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llvm-project/llvm/lib/Transforms/Scalar/MergeICmps.cpp

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//===- MergeICmps.cpp - Optimize chains of integer comparisons ------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass turns chains of integer comparisons into memcmp (the memcmp is
// later typically inlined as a chain of efficient hardware comparisons). This
// typically benefits c++ member or nonmember operator==().
//
// The basic idea is to replace a longer chain of integer comparisons loaded
// from contiguous memory locations into a shorter chain of larger integer
// comparisons. Benefits are double:
// - There are less jumps, and therefore less opportunities for mispredictions
// and I-cache misses.
// - Code size is smaller, both because jumps are removed and because the
// encoding of a 2*n byte compare is smaller than that of two n-byte
// compares.
//
// Example:
//
// struct S {
// int a;
// char b;
// char c;
// uint16_t d;
// bool operator==(const S& o) const {
// return a == o.a && b == o.b && c == o.c && d == o.d;
// }
// };
//
// Is optimized as :
//
// bool S::operator==(const S& o) const {
// return memcmp(this, &o, 8) == 0;
// }
//
// Which will later be expanded (ExpandMemCmp) as a single 8-bytes icmp.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/MergeICmps.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include <algorithm>
#include <numeric>
#include <utility>
#include <vector>
using namespace llvm;
namespace {
#define DEBUG_TYPE "mergeicmps"
// Returns true if the instruction is a simple load or a simple store
static bool isSimpleLoadOrStore(const Instruction *I) {
if (const LoadInst *LI = dyn_cast<LoadInst>(I))
return LI->isSimple();
if (const StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isSimple();
return false;
}
// A BCE atom "Binary Compare Expression Atom" represents an integer load
// that is a constant offset from a base value, e.g. `a` or `o.c` in the example
// at the top.
struct BCEAtom {
BCEAtom() = default;
BCEAtom(GetElementPtrInst *GEP, LoadInst *LoadI, int BaseId, APInt Offset)
: GEP(GEP), LoadI(LoadI), BaseId(BaseId), Offset(Offset) {}
BCEAtom(const BCEAtom &) = delete;
BCEAtom &operator=(const BCEAtom &) = delete;
BCEAtom(BCEAtom &&that) = default;
BCEAtom &operator=(BCEAtom &&that) {
if (this == &that)
return *this;
GEP = that.GEP;
LoadI = that.LoadI;
BaseId = that.BaseId;
Offset = std::move(that.Offset);
return *this;
}
// We want to order BCEAtoms by (Base, Offset). However we cannot use
// the pointer values for Base because these are non-deterministic.
// To make sure that the sort order is stable, we first assign to each atom
// base value an index based on its order of appearance in the chain of
// comparisons. We call this index `BaseOrdering`. For example, for:
// b[3] == c[2] && a[1] == d[1] && b[4] == c[3]
// | block 1 | | block 2 | | block 3 |
// b gets assigned index 0 and a index 1, because b appears as LHS in block 1,
// which is before block 2.
// We then sort by (BaseOrdering[LHS.Base()], LHS.Offset), which is stable.
bool operator<(const BCEAtom &O) const {
return BaseId != O.BaseId ? BaseId < O.BaseId : Offset.slt(O.Offset);
}
GetElementPtrInst *GEP = nullptr;
LoadInst *LoadI = nullptr;
unsigned BaseId = 0;
APInt Offset;
};
// A class that assigns increasing ids to values in the order in which they are
// seen. See comment in `BCEAtom::operator<()``.
class BaseIdentifier {
public:
// Returns the id for value `Base`, after assigning one if `Base` has not been
// seen before.
int getBaseId(const Value *Base) {
assert(Base && "invalid base");
const auto Insertion = BaseToIndex.try_emplace(Base, Order);
if (Insertion.second)
++Order;
return Insertion.first->second;
}
private:
unsigned Order = 1;
DenseMap<const Value*, int> BaseToIndex;
};
// If this value is a load from a constant offset w.r.t. a base address, and
// there are no other users of the load or address, returns the base address and
// the offset.
