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llvm-project/llvm/lib/Target/AArch64/AArch64ConditionOptimizer.cpp

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//=- AArch64ConditionOptimizer.cpp - Remove useless comparisons for AArch64 -=//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass tries to make consecutive compares of values use same operands to
// allow CSE pass to remove duplicated instructions. For this it analyzes
// branches and adjusts comparisons with immediate values by converting:
// * GE -> GT
// * GT -> GE
// * LT -> LE
// * LE -> LT
// and adjusting immediate values appropriately. It basically corrects two
// immediate values towards each other to make them equal.
//
// Consider the following example in C:
//
// if ((a < 5 && ...) || (a > 5 && ...)) {
// ~~~~~ ~~~~~
// ^ ^
// x y
//
// Here both "x" and "y" expressions compare "a" with "5". When "x" evaluates
// to "false", "y" can just check flags set by the first comparison. As a
// result of the canonicalization employed by
// SelectionDAGBuilder::visitSwitchCase, DAGCombine, and other target-specific
// code, assembly ends up in the form that is not CSE friendly:
//
// ...
// cmp w8, #4
// b.gt .LBB0_3
// ...
// .LBB0_3:
// cmp w8, #6
// b.lt .LBB0_6
// ...
//
// Same assembly after the pass:
//
// ...
// cmp w8, #5
// b.ge .LBB0_3
// ...
// .LBB0_3:
// cmp w8, #5 // <-- CSE pass removes this instruction
// b.le .LBB0_6
// ...
//
// Currently only SUBS and ADDS followed by b.?? are supported.
//
// TODO: maybe handle TBNZ/TBZ the same way as CMP when used instead for "a < 0"
// TODO: handle other conditional instructions (e.g. CSET)
// TODO: allow second branching to be anything if it doesn't require adjusting
//
//===----------------------------------------------------------------------===//
#include "AArch64.h"
#include "MCTargetDesc/AArch64AddressingModes.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <cstdlib>
#include <tuple>
using namespace llvm;
#define DEBUG_TYPE "aarch64-condopt"
STATISTIC(NumConditionsAdjusted, "Number of conditions adjusted");
namespace {
class AArch64ConditionOptimizer : public MachineFunctionPass {
const TargetInstrInfo *TII;
MachineDominatorTree *DomTree;
const MachineRegisterInfo *MRI;
public:
// Stores immediate, compare instruction opcode and branch condition (in this
// order) of adjusted comparison.
typedef std::tuple<int, unsigned, AArch64CC::CondCode> CmpInfo;
static char ID;
AArch64ConditionOptimizer() : MachineFunctionPass(ID) {
initializeAArch64ConditionOptimizerPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override;
MachineInstr *findSuitableCompare(MachineBasicBlock *MBB);
CmpInfo adjustCmp(MachineInstr *CmpMI, AArch64CC::CondCode Cmp);
void modifyCmp(MachineInstr *CmpMI, const CmpInfo &Info);
bool adjustTo(MachineInstr *CmpMI, AArch64CC::CondCode Cmp, MachineInstr *To,
int ToImm);
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override {
return "AArch64 Condition Optimizer";
}
};
} // end anonymous namespace
char AArch64ConditionOptimizer::ID = 0;
INITIALIZE_PASS_BEGIN(AArch64ConditionOptimizer, "aarch64-condopt",
"AArch64 CondOpt Pass", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_END(AArch64ConditionOptimizer, "aarch64-condopt",
"AArch64 CondOpt Pass", false, false)
FunctionPass *llvm::createAArch64ConditionOptimizerPass() {
return new AArch64ConditionOptimizer();
}
void AArch64ConditionOptimizer::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
// Finds compare instruction that corresponds to supported types of branching.
// Returns the instruction or nullptr on failures or detecting unsupported
// instructions.
MachineInstr *AArch64ConditionOptimizer::findSuitableCompare(
MachineBasicBlock *MBB) {
MachineBasicBlock::iterator I = MBB->getFirstTerminator();
if (I == MBB->end())
return nullptr;
if (I->getOpcode() != AArch64::Bcc)
return nullptr;
// Since we may modify cmp of this MBB, make sure NZCV does not live out.
for (auto SuccBB : MBB->successors())
if (SuccBB->isLiveIn(AArch64::NZCV))
return nullptr;
// Now find the instruction controlling the terminator.
for (MachineBasicBlock::iterator B = MBB->begin(); I != B;) {
--I;
assert(!I->isTerminator() && "Spurious terminator");
// Check if there is any use of NZCV between CMP and Bcc.
if (I->readsRegister(AArch64::NZCV))
return nullptr;
switch (I->getOpcode()) {
// cmp is an alias for subs with a dead destination register.
case AArch64::SUBSWri:
case AArch64::SUBSXri:
// cmn is an alias for adds with a dead destination register.
case AArch64::ADDSWri:
case AArch64::ADDSXri: {
unsigned ShiftAmt = AArch64_AM::getShiftValue(I->getOperand(3).getImm());
if (!I->getOperand(2).isImm()) {
DEBUG(dbgs() << "Immediate of cmp is symbolic, " << *I << '\n');
return nullptr;
} else if (I->getOperand(2).getImm() << ShiftAmt >= 0xfff) {
DEBUG(dbgs() << "Immediate of cmp may be out of range, " << *I << '\n');
return nullptr;
} else if (!MRI->use_empty(I->getOperand(0).getReg())) {
DEBUG(dbgs() << "Destination of cmp is not dead, " << *I << '\n');
return nullptr;
}
return &*I;
}
// Prevent false positive case like:
// cmp w19, #0
// cinc w0, w19, gt
// ...
