//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass identifies expensive constants to hoist and coalesces them to
// better prepare it for SelectionDAG-based code generation. This works around
// the limitations of the basic-block-at-a-time approach.
//
// First it scans all instructions for integer constants and calculates its
// cost. If the constant can be folded into the instruction (the cost is
// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
// consider it expensive and leave it alone. This is the default behavior and
// the default implementation of getIntImmCost will always return TCC_Free.
//
// If the cost is more than TCC_BASIC, then the integer constant can't be folded
// into the instruction and it might be beneficial to hoist the constant.
// Similar constants are coalesced to reduce register pressure and
// materialization code.
//
// When a constant is hoisted, it is also hidden behind a bitcast to force it to
// be live-out of the basic block. Otherwise the constant would be just
// duplicated and each basic block would have its own copy in the SelectionDAG.
// The SelectionDAG recognizes such constants as opaque and doesn't perform
// certain transformations on them, which would create a new expensive constant.
//
// This optimization is only applied to integer constants in instructions and
// simple (this means not nested) constant cast expressions. For example:
// %0 = load i64* inttoptr (i64 big_constant to i64*)
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/ConstantHoisting.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <tuple>
#include <utility>
using namespace llvm;
using namespace consthoist;
#define DEBUG_TYPE "consthoist"
STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
STATISTIC(NumConstantsRebased, "Number of constants rebased");
static cl::opt<bool> ConstHoistWithBlockFrequency(
"consthoist-with-block-frequency", cl::init(true), cl::Hidden,
cl::desc("Enable the use of the block frequency analysis to reduce the "
"chance to execute const materialization more frequently than "
"without hoisting."));
namespace {
/// The constant hoisting pass.
class ConstantHoistingLegacyPass : public FunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
ConstantHoistingLegacyPass() : FunctionPass(ID) {
initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &Fn) override;
StringRef getPassName() const override { return "Constant Hoisting"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
if (ConstHoistWithBlockFrequency)
AU.addRequired<BlockFrequencyInfoWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
}
void releaseMemory() override { Impl.releaseMemory(); }
private:
ConstantHoistingPass Impl;
};
} // end anonymous namespace
char ConstantHoistingLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist",
"Constant Hoisting", false, false)
INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist",
"Constant Hoisting", false, false)
FunctionPass *llvm::createConstantHoistingPass() {
return new ConstantHoistingLegacyPass();
}
/// Perform the constant hoisting optimization for the given function.
bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) {
if (skipFunction(Fn))
return false;
LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
bool MadeChange =
Impl.runImpl(Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn),
getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
ConstHoistWithBlockFrequency
? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI()
: nullptr,
Fn.getEntryBlock());
if (MadeChange) {
LLVM_DEBUG(dbgs() << "********** Function after Constant Hoisting: "
<< Fn.getName() << '\n');
LLVM_DEBUG(dbgs() << Fn);
}
LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
return MadeChange;
}
/// Find the constant materialization insertion point.
Instruction *ConstantHoistingPass::findMatInsertPt(Instruction *Inst,
unsigned Idx) const {
// If the operand is a cast instruction, then we have to materialize the
// constant before the cast instruction.
if (Idx != ~0U) {
Value *Opnd = Inst->getOperand(Idx);
if (auto CastInst = dyn_cast<Instruction>(Opnd))
if (CastInst->isCast())
return CastInst;
}
// The simple and common case. This also includes constant expressions.
if (!isa<PHINode>(Inst) && !Inst->isEHPad())
return Inst;
// We can't insert directly before a phi node or an eh pad. Insert before
// the terminator of the incoming or dominating block.
assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
if (Idx != ~0U && isa<PHINode>(Inst))
return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
// This must be an EH pad. Iterate over immediate dominators until we find a
// non-EH pad. We need to skip over catchswitch blocks, which are both EH pads
// and terminators.
auto IDom = DT->getNode(Inst->getParent())->getIDom();
while (IDom->getBlock()->isEHPad()) {
assert(Entry != IDom->getBlock() && "eh pad in entry block");
IDom = IDom->getIDom();
}
return IDom->getBlock()->getTerminator();
}
/// Given \p BBs as input, find another set of BBs which collectively
/// dominates \p BBs and have the minimal sum of frequencies. Return the BB
/// set found in \p BBs.
static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI,
BasicBlock *Entry,
SmallPtrSet<BasicBlock *, 8> &BBs) {
assert(!BBs.count(Entry) && "Assume Entry is not in BBs");
// Nodes on the current path to the root.
SmallPtrSet<BasicBlock *, 8> Path;
// Candidates includes any block 'BB' in set 'BBs' that is not strictly
// dominated by any other blocks in set 'BBs', and all nodes in the path
// in the dominator tree from Entry to 'BB'.
SmallPtrSet<BasicBlock *, 16> Candidates;
for (auto BB : BBs) {
Path.clear();
// Walk up the dominator tree until Entry or another BB in BBs
// is reached. Insert the nodes on the way to the Path.