BCEAtom visitICmpLoadOperand(Value *const Val, BaseIdentifier &BaseId) {
auto *const LoadI = dyn_cast<LoadInst>(Val);
if (!LoadI)
return {};
LLVM_DEBUG(dbgs() << "load\n");
if (LoadI->isUsedOutsideOfBlock(LoadI->getParent())) {
LLVM_DEBUG(dbgs() << "used outside of block\n");
return {};
}
// Do not optimize atomic loads to non-atomic memcmp
if (!LoadI->isSimple()) {
LLVM_DEBUG(dbgs() << "volatile or atomic\n");
return {};
}
Value *const Addr = LoadI->getOperand(0);
auto *const GEP = dyn_cast<GetElementPtrInst>(Addr);
if (!GEP)
return {};
LLVM_DEBUG(dbgs() << "GEP\n");
if (GEP->isUsedOutsideOfBlock(LoadI->getParent())) {
LLVM_DEBUG(dbgs() << "used outside of block\n");
return {};
}
const auto &DL = GEP->getModule()->getDataLayout();
if (!isDereferenceablePointer(GEP, LoadI->getType(), DL)) {
LLVM_DEBUG(dbgs() << "not dereferenceable\n");
// We need to make sure that we can do comparison in any order, so we
// require memory to be unconditionnally dereferencable.
return {};
}
APInt Offset = APInt(DL.getPointerTypeSizeInBits(GEP->getType()), 0);
if (!GEP->accumulateConstantOffset(DL, Offset))
return {};
return BCEAtom(GEP, LoadI, BaseId.getBaseId(GEP->getPointerOperand()),
Offset);
}
// A basic block with a comparison between two BCE atoms, e.g. `a == o.a` in the
// example at the top.
// The block might do extra work besides the atom comparison, in which case
// doesOtherWork() returns true. Under some conditions, the block can be
// split into the atom comparison part and the "other work" part
// (see canSplit()).
// Note: the terminology is misleading: the comparison is symmetric, so there
// is no real {l/r}hs. What we want though is to have the same base on the
// left (resp. right), so that we can detect consecutive loads. To ensure this
// we put the smallest atom on the left.
class BCECmpBlock {
public:
BCECmpBlock() {}
BCECmpBlock(BCEAtom L, BCEAtom R, int SizeBits)
: Lhs_(std::move(L)), Rhs_(std::move(R)), SizeBits_(SizeBits) {
if (Rhs_ < Lhs_) std::swap(Rhs_, Lhs_);
}
bool IsValid() const { return Lhs_.BaseId != 0 && Rhs_.BaseId != 0; }
// Assert the block is consistent: If valid, it should also have
// non-null members besides Lhs_ and Rhs_.
void AssertConsistent() const {
if (IsValid()) {
assert(BB);
assert(CmpI);
assert(BranchI);
}
}
const BCEAtom &Lhs() const { return Lhs_; }
const BCEAtom &Rhs() const { return Rhs_; }
int SizeBits() const { return SizeBits_; }
// Returns true if the block does other works besides comparison.
bool doesOtherWork() const;
// Returns true if the non-BCE-cmp instructions can be separated from BCE-cmp
// instructions in the block.
bool canSplit(AliasAnalysis &AA) const;
// Return true if this all the relevant instructions in the BCE-cmp-block can
// be sunk below this instruction. By doing this, we know we can separate the
// BCE-cmp-block instructions from the non-BCE-cmp-block instructions in the
// block.
bool canSinkBCECmpInst(const Instruction *, DenseSet<Instruction *> &,
AliasAnalysis &AA) const;
// We can separate the BCE-cmp-block instructions and the non-BCE-cmp-block
// instructions. Split the old block and move all non-BCE-cmp-insts into the
// new parent block.
void split(BasicBlock *NewParent, AliasAnalysis &AA) const;
// The basic block where this comparison happens.
BasicBlock *BB = nullptr;
// The ICMP for this comparison.
ICmpInst *CmpI = nullptr;
// The terminating branch.
BranchInst *BranchI = nullptr;
// The block requires splitting.
bool RequireSplit = false;
private:
BCEAtom Lhs_;
BCEAtom Rhs_;
int SizeBits_ = 0;
};
bool BCECmpBlock::canSinkBCECmpInst(const Instruction *Inst,
DenseSet<Instruction *> &BlockInsts,
AliasAnalysis &AA) const {
// If this instruction has side effects and its in middle of the BCE cmp block
// instructions, then bail for now.
if (Inst->mayHaveSideEffects()) {
// Bail if this is not a simple load or store
if (!isSimpleLoadOrStore(Inst))
return false;
// Disallow stores that might alias the BCE operands
MemoryLocation LLoc = MemoryLocation::get(Lhs_.LoadI);
MemoryLocation RLoc = MemoryLocation::get(Rhs_.LoadI);
if (isModSet(AA.getModRefInfo(Inst, LLoc)) ||
isModSet(AA.getModRefInfo(Inst, RLoc)))
return false;
}
// Make sure this instruction does not use any of the BCE cmp block
// instructions as operand.
