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//===-- LoopSink.cpp - Loop Sink Pass -------------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass does the inverse transformation of what LICM does.
// It traverses all of the instructions in the loop's preheader and sinks
// them to the loop body where frequency is lower than the loop's preheader.
// This pass is a reverse-transformation of LICM. It differs from the Sink
// pass in the following ways:
// * It only handles sinking of instructions from the loop's preheader to the
// loop's body
// * It uses alias set tracker to get more accurate alias info
// * It uses block frequency info to find the optimal sinking locations
// Overall algorithm:
// For I in Preheader:
// InsertBBs = BBs that uses I
// For BB in sorted(LoopBBs):
// DomBBs = BBs in InsertBBs that are dominated by BB
// if freq(DomBBs) > freq(BB)
// InsertBBs = UseBBs - DomBBs + BB
// For BB in InsertBBs:
// Insert I at BB's beginning
#include "llvm/Transforms/Scalar/LoopSink.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loopsink"
STATISTIC(NumLoopSunk, "Number of instructions sunk into loop");
STATISTIC(NumLoopSunkCloned, "Number of cloned instructions sunk into loop");
static cl::opt<unsigned> SinkFrequencyPercentThreshold(
"sink-freq-percent-threshold", cl::Hidden, cl::init(90),
cl::desc("Do not sink instructions that require cloning unless they "
"execute less than this percent of the time."));
static cl::opt<unsigned> MaxNumberOfUseBBsForSinking(
"max-uses-for-sinking", cl::Hidden, cl::init(30),
cl::desc("Do not sink instructions that have too many uses."));
/// Return adjusted total frequency of \p BBs.
/// * If there is only one BB, sinking instruction will not introduce code
/// size increase. Thus there is no need to adjust the frequency.
/// * If there are more than one BB, sinking would lead to code size increase.
/// In this case, we add some "tax" to the total frequency to make it harder
/// to sink. E.g.
/// Freq(Preheader) = 100
/// Freq(BBs) = sum(50, 49) = 99
/// Even if Freq(BBs) < Freq(Preheader), we will not sink from Preheade to
/// BBs as the difference is too small to justify the code size increase.
/// To model this, The adjusted Freq(BBs) will be:
/// AdjustedFreq(BBs) = 99 / SinkFrequencyPercentThreshold%
static BlockFrequency adjustedSumFreq(SmallPtrSetImpl<BasicBlock *> &BBs,
BlockFrequencyInfo &BFI) {
BlockFrequency T = 0;
for (BasicBlock *B : BBs)
T += BFI.getBlockFreq(B);
if (BBs.size() > 1)
T /= BranchProbability(SinkFrequencyPercentThreshold, 100);
return T;
/// Return a set of basic blocks to insert sinked instructions.
/// The returned set of basic blocks (BBsToSinkInto) should satisfy:
/// * Inside the loop \p L
/// * For each UseBB in \p UseBBs, there is at least one BB in BBsToSinkInto
/// that domintates the UseBB
/// * Has minimum total frequency that is no greater than preheader frequency
/// The purpose of the function is to find the optimal sinking points to
/// minimize execution cost, which is defined as "sum of frequency of
/// BBsToSinkInto".
/// As a result, the returned BBsToSinkInto needs to have minimum total
/// frequency.
/// Additionally, if the total frequency of BBsToSinkInto exceeds preheader
/// frequency, the optimal solution is not sinking (return empty set).
/// \p ColdLoopBBs is used to help find the optimal sinking locations.
/// It stores a list of BBs that is:
/// * Inside the loop \p L
/// * Has a frequency no larger than the loop's preheader
/// * Sorted by BB frequency
/// The complexity of the function is O(UseBBs.size() * ColdLoopBBs.size()).
/// To avoid expensive computation, we cap the maximum UseBBs.size() in its
/// caller.
static SmallPtrSet<BasicBlock *, 2>
findBBsToSinkInto(const Loop &L, const SmallPtrSetImpl<BasicBlock *> &UseBBs,
const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
DominatorTree &DT, BlockFrequencyInfo &BFI) {
SmallPtrSet<BasicBlock *, 2> BBsToSinkInto;
if (UseBBs.size() == 0)
return BBsToSinkInto;
BBsToSinkInto.insert(UseBBs.begin(), UseBBs.end());
SmallPtrSet<BasicBlock *, 2> BBsDominatedByColdestBB;
// For every iteration:
// * Pick the ColdestBB from ColdLoopBBs
// * Find the set BBsDominatedByColdestBB that satisfy:
// - BBsDominatedByColdestBB is a subset of BBsToSinkInto
// - Every BB in BBsDominatedByColdestBB is dominated by ColdestBB
// * If Freq(ColdestBB) < Freq(BBsDominatedByColdestBB), remove
// BBsDominatedByColdestBB from BBsToSinkInto, add ColdestBB to
// BBsToSinkInto
for (BasicBlock *ColdestBB : ColdLoopBBs) {
for (BasicBlock *SinkedBB : BBsToSinkInto)
if (DT.dominates(ColdestBB, SinkedBB))
if (BBsDominatedByColdestBB.size() == 0)
if (adjustedSumFreq(BBsDominatedByColdestBB, BFI) >
BFI.getBlockFreq(ColdestBB)) {
for (BasicBlock *DominatedBB : BBsDominatedByColdestBB) {
// Can't sink into blocks that have no valid insertion point.