BasicBlock *Node = BB;
// The "Path" is a candidate path to be added into Candidates set.
bool isCandidate = false;
do {
Path.insert(Node);
if (Node == Entry || Candidates.count(Node)) {
isCandidate = true;
break;
}
assert(DT.getNode(Node)->getIDom() &&
"Entry doens't dominate current Node");
Node = DT.getNode(Node)->getIDom()->getBlock();
} while (!BBs.count(Node));
// If isCandidate is false, Node is another Block in BBs dominating
// current 'BB'. Drop the nodes on the Path.
if (!isCandidate)
continue;
// Add nodes on the Path into Candidates.
Candidates.insert(Path.begin(), Path.end());
}
// Sort the nodes in Candidates in top-down order and save the nodes
// in Orders.
unsigned Idx = 0;
SmallVector<BasicBlock *, 16> Orders;
Orders.push_back(Entry);
while (Idx != Orders.size()) {
BasicBlock *Node = Orders[Idx++];
for (auto ChildDomNode : DT.getNode(Node)->getChildren()) {
if (Candidates.count(ChildDomNode->getBlock()))
Orders.push_back(ChildDomNode->getBlock());
}
}
// Visit Orders in bottom-up order.
using InsertPtsCostPair =
std::pair<SmallPtrSet<BasicBlock *, 16>, BlockFrequency>;
// InsertPtsMap is a map from a BB to the best insertion points for the
// subtree of BB (subtree not including the BB itself).
DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap;
InsertPtsMap.reserve(Orders.size() + 1);
for (auto RIt = Orders.rbegin(); RIt != Orders.rend(); RIt++) {
BasicBlock *Node = *RIt;
bool NodeInBBs = BBs.count(Node);
SmallPtrSet<BasicBlock *, 16> &InsertPts = InsertPtsMap[Node].first;
BlockFrequency &InsertPtsFreq = InsertPtsMap[Node].second;
// Return the optimal insert points in BBs.
if (Node == Entry) {
BBs.clear();
if (InsertPtsFreq > BFI.getBlockFreq(Node) ||
(InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1))
BBs.insert(Entry);
else
BBs.insert(InsertPts.begin(), InsertPts.end());
break;
}
BasicBlock *Parent = DT.getNode(Node)->getIDom()->getBlock();
// Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child
// will update its parent's ParentInsertPts and ParentPtsFreq.
SmallPtrSet<BasicBlock *, 16> &ParentInsertPts = InsertPtsMap[Parent].first;
BlockFrequency &ParentPtsFreq = InsertPtsMap[Parent].second;
// Choose to insert in Node or in subtree of Node.
// Don't hoist to EHPad because we may not find a proper place to insert
// in EHPad.
// If the total frequency of InsertPts is the same as the frequency of the
// target Node, and InsertPts contains more than one nodes, choose hoisting
// to reduce code size.
if (NodeInBBs ||
(!Node->isEHPad() &&
(InsertPtsFreq > BFI.getBlockFreq(Node) ||
(InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)))) {
ParentInsertPts.insert(Node);
ParentPtsFreq += BFI.getBlockFreq(Node);
} else {
ParentInsertPts.insert(InsertPts.begin(), InsertPts.end());
ParentPtsFreq += InsertPtsFreq;
}
}
}
/// Find an insertion point that dominates all uses.
SmallPtrSet<Instruction *, 8> ConstantHoistingPass::findConstantInsertionPoint(
const ConstantInfo &ConstInfo) const {
assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
// Collect all basic blocks.
SmallPtrSet<BasicBlock *, 8> BBs;
SmallPtrSet<Instruction *, 8> InsertPts;
for (auto const &RCI : ConstInfo.RebasedConstants)
for (auto const &U : RCI.Uses)
BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
if (BBs.count(Entry)) {
InsertPts.insert(&Entry->front());
return InsertPts;
}
if (BFI) {
findBestInsertionSet(*DT, *BFI, Entry, BBs);
for (auto BB : BBs) {
BasicBlock::iterator InsertPt = BB->begin();
for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
;
InsertPts.insert(&*InsertPt);
}
return InsertPts;
}
while (BBs.size() >= 2) {
BasicBlock *BB, *BB1, *BB2;
BB1 = *BBs.begin();
BB2 = *std::next(BBs.begin());
BB = DT->findNearestCommonDominator(BB1, BB2);
if (BB == Entry) {
InsertPts.insert(&Entry->front());
return InsertPts;
}
BBs.erase(BB1);
BBs.erase(BB2);
BBs.insert(BB);
}
assert((BBs.size() == 1) && "Expected only one element.");
Instruction &FirstInst = (*BBs.begin())->front();
InsertPts.insert(findMatInsertPt(&FirstInst));
return InsertPts;
}
/// Record constant integer ConstInt for instruction Inst at operand
/// index Idx.