for (auto BI : BlockInsts) {
if (is_contained(Inst->operands(), BI))
return false;
}
return true;
}
void BCECmpBlock::split(BasicBlock *NewParent, AliasAnalysis &AA) const {
DenseSet<Instruction *> BlockInsts(
{Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
llvm::SmallVector<Instruction *, 4> OtherInsts;
for (Instruction &Inst : *BB) {
if (BlockInsts.count(&Inst))
continue;
assert(canSinkBCECmpInst(&Inst, BlockInsts, AA) &&
"Split unsplittable block");
// This is a non-BCE-cmp-block instruction. And it can be separated
// from the BCE-cmp-block instruction.
OtherInsts.push_back(&Inst);
}
// Do the actual spliting.
for (Instruction *Inst : reverse(OtherInsts)) {
Inst->moveBefore(&*NewParent->begin());
}
}
bool BCECmpBlock::canSplit(AliasAnalysis &AA) const {
DenseSet<Instruction *> BlockInsts(
{Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
for (Instruction &Inst : *BB) {
if (!BlockInsts.count(&Inst)) {
if (!canSinkBCECmpInst(&Inst, BlockInsts, AA))
return false;
}
}
return true;
}
bool BCECmpBlock::doesOtherWork() const {
AssertConsistent();
// All the instructions we care about in the BCE cmp block.
DenseSet<Instruction *> BlockInsts(
{Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
// TODO(courbet): Can we allow some other things ? This is very conservative.
// We might be able to get away with anything does not have any side
// effects outside of the basic block.
// Note: The GEPs and/or loads are not necessarily in the same block.
for (const Instruction &Inst : *BB) {
if (!BlockInsts.count(&Inst))
return true;
}
return false;
}
// Visit the given comparison. If this is a comparison between two valid
// BCE atoms, returns the comparison.
BCECmpBlock visitICmp(const ICmpInst *const CmpI,
const ICmpInst::Predicate ExpectedPredicate,
BaseIdentifier &BaseId) {
// The comparison can only be used once:
// - For intermediate blocks, as a branch condition.
// - For the final block, as an incoming value for the Phi.
// If there are any other uses of the comparison, we cannot merge it with
// other comparisons as we would create an orphan use of the value.
if (!CmpI->hasOneUse()) {
LLVM_DEBUG(dbgs() << "cmp has several uses\n");
return {};
}
if (CmpI->getPredicate() != ExpectedPredicate)
return {};
LLVM_DEBUG(dbgs() << "cmp "
<< (ExpectedPredicate == ICmpInst::ICMP_EQ ? "eq" : "ne")
<< "\n");
auto Lhs = visitICmpLoadOperand(CmpI->getOperand(0), BaseId);
if (!Lhs.BaseId)
return {};
auto Rhs = visitICmpLoadOperand(CmpI->getOperand(1), BaseId);
if (!Rhs.BaseId)
return {};
const auto &DL = CmpI->getModule()->getDataLayout();
return BCECmpBlock(std::move(Lhs), std::move(Rhs),
DL.getTypeSizeInBits(CmpI->getOperand(0)->getType()));
}
// Visit the given comparison block. If this is a comparison between two valid
// BCE atoms, returns the comparison.
BCECmpBlock visitCmpBlock(Value *const Val, BasicBlock *const Block,
const BasicBlock *const PhiBlock,
BaseIdentifier &BaseId) {
if (Block->empty()) return {};
auto *const BranchI = dyn_cast<BranchInst>(Block->getTerminator());
if (!BranchI) return {};
LLVM_DEBUG(dbgs() << "branch\n");
if (BranchI->isUnconditional()) {
// In this case, we expect an incoming value which is the result of the
// comparison. This is the last link in the chain of comparisons (note
// that this does not mean that this is the last incoming value, blocks
// can be reordered).
auto *const CmpI = dyn_cast<ICmpInst>(Val);
if (!CmpI) return {};
LLVM_DEBUG(dbgs() << "icmp\n");
auto Result = visitICmp(CmpI, ICmpInst::ICMP_EQ, BaseId);
Result.CmpI = CmpI;
Result.BranchI = BranchI;
return Result;
} else {
// In this case, we expect a constant incoming value (the comparison is
// chained).
