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llvm-project/llvm/lib/Target/X86/X86CondBrFolding.cpp

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//===---- X86CondBrFolding.cpp - optimize conditional branches ------------===//
//
// 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 file defines a pass that optimizes condition branches on x86 by taking
// advantage of the three-way conditional code generated by compare
// instructions.
// Currently, it tries to hoisting EQ and NE conditional branch to a dominant
// conditional branch condition where the same EQ/NE conditional code is
// computed. An example:
// bb_0:
// cmp %0, 19
// jg bb_1
// jmp bb_2
// bb_1:
// cmp %0, 40
// jg bb_3
// jmp bb_4
// bb_4:
// cmp %0, 20
// je bb_5
// jmp bb_6
// Here we could combine the two compares in bb_0 and bb_4 and have the
// following code:
// bb_0:
// cmp %0, 20
// jg bb_1
// jl bb_2
// jmp bb_5
// bb_1:
// cmp %0, 40
// jg bb_3
// jmp bb_6
// For the case of %0 == 20 (bb_5), we eliminate two jumps, and the control
// height for bb_6 is also reduced. bb_4 is gone after the optimization.
//
// There are plenty of this code patterns, especially from the switch case
// lowing where we generate compare of "pivot-1" for the inner nodes in the
// binary search tree.
//===----------------------------------------------------------------------===//
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/BranchProbability.h"
using namespace llvm;
#define DEBUG_TYPE "x86-condbr-folding"
STATISTIC(NumFixedCondBrs, "Number of x86 condbr folded");
namespace {
class X86CondBrFoldingPass : public MachineFunctionPass {
public:
X86CondBrFoldingPass() : MachineFunctionPass(ID) {
initializeX86CondBrFoldingPassPass(*PassRegistry::getPassRegistry());
}
StringRef getPassName() const override { return "X86 CondBr Folding"; }
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<MachineBranchProbabilityInfo>();
}
public:
static char ID;
};
} // namespace
char X86CondBrFoldingPass::ID = 0;
INITIALIZE_PASS(X86CondBrFoldingPass, "X86CondBrFolding", "X86CondBrFolding", false, false)
FunctionPass *llvm::createX86CondBrFolding() {
return new X86CondBrFoldingPass();
}
namespace {
// A class the stores the auxiliary information for each MBB.
struct TargetMBBInfo {
MachineBasicBlock *TBB;
MachineBasicBlock *FBB;
MachineInstr *BrInstr;
MachineInstr *CmpInstr;
X86::CondCode BranchCode;
unsigned SrcReg;
int CmpValue;
bool Modified;
bool CmpBrOnly;
};
// A class that optimizes the conditional branch by hoisting and merge CondCode.
class X86CondBrFolding {
public:
X86CondBrFolding(const X86InstrInfo *TII,
const MachineBranchProbabilityInfo *MBPI,
MachineFunction &MF)
: TII(TII), MBPI(MBPI), MF(MF) {}
bool optimize();
private:
const X86InstrInfo *TII;
const MachineBranchProbabilityInfo *MBPI;
MachineFunction &MF;
std::vector<std::unique_ptr<TargetMBBInfo>> MBBInfos;
SmallVector<MachineBasicBlock *, 4> RemoveList;
void optimizeCondBr(MachineBasicBlock &MBB,
SmallVectorImpl<MachineBasicBlock *> &BranchPath);
void fixBranchProb(MachineBasicBlock *NextMBB, MachineBasicBlock *RootMBB,
SmallVectorImpl<MachineBasicBlock *> &BranchPath);
void replaceBrDest(MachineBasicBlock *MBB, MachineBasicBlock *OrigDest,
MachineBasicBlock *NewDest);
void fixupModifiedCond(MachineBasicBlock *MBB);
std::unique_ptr<TargetMBBInfo> analyzeMBB(MachineBasicBlock &MBB);
static bool analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
int &CmpValue);
bool findPath(MachineBasicBlock *MBB,
SmallVectorImpl<MachineBasicBlock *> &BranchPath);
TargetMBBInfo *getMBBInfo(MachineBasicBlock *MBB) const {
return MBBInfos[MBB->getNumber()].get();
}
};
} // namespace
// Find a valid path that we can reuse the CondCode.
// The resulted path (if return true) is stored in BranchPath.