for (BasicBlock *BB : BBsToSinkInto) {
if (BB->getFirstInsertionPt() == BB->end()) {
// If the total frequency of BBsToSinkInto is larger than preheader frequency,
// do not sink.
if (adjustedSumFreq(BBsToSinkInto, BFI) >
return BBsToSinkInto;
// Sinks \p I from the loop \p L's preheader to its uses. Returns true if
// sinking is successful.
// \p LoopBlockNumber is used to sort the insertion blocks to ensure
// determinism.
static bool sinkInstruction(
Loop &L, Instruction &I, const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber, LoopInfo &LI,
DominatorTree &DT, BlockFrequencyInfo &BFI, MemorySSAUpdater *MSSAU) {
// Compute the set of blocks in loop L which contain a use of I.
SmallPtrSet<BasicBlock *, 2> BBs;
for (auto &U : I.uses()) {
Instruction *UI = cast<Instruction>(U.getUser());
// We cannot sink I to PHI-uses.
if (isa<PHINode>(UI))
return false;
// We cannot sink I if it has uses outside of the loop.
if (!L.contains(LI.getLoopFor(UI->getParent())))
return false;
// findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
// BBs.size() to avoid expensive computation.
// FIXME: Handle code size growth for min_size and opt_size.
if (BBs.size() > MaxNumberOfUseBBsForSinking)
return false;
// Find the set of BBs that we should insert a copy of I.
SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI);
if (BBsToSinkInto.empty())
return false;
// Return if any of the candidate blocks to sink into is non-cold.
if (BBsToSinkInto.size() > 1 &&
!llvm::set_is_subset(BBsToSinkInto, LoopBlockNumber))
return false;
// Copy the final BBs into a vector and sort them using the total ordering
// of the loop block numbers as iterating the set doesn't give a useful
// order. No need to stable sort as the block numbers are a total ordering.
SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
llvm::append_range(SortedBBsToSinkInto, BBsToSinkInto);
llvm::sort(SortedBBsToSinkInto, [&](BasicBlock *A, BasicBlock *B) {
return LoopBlockNumber.find(A)->second < LoopBlockNumber.find(B)->second;
BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
// FIXME: Optimize the efficiency for cloned value replacement. The current
// implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
for (BasicBlock *N : ArrayRef(SortedBBsToSinkInto).drop_front(1)) {
assert(LoopBlockNumber.find(N)->second >
LoopBlockNumber.find(MoveBB)->second &&
"BBs not sorted!");
// Clone I and replace its uses.
Instruction *IC = I.clone();
if (MSSAU && MSSAU->getMemorySSA()->getMemoryAccess(&I)) {
// Create a new MemoryAccess and let MemorySSA set its defining access.
MemoryAccess *NewMemAcc =
MSSAU->createMemoryAccessInBB(IC, nullptr, N, MemorySSA::Beginning);
if (NewMemAcc) {
if (auto *MemDef = dyn_cast<MemoryDef>(NewMemAcc))
MSSAU->insertDef(MemDef, /*RenameUses=*/true);
else {
auto *MemUse = cast<MemoryUse>(NewMemAcc);
MSSAU->insertUse(MemUse, /*RenameUses=*/true);
// Replaces uses of I with IC in N
I.replaceUsesWithIf(IC, [N](Use &U) {
return cast<Instruction>(U.getUser())->getParent() == N;
// Replaces uses of I with IC in blocks dominated by N
replaceDominatedUsesWith(&I, IC, DT, N);
LLVM_DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
<< '\n');
LLVM_DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
if (MSSAU)
if (MemoryUseOrDef *OldMemAcc = cast_or_null<MemoryUseOrDef>(
MSSAU->moveToPlace(OldMemAcc, MoveBB, MemorySSA::Beginning);
return true;
/// Sinks instructions from loop's preheader to the loop body if the
/// sum frequency of inserted copy is smaller than preheader's frequency.