///
/// The operand at index Idx is not necessarily the constant integer itself. It
/// could also be a cast instruction or a constant expression that uses the
// constant integer.
void ConstantHoistingPass::collectConstantCandidates(
ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
ConstantInt *ConstInt) {
unsigned Cost;
// Ask the target about the cost of materializing the constant for the given
// instruction and operand index.
if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
ConstInt->getValue(), ConstInt->getType());
else
Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
ConstInt->getType());
// Ignore cheap integer constants.
if (Cost > TargetTransformInfo::TCC_Basic) {
ConstCandMapType::iterator Itr;
bool Inserted;
std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
if (Inserted) {
ConstCandVec.push_back(ConstantCandidate(ConstInt));
Itr->second = ConstCandVec.size() - 1;
}
ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
LLVM_DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs()
<< "Collect constant " << *ConstInt << " from " << *Inst
<< " with cost " << Cost << '\n';
else dbgs() << "Collect constant " << *ConstInt
<< " indirectly from " << *Inst << " via "
<< *Inst->getOperand(Idx) << " with cost " << Cost
<< '\n';);
}
}
/// Check the operand for instruction Inst at index Idx.
void ConstantHoistingPass::collectConstantCandidates(
ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) {
Value *Opnd = Inst->getOperand(Idx);
// Visit constant integers.
if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
return;
}
// Visit cast instructions that have constant integers.
if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
// Only visit cast instructions, which have been skipped. All other
// instructions should have already been visited.
if (!CastInst->isCast())
return;
if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
// Pretend the constant is directly used by the instruction and ignore
// the cast instruction.
collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
return;
}
}
// Visit constant expressions that have constant integers.
if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
// 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::getIntImmCost`) 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) || isa<IntrinsicInst>(Inst)) {
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)
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;
if(!Entry->getParent()->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->getIntImmCost(Opcode, OpndIdx, Value, Ty);
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) {
auto MaxCostItr = S;
unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr);
// Don't hoist constants that have only one use.
if (NumUses <= 1)
return;
ConstantInfo ConstInfo;
ConstInfo.BaseConstant = MaxCostItr->ConstInt;
Type *Ty = ConstInfo.BaseConstant->getType();
// Rebase the constants with respect to the base constant.
for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
APInt Diff = ConstCand->ConstInt->getValue() -
ConstInfo.BaseConstant->getValue();
Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
ConstInfo.RebasedConstants.push_back(
RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
}
ConstantVec.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() {
// Sort the constants by value and type. This invalidates the mapping!
llvm::sort(ConstCandVec.begin(), ConstCandVec.end(),
[](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()) {
// 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()))
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);
// Start a new base constant search.
MinValItr = CC;
}
// Finalize the last base constant search.
findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
}
/// 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,
const ConstantUser &ConstUser) {
Instruction *Mat = Base;
if (Offset) {
Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
ConstUser.OpndIdx);
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)) {
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() {
bool MadeChange = false;
for (auto const &ConstInfo : ConstantVec) {
// Hoist and hide the base constant behind a bitcast.
SmallPtrSet<Instruction *, 8> IPSet = findConstantInsertionPoint(ConstInfo);
assert(!IPSet.empty() && "IPSet is empty");
unsigned UsesNum = 0;
unsigned ReBasesNum = 0;
for (Instruction *IP : IPSet) {
IntegerType *Ty = ConstInfo.BaseConstant->getType();
Instruction *Base =
new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
Base->setDebugLoc(IP->getDebugLoc());
LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant
<< ") to BB " << IP->getParent()->getName() << '\n'
<< *Base << '\n');
// Emit materialization code for all rebased constants.
unsigned Uses = 0;
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(Base->getParent(), OrigMatInsertBB)) {
emitBaseConstants(Base, RCI.Offset, U);
ReBasesNum++;
}
Base->setDebugLoc(DILocation::getMergedLocation(Base->getDebugLoc(), U.Inst->getDebugLoc()));
}
}
UsesNum = Uses;
// Use the same debug location as the last user of the constant.
assert(!Base->use_empty() && "The use list is empty!?");
assert(isa<Instruction>(Base->user_back()) &&
"All uses should be instructions.");
}
(void)UsesNum;
(void)ReBasesNum;
// Expect all uses are rebased after rebase is done.
assert(UsesNum == ReBasesNum && "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) {
this->TTI = &TTI;
this->DT = &DT;
this->BFI = BFI;
this->Entry = &Entry;
// Collect all constant candidates.
collectConstantCandidates(Fn);
// There are no constant candidates to worry about.
if (ConstCandVec.empty())
return false;
// Combine constants that can be easily materialized with an add from a common
// base constant.
findBaseConstants();
// There are no constants to emit.
if (ConstantVec.empty())
return false;
// Finally hoist the base constant and emit materialization code for dependent
// constants.
bool MadeChange = emitBaseConstants();
// Cleanup dead instructions.
deleteDeadCastInst();
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;
if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock()))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
return PA;
}