// Return value:
// false: is no valid path is found.
// true: a valid path is found and the targetBB can be reached.
bool X86CondBrFolding::findPath(
MachineBasicBlock *MBB, SmallVectorImpl<MachineBasicBlock *> &BranchPath) {
TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
assert(MBBInfo && "Expecting a candidate MBB");
int CmpValue = MBBInfo->CmpValue;
MachineBasicBlock *PredMBB = *MBB->pred_begin();
MachineBasicBlock *SaveMBB = MBB;
while (PredMBB) {
TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
if (!PredMBBInfo || PredMBBInfo->SrcReg != MBBInfo->SrcReg)
return false;
assert(SaveMBB == PredMBBInfo->TBB || SaveMBB == PredMBBInfo->FBB);
bool IsFalseBranch = (SaveMBB == PredMBBInfo->FBB);
X86::CondCode CC = PredMBBInfo->BranchCode;
assert(CC == X86::COND_L || CC == X86::COND_G || CC == X86::COND_E);
int PredCmpValue = PredMBBInfo->CmpValue;
bool ValueCmpTrue = ((CmpValue < PredCmpValue && CC == X86::COND_L) ||
(CmpValue > PredCmpValue && CC == X86::COND_G) ||
(CmpValue == PredCmpValue && CC == X86::COND_E));
// Check if both the result of value compare and the branch target match.
if (!(ValueCmpTrue ^ IsFalseBranch)) {
LLVM_DEBUG(dbgs() << "Dead BB detected!\n");
return false;
}
BranchPath.push_back(PredMBB);
// These are the conditions on which we could combine the compares.
if ((CmpValue == PredCmpValue) ||
(CmpValue == PredCmpValue - 1 && CC == X86::COND_L) ||
(CmpValue == PredCmpValue + 1 && CC == X86::COND_G))
return true;
// If PredMBB has more than on preds, or not a pure cmp and br, we bailout.
if (PredMBB->pred_size() != 1 || !PredMBBInfo->CmpBrOnly)
return false;
SaveMBB = PredMBB;
PredMBB = *PredMBB->pred_begin();
}
return false;
}
// Fix up any PHI node in the successor of MBB.
static void fixPHIsInSucc(MachineBasicBlock *MBB, MachineBasicBlock *OldMBB,
MachineBasicBlock *NewMBB) {
if (NewMBB == OldMBB)
return;
for (auto MI = MBB->instr_begin(), ME = MBB->instr_end();
MI != ME && MI->isPHI(); ++MI)
for (unsigned i = 2, e = MI->getNumOperands() + 1; i != e; i += 2) {
MachineOperand &MO = MI->getOperand(i);
if (MO.getMBB() == OldMBB)
MO.setMBB(NewMBB);
}
}
// Utility function to set branch probability for edge MBB->SuccMBB.
static inline bool setBranchProb(MachineBasicBlock *MBB,
MachineBasicBlock *SuccMBB,
BranchProbability Prob) {
auto MBBI = std::find(MBB->succ_begin(), MBB->succ_end(), SuccMBB);
if (MBBI == MBB->succ_end())
return false;
MBB->setSuccProbability(MBBI, Prob);
return true;
}
// Utility function to find the unconditional br instruction in MBB.
static inline MachineBasicBlock::iterator
findUncondBrI(MachineBasicBlock *MBB) {
return std::find_if(MBB->begin(), MBB->end(), [](MachineInstr &MI) -> bool {
return MI.getOpcode() == X86::JMP_1;
});
}
// Replace MBB's original successor, OrigDest, with NewDest.
// Also update the MBBInfo for MBB.
void X86CondBrFolding::replaceBrDest(MachineBasicBlock *MBB,
MachineBasicBlock *OrigDest,
MachineBasicBlock *NewDest) {
TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
MachineInstr *BrMI;
if (MBBInfo->TBB == OrigDest) {
BrMI = MBBInfo->BrInstr;
unsigned JNCC = GetCondBranchFromCond(MBBInfo->BranchCode);
MachineInstrBuilder MIB =
BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI), TII->get(JNCC))
.addMBB(NewDest);
MBBInfo->TBB = NewDest;
MBBInfo->BrInstr = MIB.getInstr();
} else { // Should be the unconditional jump stmt.
MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
.addMBB(NewDest);
MBBInfo->FBB = NewDest;
BrMI = &*UncondBrI;
}
fixPHIsInSucc(NewDest, OrigDest, MBB);
BrMI->eraseFromParent();
MBB->addSuccessor(NewDest);
setBranchProb(MBB, NewDest, MBPI->getEdgeProbability(MBB, OrigDest));
MBB->removeSuccessor(OrigDest);
}
// Change the CondCode and BrInstr according to MBBInfo.
void X86CondBrFolding::fixupModifiedCond(MachineBasicBlock *MBB) {
TargetMBBInfo *MBBInfo = getMBBInfo(MBB);
if (!MBBInfo->Modified)
return;
MachineInstr *BrMI = MBBInfo->BrInstr;
X86::CondCode CC = MBBInfo->BranchCode;
MachineInstrBuilder MIB = BuildMI(*MBB, BrMI, MBB->findDebugLoc(BrMI),
TII->get(GetCondBranchFromCond(CC)))
.addMBB(MBBInfo->TBB);
BrMI->eraseFromParent();
MBBInfo->BrInstr = MIB.getInstr();
MachineBasicBlock::iterator UncondBrI = findUncondBrI(MBB);
BuildMI(*MBB, UncondBrI, MBB->findDebugLoc(UncondBrI), TII->get(X86::JMP_1))
.addMBB(MBBInfo->FBB);
MBB->erase(UncondBrI);
MBBInfo->Modified = false;
}
//
// Apply the transformation:
// RootMBB -1-> ... PredMBB -3-> MBB -5-> TargetMBB
// \-2-> \-4-> \-6-> FalseMBB
// ==>
// RootMBB -1-> ... PredMBB -7-> FalseMBB
// TargetMBB <-8-/ \-2-> \-4->
//
// Note that PredMBB and RootMBB could be the same.
// And in the case of dead TargetMBB, we will not have TargetMBB and edge 8.
//
// There are some special handling where the RootMBB is COND_E in which case
// we directly short-cycle the brinstr.
//
void X86CondBrFolding::optimizeCondBr(
MachineBasicBlock &MBB, SmallVectorImpl<MachineBasicBlock *> &BranchPath) {
X86::CondCode CC;
TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
assert(MBBInfo && "Expecting a candidate MBB");
MachineBasicBlock *TargetMBB = MBBInfo->TBB;
BranchProbability TargetProb = MBPI->getEdgeProbability(&MBB, MBBInfo->TBB);
// Forward the jump from MBB's predecessor to MBB's false target.
MachineBasicBlock *PredMBB = BranchPath.front();
TargetMBBInfo *PredMBBInfo = getMBBInfo(PredMBB);
assert(PredMBBInfo && "Expecting a candidate MBB");
if (PredMBBInfo->Modified)
fixupModifiedCond(PredMBB);
CC = PredMBBInfo->BranchCode;
// Don't do this if depth of BranchPath is 1 and PredMBB is of COND_E.
// We will short-cycle directly for this case.
if (!(CC == X86::COND_E && BranchPath.size() == 1))
replaceBrDest(PredMBB, &MBB, MBBInfo->FBB);
MachineBasicBlock *RootMBB = BranchPath.back();
TargetMBBInfo *RootMBBInfo = getMBBInfo(RootMBB);
assert(RootMBBInfo && "Expecting a candidate MBB");
if (RootMBBInfo->Modified)
fixupModifiedCond(RootMBB);
CC = RootMBBInfo->BranchCode;
if (CC != X86::COND_E) {
MachineBasicBlock::iterator UncondBrI = findUncondBrI(RootMBB);
// RootMBB: Cond jump to the original not-taken MBB.
X86::CondCode NewCC;
switch (CC) {
case X86::COND_L:
NewCC = X86::COND_G;
break;
case X86::COND_G:
NewCC = X86::COND_L;
break;
default:
llvm_unreachable("unexpected condtional code.");
}
BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
TII->get(GetCondBranchFromCond(NewCC)))
.addMBB(RootMBBInfo->FBB);
// RootMBB: Jump to TargetMBB
BuildMI(*RootMBB, UncondBrI, RootMBB->findDebugLoc(UncondBrI),
TII->get(X86::JMP_1))
.addMBB(TargetMBB);
RootMBB->addSuccessor(TargetMBB);
fixPHIsInSucc(TargetMBB, &MBB, RootMBB);
RootMBB->erase(UncondBrI);
} else {
replaceBrDest(RootMBB, RootMBBInfo->TBB, TargetMBB);
}
// Fix RootMBB's CmpValue to MBB's CmpValue to TargetMBB. Don't set Imm
// directly. Move MBB's stmt to here as the opcode might be different.