static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
DominatorTree &DT,
BlockFrequencyInfo &BFI,
MemorySSA &MSSA,
ScalarEvolution *SE) {
BasicBlock *Preheader = L.getLoopPreheader();
assert(Preheader && "Expected loop to have preheader");
assert(Preheader->getParent()->hasProfileData() &&
"Unexpected call when profile data unavailable.");
const BlockFrequency PreheaderFreq = BFI.getBlockFreq(Preheader);
// If there are no basic blocks with lower frequency than the preheader then
// we can avoid the detailed analysis as we will never find profitable sinking
// opportunities.
if (all_of(L.blocks(), [&](const BasicBlock *BB) {
return BFI.getBlockFreq(BB) > PreheaderFreq;
return false;
MemorySSAUpdater MSSAU(&MSSA);
SinkAndHoistLICMFlags LICMFlags(/*IsSink=*/true, &L, &MSSA);
bool Changed = false;
// Sort loop's basic blocks by frequency
SmallVector<BasicBlock *, 10> ColdLoopBBs;
SmallDenseMap<BasicBlock *, int, 16> LoopBlockNumber;
int i = 0;
for (BasicBlock *B : L.blocks())
if (BFI.getBlockFreq(B) < BFI.getBlockFreq(L.getLoopPreheader())) {
LoopBlockNumber[B] = ++i;
llvm::stable_sort(ColdLoopBBs, [&](BasicBlock *A, BasicBlock *B) {
return BFI.getBlockFreq(A) < BFI.getBlockFreq(B);
// Traverse preheader's instructions in reverse order because if A depends
// on B (A appears after B), A needs to be sunk first before B can be
// sinked.
for (Instruction &I : llvm::make_early_inc_range(llvm::reverse(*Preheader))) {
if (isa<PHINode>(&I))
// No need to check for instruction's operands are loop invariant.
assert(L.hasLoopInvariantOperands(&I) &&
"Insts in a loop's preheader should have loop invariant operands!");
if (!canSinkOrHoistInst(I, &AA, &DT, &L, MSSAU, false, LICMFlags))
if (sinkInstruction(L, I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI,
&MSSAU)) {
Changed = true;
if (SE)
return Changed;
PreservedAnalyses LoopSinkPass::run(Function &F, FunctionAnalysisManager &FAM) {
LoopInfo &LI = FAM.getResult<LoopAnalysis>(F);
// Nothing to do if there are no loops.
if (LI.empty())
return PreservedAnalyses::all();
AAResults &AA = FAM.getResult<AAManager>(F);
DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
BlockFrequencyInfo &BFI = FAM.getResult<BlockFrequencyAnalysis>(F);
MemorySSA &MSSA = FAM.getResult<MemorySSAAnalysis>(F).getMSSA();
// We want to do a postorder walk over the loops. Since loops are a tree this
// is equivalent to a reversed preorder walk and preorder is easy to compute
// without recursion. Since we reverse the preorder, we will visit siblings
// in reverse program order. This isn't expected to matter at all but is more
// consistent with sinking algorithms which generally work bottom-up.
SmallVector<Loop *, 4> PreorderLoops = LI.getLoopsInPreorder();
bool Changed = false;
do {
Loop &L = *PreorderLoops.pop_back_val();
BasicBlock *Preheader = L.getLoopPreheader();
if (!Preheader)
// Enable LoopSink only when runtime profile is available.
// With static profile, the sinking decision may be sub-optimal.
if (!Preheader->getParent()->hasProfileData())
// Note that we don't pass SCEV here because it is only used to invalidate
// loops in SCEV and we don't preserve (or request) SCEV at all making that
// unnecessary.
Changed |= sinkLoopInvariantInstructions(L, AA, LI, DT, BFI, MSSA,
/*ScalarEvolution*/ nullptr);
} while (!PreorderLoops.empty());
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
if (VerifyMemorySSA)
return PA;
namespace {
struct LegacyLoopSinkPass : public LoopPass {
static char ID;
LegacyLoopSinkPass() : LoopPass(ID) {
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader)
return false;
// Enable LoopSink only when runtime profile is available.
// With static profile, the sinking decision may be sub-optimal.
if (!Preheader->getParent()->hasProfileData())
return false;
AAResults &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
bool Changed = sinkLoopInvariantInstructions(
*L, AA, getAnalysis<LoopInfoWrapperPass>().getLoopInfo(),
MSSA, SE ? &SE->getSE() : nullptr);
if (VerifyMemorySSA)
return Changed;
void getAnalysisUsage(AnalysisUsage &AU) const override {
char LegacyLoopSinkPass::ID = 0;
INITIALIZE_PASS_BEGIN(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false,
INITIALIZE_PASS_END(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false, false)
Pass *llvm::createLoopSinkPass() { return new LegacyLoopSinkPass(); }