const auto *const Const = cast<ConstantInt>(Val);
LLVM_DEBUG(dbgs() << "const\n");
if (!Const->isZero()) return {};
LLVM_DEBUG(dbgs() << "false\n");
auto *const CmpI = dyn_cast<ICmpInst>(BranchI->getCondition());
if (!CmpI) return {};
LLVM_DEBUG(dbgs() << "icmp\n");
assert(BranchI->getNumSuccessors() == 2 && "expecting a cond branch");
BasicBlock *const FalseBlock = BranchI->getSuccessor(1);
auto Result = visitICmp(
CmpI, FalseBlock == PhiBlock ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE,
BaseId);
Result.CmpI = CmpI;
Result.BranchI = BranchI;
return Result;
}
return {};
}
static inline void enqueueBlock(std::vector<BCECmpBlock> &Comparisons,
BCECmpBlock &&Comparison) {
LLVM_DEBUG(dbgs() << "Block '" << Comparison.BB->getName()
<< "': Found cmp of " << Comparison.SizeBits()
<< " bits between " << Comparison.Lhs().BaseId << " + "
<< Comparison.Lhs().Offset << " and "
<< Comparison.Rhs().BaseId << " + "
<< Comparison.Rhs().Offset << "\n");
LLVM_DEBUG(dbgs() << "\n");
Comparisons.push_back(std::move(Comparison));
}
// A chain of comparisons.
class BCECmpChain {
public:
BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi,
AliasAnalysis &AA);
int size() const { return Comparisons_.size(); }
#ifdef MERGEICMPS_DOT_ON
void dump() const;
#endif // MERGEICMPS_DOT_ON
bool simplify(const TargetLibraryInfo &TLI, AliasAnalysis &AA,
DomTreeUpdater &DTU);
private:
static bool IsContiguous(const BCECmpBlock &First,
const BCECmpBlock &Second) {
return First.Lhs().BaseId == Second.Lhs().BaseId &&
First.Rhs().BaseId == Second.Rhs().BaseId &&
First.Lhs().Offset + First.SizeBits() / 8 == Second.Lhs().Offset &&
First.Rhs().Offset + First.SizeBits() / 8 == Second.Rhs().Offset;
}
PHINode &Phi_;
std::vector<BCECmpBlock> Comparisons_;
// The original entry block (before sorting);
BasicBlock *EntryBlock_;
};
BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi,
AliasAnalysis &AA)
: Phi_(Phi) {
assert(!Blocks.empty() && "a chain should have at least one block");
// Now look inside blocks to check for BCE comparisons.
std::vector<BCECmpBlock> Comparisons;
BaseIdentifier BaseId;
for (size_t BlockIdx = 0; BlockIdx < Blocks.size(); ++BlockIdx) {
BasicBlock *const Block = Blocks[BlockIdx];
assert(Block && "invalid block");
BCECmpBlock Comparison = visitCmpBlock(Phi.getIncomingValueForBlock(Block),
Block, Phi.getParent(), BaseId);
Comparison.BB = Block;
if (!Comparison.IsValid()) {
LLVM_DEBUG(dbgs() << "chain with invalid BCECmpBlock, no merge.\n");
return;
}
if (Comparison.doesOtherWork()) {
LLVM_DEBUG(dbgs() << "block '" << Comparison.BB->getName()
<< "' does extra work besides compare\n");
if (Comparisons.empty()) {
// This is the initial block in the chain, in case this block does other
// work, we can try to split the block and move the irrelevant
// instructions to the predecessor.
//
// If this is not the initial block in the chain, splitting it wont
// work.
//
// As once split, there will still be instructions before the BCE cmp
// instructions that do other work in program order, i.e. within the
// chain before sorting. Unless we can abort the chain at this point
// and start anew.
//
// NOTE: we only handle blocks a with single predecessor for now.
if (Comparison.canSplit(AA)) {
LLVM_DEBUG(dbgs()
<< "Split initial block '" << Comparison.BB->getName()
<< "' that does extra work besides compare\n");
Comparison.RequireSplit = true;
enqueueBlock(Comparisons, std::move(Comparison));
} else {
LLVM_DEBUG(dbgs()
<< "ignoring initial block '" << Comparison.BB->getName()
<< "' that does extra work besides compare\n");
}
continue;
}
// TODO(courbet): Right now we abort the whole chain. We could be
// merging only the blocks that don't do other work and resume the
// chain from there. For example:
// if (a[0] == b[0]) { // bb1
// if (a[1] == b[1]) { // bb2
// some_value = 3; //bb3
// if (a[2] == b[2]) { //bb3
// do a ton of stuff //bb4
// }
// }
// }
//
// This is:
//
// bb1 --eq--> bb2 --eq--> bb3* -eq--> bb4 --+
// \ \ \ \
// ne ne ne \
// \ \ \ v
// +------------+-----------+----------> bb_phi
//
// We can only merge the first two comparisons, because bb3* does
// "other work" (setting some_value to 3).