if (RootMBBInfo->CmpValue != MBBInfo->CmpValue) {
MachineInstr *NewCmp = MBBInfo->CmpInstr;
NewCmp->removeFromParent();
RootMBB->insert(RootMBBInfo->CmpInstr, NewCmp);
RootMBBInfo->CmpInstr->eraseFromParent();
}
// Fix branch Probabilities.
auto fixBranchProb = [&](MachineBasicBlock *NextMBB) {
BranchProbability Prob;
for (auto &I : BranchPath) {
MachineBasicBlock *ThisMBB = I;
if (!ThisMBB->hasSuccessorProbabilities() ||
!ThisMBB->isSuccessor(NextMBB))
break;
Prob = MBPI->getEdgeProbability(ThisMBB, NextMBB);
if (Prob.isUnknown())
break;
TargetProb = Prob * TargetProb;
Prob = Prob - TargetProb;
setBranchProb(ThisMBB, NextMBB, Prob);
if (ThisMBB == RootMBB) {
setBranchProb(ThisMBB, TargetMBB, TargetProb);
}
ThisMBB->normalizeSuccProbs();
if (ThisMBB == RootMBB)
break;
NextMBB = ThisMBB;
}
return true;
};
if (CC != X86::COND_E && !TargetProb.isUnknown())
fixBranchProb(MBBInfo->FBB);
if (CC != X86::COND_E)
RemoveList.push_back(&MBB);
// Invalidate MBBInfo just in case.
MBBInfos[MBB.getNumber()] = nullptr;
MBBInfos[RootMBB->getNumber()] = nullptr;
LLVM_DEBUG(dbgs() << "After optimization:\nRootMBB is: " << *RootMBB << "\n");
if (BranchPath.size() > 1)
LLVM_DEBUG(dbgs() << "PredMBB is: " << *(BranchPath[0]) << "\n");
}
// Driver function for optimization: find the valid candidate and apply
// the transformation.
bool X86CondBrFolding::optimize() {
bool Changed = false;
LLVM_DEBUG(dbgs() << "***** X86CondBr Folding on Function: " << MF.getName()
<< " *****\n");
// Setup data structures.
MBBInfos.resize(MF.getNumBlockIDs());
for (auto &MBB : MF)
MBBInfos[MBB.getNumber()] = analyzeMBB(MBB);
for (auto &MBB : MF) {
TargetMBBInfo *MBBInfo = getMBBInfo(&MBB);
if (!MBBInfo || !MBBInfo->CmpBrOnly)
continue;
if (MBB.pred_size() != 1)
continue;
LLVM_DEBUG(dbgs() << "Work on MBB." << MBB.getNumber()
<< " CmpValue: " << MBBInfo->CmpValue << "\n");
SmallVector<MachineBasicBlock *, 4> BranchPath;
if (!findPath(&MBB, BranchPath))
continue;
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << "Found one path (len=" << BranchPath.size() << "):\n");
int Index = 1;
LLVM_DEBUG(dbgs() << "Target MBB is: " << MBB << "\n");
for (auto I = BranchPath.rbegin(); I != BranchPath.rend(); ++I, ++Index) {
MachineBasicBlock *PMBB = *I;
TargetMBBInfo *PMBBInfo = getMBBInfo(PMBB);
LLVM_DEBUG(dbgs() << "Path MBB (" << Index << " of " << BranchPath.size()
<< ") is " << *PMBB);
LLVM_DEBUG(dbgs() << "CC=" << PMBBInfo->BranchCode
<< " Val=" << PMBBInfo->CmpValue
<< " CmpBrOnly=" << PMBBInfo->CmpBrOnly << "\n\n");
}
#endif
optimizeCondBr(MBB, BranchPath);
Changed = true;
}
NumFixedCondBrs += RemoveList.size();
for (auto MBBI : RemoveList) {
while (!MBBI->succ_empty())
MBBI->removeSuccessor(MBBI->succ_end() - 1);
MBBI->eraseFromParent();
}
return Changed;
}
// Analyze instructions that generate CondCode and extract information.
