {
|
|
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
|
|
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
|
|
auto BFI = ConstHoistWithBlockFrequency
|
|
? &AM.getResult<BlockFrequencyAnalysis>(F)
|
|
: nullptr;
|
|
auto &MAMProxy = AM.getResult<ModuleAnalysisManagerFunctionProxy>(F);
|
|
auto *PSI = MAMProxy.getCachedResult<ProfileSummaryAnalysis>(*F.getParent());
|
|
if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock(), PSI))
|
|
return PreservedAnalyses::all();
|
|
|
|
PreservedAnalyses PA;
|
|
PA.preserveSet<CFGAnalyses>();
|
|
return PA;
|
|
}
|