// We could still merge bb1 and bb2 though.
return;
}
enqueueBlock(Comparisons, std::move(Comparison));
}
// It is possible we have no suitable comparison to merge.
if (Comparisons.empty()) {
LLVM_DEBUG(dbgs() << "chain with no BCE basic blocks, no merge\n");
return;
}
EntryBlock_ = Comparisons[0].BB;
Comparisons_ = std::move(Comparisons);
#ifdef MERGEICMPS_DOT_ON
errs() << "BEFORE REORDERING:\n\n";
dump();
#endif // MERGEICMPS_DOT_ON
// Reorder blocks by LHS. We can do that without changing the
// semantics because we are only accessing dereferencable memory.
llvm::sort(Comparisons_,
[](const BCECmpBlock &LhsBlock, const BCECmpBlock &RhsBlock) {
return std::tie(LhsBlock.Lhs(), LhsBlock.Rhs()) <
std::tie(RhsBlock.Lhs(), RhsBlock.Rhs());
});
#ifdef MERGEICMPS_DOT_ON
errs() << "AFTER REORDERING:\n\n";
dump();
#endif // MERGEICMPS_DOT_ON
}
#ifdef MERGEICMPS_DOT_ON
void BCECmpChain::dump() const {
errs() << "digraph dag {\n";
errs() << " graph [bgcolor=transparent];\n";
errs() << " node [color=black,style=filled,fillcolor=lightyellow];\n";
errs() << " edge [color=black];\n";
for (size_t I = 0; I < Comparisons_.size(); ++I) {
const auto &Comparison = Comparisons_[I];
errs() << " \"" << I << "\" [label=\"%"
<< Comparison.Lhs().Base()->getName() << " + "
<< Comparison.Lhs().Offset << " == %"
<< Comparison.Rhs().Base()->getName() << " + "
<< Comparison.Rhs().Offset << " (" << (Comparison.SizeBits() / 8)
<< " bytes)\"];\n";
const Value *const Val = Phi_.getIncomingValueForBlock(Comparison.BB);
if (I > 0) errs() << " \"" << (I - 1) << "\" -> \"" << I << "\";\n";
errs() << " \"" << I << "\" -> \"Phi\" [label=\"" << *Val << "\"];\n";
}
errs() << " \"Phi\" [label=\"Phi\"];\n";
errs() << "}\n\n";
}
#endif // MERGEICMPS_DOT_ON
namespace {
// A class to compute the name of a set of merged basic blocks.
// This is optimized for the common case of no block names.
class MergedBlockName {
// Storage for the uncommon case of several named blocks.
SmallString<16> Scratch;
public:
explicit MergedBlockName(ArrayRef<BCECmpBlock> Comparisons)
: Name(makeName(Comparisons)) {}
const StringRef Name;
private:
StringRef makeName(ArrayRef<BCECmpBlock> Comparisons) {
assert(!Comparisons.empty() && "no basic block");
// Fast path: only one block, or no names at all.
if (Comparisons.size() == 1)
return Comparisons[0].BB->getName();
const int size = std::accumulate(Comparisons.begin(), Comparisons.end(), 0,
[](int i, const BCECmpBlock &Cmp) {
return i + Cmp.BB->getName().size();
});
if (size == 0)
return StringRef("", 0);
// Slow path: at least two blocks, at least one block with a name.
Scratch.clear();
// We'll have `size` bytes for name and `Comparisons.size() - 1` bytes for
// separators.
Scratch.reserve(size + Comparisons.size() - 1);
const auto append = [this](StringRef str) {
Scratch.append(str.begin(), str.end());
};
append(Comparisons[0].BB->getName());
for (int I = 1, E = Comparisons.size(); I < E; ++I) {
const BasicBlock *const BB = Comparisons[I].BB;
if (!BB->getName().empty()) {
append("+");
append(BB->getName());
}
}
return StringRef(Scratch);
}
};
} // namespace
// Merges the given contiguous comparison blocks into one memcmp block.
static BasicBlock *mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
BasicBlock *const InsertBefore,
BasicBlock *const NextCmpBlock,
PHINode &Phi, const TargetLibraryInfo &TLI,
AliasAnalysis &AA, DomTreeUpdater &DTU) {
assert(!Comparisons.empty() && "merging zero comparisons");
LLVMContext &Context = NextCmpBlock->getContext();
const BCECmpBlock &FirstCmp = Comparisons[0];
// Create a new cmp block before next cmp block.