bool X86CondBrFolding::analyzeCompare(const MachineInstr &MI, unsigned &SrcReg,
int &CmpValue) {
unsigned SrcRegIndex = 0;
unsigned ValueIndex = 0;
switch (MI.getOpcode()) {
// TODO: handle test instructions.
default:
return false;
case X86::CMP64ri32:
case X86::CMP64ri8:
case X86::CMP32ri:
case X86::CMP32ri8:
case X86::CMP16ri:
case X86::CMP16ri8:
case X86::CMP8ri:
SrcRegIndex = 0;
ValueIndex = 1;
break;
case X86::SUB64ri32:
case X86::SUB64ri8:
case X86::SUB32ri:
case X86::SUB32ri8:
case X86::SUB16ri:
case X86::SUB16ri8:
case X86::SUB8ri:
SrcRegIndex = 1;
ValueIndex = 2;
break;
}
SrcReg = MI.getOperand(SrcRegIndex).getReg();
if (!MI.getOperand(ValueIndex).isImm())
return false;
CmpValue = MI.getOperand(ValueIndex).getImm();
return true;
}
// Analyze a candidate MBB and set the extract all the information needed.
// The valid candidate will have two successors.
// It also should have a sequence of
// Branch_instr,
// CondBr,
// UnCondBr.
// Return TargetMBBInfo if MBB is a valid candidate and nullptr otherwise.
std::unique_ptr<TargetMBBInfo>
X86CondBrFolding::analyzeMBB(MachineBasicBlock &MBB) {
MachineBasicBlock *TBB;
MachineBasicBlock *FBB;
MachineInstr *BrInstr;
MachineInstr *CmpInstr;
X86::CondCode CC;
unsigned SrcReg;
int CmpValue;
bool Modified;
bool CmpBrOnly;
if (MBB.succ_size() != 2)
return nullptr;
CmpBrOnly = true;
FBB = TBB = nullptr;
CmpInstr = nullptr;
MachineBasicBlock::iterator I = MBB.end();
while (I != MBB.begin()) {
--I;
if (I->isDebugValue())
continue;
if (I->getOpcode() == X86::JMP_1) {
if (FBB)
return nullptr;
FBB = I->getOperand(0).getMBB();
continue;
}
if (I->isBranch()) {
if (TBB)
return nullptr;
CC = X86::getCondFromBranchOpc(I->getOpcode());
switch (CC) {
default:
return nullptr;
case X86::COND_E:
case X86::COND_L:
case X86::COND_G:
case X86::COND_NE:
case X86::COND_LE:
case X86::COND_GE:
break;
}
TBB = I->getOperand(0).getMBB();
BrInstr = &*I;
continue;
}
if (analyzeCompare(*I, SrcReg, CmpValue)) {
if (CmpInstr)
return nullptr;
CmpInstr = &*I;
continue;
}
CmpBrOnly = false;
break;
}
if (!TBB || !FBB || !CmpInstr)
return nullptr;
// Simplify CondCode. Note this is only to simplify the findPath logic
// and will not change the instruction here.
switch (CC) {
case X86::COND_NE:
CC = X86::COND_E;
std::swap(TBB, FBB);
Modified = true;
break;
case X86::COND_LE:
if (CmpValue == INT_MAX)
return nullptr;
CC = X86::COND_L;
CmpValue += 1;
Modified = true;
break;
case X86::COND_GE:
if (CmpValue == INT_MIN)
return nullptr;
CC = X86::COND_G;
CmpValue -= 1;
Modified = true;
break;
default:
Modified = false;
break;
}
return llvm::make_unique<TargetMBBInfo>(TargetMBBInfo{
TBB, FBB, BrInstr, CmpInstr, CC, SrcReg, CmpValue, Modified, CmpBrOnly});
}
bool X86CondBrFoldingPass::runOnMachineFunction(MachineFunction &MF) {
const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
if (!ST.threewayBranchProfitable())
return false;
const X86InstrInfo *TII = ST.getInstrInfo();
const MachineBranchProbabilityInfo *MBPI =
&getAnalysis<MachineBranchProbabilityInfo>();
X86CondBrFolding CondBr(TII, MBPI, MF);
return CondBr.optimize();
}
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