BasicBlock *const BB =
BasicBlock::Create(Context, MergedBlockName(Comparisons).Name,
NextCmpBlock->getParent(), InsertBefore);
IRBuilder<> Builder(BB);
// Add the GEPs from the first BCECmpBlock.
Value *const Lhs = Builder.Insert(FirstCmp.Lhs().GEP->clone());
Value *const Rhs = Builder.Insert(FirstCmp.Rhs().GEP->clone());
Value *IsEqual = nullptr;
LLVM_DEBUG(dbgs() << "Merging " << Comparisons.size() << " comparisons -> "
<< BB->getName() << "\n");
// If there is one block that requires splitting, we do it now, i.e.
// just before we know we will collapse the chain. The instructions
// can be executed before any of the instructions in the chain.
const auto ToSplit = llvm::find_if(
Comparisons, [](const BCECmpBlock &B) { return B.RequireSplit; });
if (ToSplit != Comparisons.end()) {
LLVM_DEBUG(dbgs() << "Splitting non_BCE work to header\n");
ToSplit->split(BB, AA);
}
if (Comparisons.size() == 1) {
LLVM_DEBUG(dbgs() << "Only one comparison, updating branches\n");
Value *const LhsLoad =
Builder.CreateLoad(FirstCmp.Lhs().LoadI->getType(), Lhs);
Value *const RhsLoad =
Builder.CreateLoad(FirstCmp.Rhs().LoadI->getType(), Rhs);
// There are no blocks to merge, just do the comparison.
IsEqual = Builder.CreateICmpEQ(LhsLoad, RhsLoad);
} else {
const unsigned TotalSizeBits = std::accumulate(
Comparisons.begin(), Comparisons.end(), 0u,
[](int Size, const BCECmpBlock &C) { return Size + C.SizeBits(); });
// Create memcmp() == 0.
const auto &DL = Phi.getModule()->getDataLayout();
Value *const MemCmpCall = emitMemCmp(
Lhs, Rhs,
ConstantInt::get(DL.getIntPtrType(Context), TotalSizeBits / 8), Builder,
DL, &TLI);
IsEqual = Builder.CreateICmpEQ(
MemCmpCall, ConstantInt::get(Type::getInt32Ty(Context), 0));
}
BasicBlock *const PhiBB = Phi.getParent();
// Add a branch to the next basic block in the chain.
if (NextCmpBlock == PhiBB) {
// Continue to phi, passing it the comparison result.
Builder.CreateBr(PhiBB);
Phi.addIncoming(IsEqual, BB);
DTU.applyUpdates({{DominatorTree::Insert, BB, PhiBB}});
} else {
// Continue to next block if equal, exit to phi else.
Builder.CreateCondBr(IsEqual, NextCmpBlock, PhiBB);
Phi.addIncoming(ConstantInt::getFalse(Context), BB);
DTU.applyUpdates({{DominatorTree::Insert, BB, NextCmpBlock},
{DominatorTree::Insert, BB, PhiBB}});
}
return BB;
}
bool BCECmpChain::simplify(const TargetLibraryInfo &TLI, AliasAnalysis &AA,
DomTreeUpdater &DTU) {
assert(Comparisons_.size() >= 2 && "simplifying trivial BCECmpChain");
// First pass to check if there is at least one merge. If not, we don't do
// anything and we keep analysis passes intact.
const auto AtLeastOneMerged = [this]() {
for (size_t I = 1; I < Comparisons_.size(); ++I) {
if (IsContiguous(Comparisons_[I - 1], Comparisons_[I]))
return true;
}
return false;
};
if (!AtLeastOneMerged())
return false;
LLVM_DEBUG(dbgs() << "Simplifying comparison chain starting at block "
<< EntryBlock_->getName() << "\n");
// Effectively merge blocks. We go in the reverse direction from the phi block
// so that the next block is always available to branch to.
const auto mergeRange = [this, &TLI, &AA, &DTU](int I, int Num,
BasicBlock *InsertBefore,
BasicBlock *Next) {
return mergeComparisons(makeArrayRef(Comparisons_).slice(I, Num),
InsertBefore, Next, Phi_, TLI, AA, DTU);
};
int NumMerged = 1;
BasicBlock *NextCmpBlock = Phi_.getParent();
for (int I = static_cast<int>(Comparisons_.size()) - 2; I >= 0; --I) {
if (IsContiguous(Comparisons_[I], Comparisons_[I + 1])) {
LLVM_DEBUG(dbgs() << "Merging block " << Comparisons_[I].BB->getName()
<< " into " << Comparisons_[I + 1].BB->getName()
<< "\n");
++NumMerged;
} else {
NextCmpBlock = mergeRange(I + 1, NumMerged, NextCmpBlock, NextCmpBlock);
NumMerged = 1;
}
}
// Insert the entry block for the new chain before the old entry block.
// If the old entry block was the function entry, this ensures that the new
// entry can become the function entry.
NextCmpBlock = mergeRange(0, NumMerged, EntryBlock_, NextCmpBlock);
// Replace the original cmp chain with the new cmp chain by pointing all
// predecessors of EntryBlock_ to NextCmpBlock instead. This makes all cmp
// blocks in the old chain unreachable.
while (!pred_empty(EntryBlock_)) {
BasicBlock* const Pred = *pred_begin(EntryBlock_);
LLVM_DEBUG(dbgs() << "Updating jump into old chain from " << Pred->getName()
<< "\n");
Pred->getTerminator()->replaceUsesOfWith(EntryBlock_, NextCmpBlock);
DTU.applyUpdates({{DominatorTree::Delete, Pred, EntryBlock_},
{DominatorTree::Insert, Pred, NextCmpBlock}});
}
// If the old cmp chain was the function entry, we need to update the function
// entry.
const bool ChainEntryIsFnEntry =
(EntryBlock_ == &EntryBlock_->getParent()->getEntryBlock());
if (ChainEntryIsFnEntry && DTU.hasDomTree()) {
LLVM_DEBUG(dbgs() << "Changing function entry from "
<< EntryBlock_->getName() << " to "
<< NextCmpBlock->getName() << "\n");
DTU.getDomTree().setNewRoot(NextCmpBlock);
DTU.applyUpdates({{DominatorTree::Delete, NextCmpBlock, EntryBlock_}});
}
EntryBlock_ = nullptr;
// Delete merged blocks. This also removes incoming values in phi.
SmallVector<BasicBlock *, 16> DeadBlocks;
for (auto &Cmp : Comparisons_) {
LLVM_DEBUG(dbgs() << "Deleting merged block " << Cmp.BB->getName() << "\n");
DeadBlocks.push_back(Cmp.BB);
}
DeleteDeadBlocks(DeadBlocks, &DTU);
Comparisons_.clear();
return true;
}
std::vector<BasicBlock *> getOrderedBlocks(PHINode &Phi,
BasicBlock *const LastBlock,
int NumBlocks) {
// Walk up from the last block to find other blocks.
std::vector<BasicBlock *> Blocks(NumBlocks);
assert(LastBlock && "invalid last block");
BasicBlock *CurBlock = LastBlock;
for (int BlockIndex = NumBlocks - 1; BlockIndex > 0; --BlockIndex) {
if (CurBlock->hasAddressTaken()) {
// Somebody is jumping to the block through an address, all bets are
// off.
LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
<< " has its address taken\n");
return {};
}
Blocks[BlockIndex] = CurBlock;
auto *SinglePredecessor = CurBlock->getSinglePredecessor();
if (!SinglePredecessor) {
// The block has two or more predecessors.
LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
<< " has two or more predecessors\n");
return {};
}
if (Phi.getBasicBlockIndex(SinglePredecessor) < 0) {
// The block does not link back to the phi.
LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
<< " does not link back to the phi\n");
return {};
}
CurBlock = SinglePredecessor;
}
Blocks[0] = CurBlock;
return Blocks;
}
bool processPhi(PHINode &Phi, const TargetLibraryInfo &TLI, AliasAnalysis &AA,
DomTreeUpdater &DTU) {
LLVM_DEBUG(dbgs() << "processPhi()\n");
if (Phi.getNumIncomingValues() <= 1) {
LLVM_DEBUG(dbgs() << "skip: only one incoming value in phi\n");
return false;
}
// We are looking for something that has the following structure:
// bb1 --eq--> bb2 --eq--> bb3 --eq--> bb4 --+
// \ \ \ \
// ne ne ne \
// \ \ \ v
// +------------+-----------+----------> bb_phi
//
// - The last basic block (bb4 here) must branch unconditionally to bb_phi.
// It's the only block that contributes a non-constant value to the Phi.
// - All other blocks (b1, b2, b3) must have exactly two successors, one of
// them being the phi block.
// - All intermediate blocks (bb2, bb3) must have only one predecessor.
// - Blocks cannot do other work besides the comparison, see doesOtherWork()
// The blocks are not necessarily ordered in the phi, so we start from the
// last block and reconstruct the order.
BasicBlock *LastBlock = nullptr;
for (unsigned I = 0; I < Phi.getNumIncomingValues(); ++I) {
if (isa<ConstantInt>(Phi.getIncomingValue(I))) continue;
if (LastBlock) {
// There are several non-constant values.
LLVM_DEBUG(dbgs() << "skip: several non-constant values\n");
return false;
}
if (!isa<ICmpInst>(Phi.getIncomingValue(I)) ||
cast<ICmpInst>(Phi.getIncomingValue(I))->getParent() !=
Phi.getIncomingBlock(I)) {
// Non-constant incoming value is not from a cmp instruction or not
// produced by the last block. We could end up processing the value
// producing block more than once.
//
// This is an uncommon case, so we bail.
LLVM_DEBUG(
dbgs()
<< "skip: non-constant value not from cmp or not from last block.\n");
return false;
}
LastBlock = Phi.getIncomingBlock(I);
}
if (!LastBlock) {
// There is no non-constant block.
LLVM_DEBUG(dbgs() << "skip: no non-constant block\n");
return false;
}
if (LastBlock->getSingleSuccessor() != Phi.getParent()) {
LLVM_DEBUG(dbgs() << "skip: last block non-phi successor\n");
return false;
}
const auto Blocks =
getOrderedBlocks(Phi, LastBlock, Phi.getNumIncomingValues());
if (Blocks.empty()) return false;
BCECmpChain CmpChain(Blocks, Phi, AA);
if (CmpChain.size() < 2) {
LLVM_DEBUG(dbgs() << "skip: only one compare block\n");
return false;
}
return CmpChain.simplify(TLI, AA, DTU);
}
static bool runImpl(Function &F, const TargetLibraryInfo &TLI,
const TargetTransformInfo &TTI, AliasAnalysis &AA,
DominatorTree *DT) {
LLVM_DEBUG(dbgs() << "MergeICmpsLegacyPass: " << F.getName() << "\n");
// We only try merging comparisons if the target wants to expand memcmp later.
// The rationale is to avoid turning small chains into memcmp calls.
if (!TTI.enableMemCmpExpansion(F.hasOptSize(), true))
return false;
// If we don't have memcmp avaiable we can't emit calls to it.
if (!TLI.has(LibFunc_memcmp))
return false;
DomTreeUpdater DTU(DT, /*PostDominatorTree*/ nullptr,
DomTreeUpdater::UpdateStrategy::Eager);
bool MadeChange = false;
for (auto BBIt = ++F.begin(); BBIt != F.end(); ++BBIt) {
// A Phi operation is always first in a basic block.
if (auto *const Phi = dyn_cast<PHINode>(&*BBIt->begin()))
MadeChange |= processPhi(*Phi, TLI, AA, DTU);
}
return MadeChange;
}
class MergeICmpsLegacyPass : public FunctionPass {
public:
static char ID;
MergeICmpsLegacyPass() : FunctionPass(ID) {
initializeMergeICmpsLegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F)) return false;
const auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
const auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
// MergeICmps does not need the DominatorTree, but we update it if it's
// already available.
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
return runImpl(F, TLI, TTI, AA, DTWP ? &DTWP->getDomTree() : nullptr);
}
private:
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
};
} // namespace
char MergeICmpsLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(MergeICmpsLegacyPass, "mergeicmps",
"Merge contiguous icmps into a memcmp", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MergeICmpsLegacyPass, "mergeicmps",
"Merge contiguous icmps into a memcmp", false, false)
Pass *llvm::createMergeICmpsLegacyPass() { return new MergeICmpsLegacyPass(); }
PreservedAnalyses MergeICmpsPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
const bool MadeChanges = runImpl(F, TLI, TTI, AA, DT);
if (!MadeChanges)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<GlobalsAA>();
PA.preserve<DominatorTreeAnalysis>();
return PA;
}
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