// fcmp d8, #0.0
// b.gt .LBB0_5
case AArch64::FCMPDri:
case AArch64::FCMPSri:
case AArch64::FCMPESri:
case AArch64::FCMPEDri:
case AArch64::SUBSWrr:
case AArch64::SUBSXrr:
case AArch64::ADDSWrr:
case AArch64::ADDSXrr:
case AArch64::FCMPSrr:
case AArch64::FCMPDrr:
case AArch64::FCMPESrr:
case AArch64::FCMPEDrr:
// Skip comparison instructions without immediate operands.
return nullptr;
}
}
DEBUG(dbgs() << "Flags not defined in BB#" << MBB->getNumber() << '\n');
return nullptr;
}
// Changes opcode adds <-> subs considering register operand width.
static int getComplementOpc(int Opc) {
switch (Opc) {
case AArch64::ADDSWri: return AArch64::SUBSWri;
case AArch64::ADDSXri: return AArch64::SUBSXri;
case AArch64::SUBSWri: return AArch64::ADDSWri;
case AArch64::SUBSXri: return AArch64::ADDSXri;
default:
llvm_unreachable("Unexpected opcode");
}
}
// Changes form of comparison inclusive <-> exclusive.
static AArch64CC::CondCode getAdjustedCmp(AArch64CC::CondCode Cmp) {
switch (Cmp) {
case AArch64CC::GT: return AArch64CC::GE;
case AArch64CC::GE: return AArch64CC::GT;
case AArch64CC::LT: return AArch64CC::LE;
case AArch64CC::LE: return AArch64CC::LT;
default:
llvm_unreachable("Unexpected condition code");
}
}
// Transforms GT -> GE, GE -> GT, LT -> LE, LE -> LT by updating comparison
// operator and condition code.
AArch64ConditionOptimizer::CmpInfo AArch64ConditionOptimizer::adjustCmp(
MachineInstr *CmpMI, AArch64CC::CondCode Cmp) {
unsigned Opc = CmpMI->getOpcode();
// CMN (compare with negative immediate) is an alias to ADDS (as
// "operand - negative" == "operand + positive")
bool Negative = (Opc == AArch64::ADDSWri || Opc == AArch64::ADDSXri);
int Correction = (Cmp == AArch64CC::GT) ? 1 : -1;
// Negate Correction value for comparison with negative immediate (CMN).
if (Negative) {
Correction = -Correction;
}
const int OldImm = (int)CmpMI->getOperand(2).getImm();
const int NewImm = std::abs(OldImm + Correction);
// Handle +0 -> -1 and -0 -> +1 (CMN with 0 immediate) transitions by
// adjusting compare instruction opcode.
if (OldImm == 0 && ((Negative && Correction == 1) ||
(!Negative && Correction == -1))) {
Opc = getComplementOpc(Opc);
}
return CmpInfo(NewImm, Opc, getAdjustedCmp(Cmp));
}
// Applies changes to comparison instruction suggested by adjustCmp().
void AArch64ConditionOptimizer::modifyCmp(MachineInstr *CmpMI,
const CmpInfo &Info) {
int Imm;
unsigned Opc;
AArch64CC::CondCode Cmp;
std::tie(Imm, Opc, Cmp) = Info;
MachineBasicBlock *const MBB = CmpMI->getParent();
// Change immediate in comparison instruction (ADDS or SUBS).
BuildMI(*MBB, CmpMI, CmpMI->getDebugLoc(), TII->get(Opc))
.add(CmpMI->getOperand(0))
.add(CmpMI->getOperand(1))
.addImm(Imm)
.add(CmpMI->getOperand(3));
CmpMI->eraseFromParent();
// The fact that this comparison was picked ensures that it's related to the
// first terminator instruction.
MachineInstr &BrMI = *MBB->getFirstTerminator();
// Change condition in branch instruction.
BuildMI(*MBB, BrMI, BrMI.getDebugLoc(), TII->get(AArch64::Bcc))
.addImm(Cmp)
.add(BrMI.getOperand(1));
BrMI.eraseFromParent();
MBB->updateTerminator();
++NumConditionsAdjusted;
}
// Parse a condition code returned by AnalyzeBranch, and compute the CondCode
// corresponding to TBB.
// Returns true if parsing was successful, otherwise false is returned.
static bool parseCond(ArrayRef<MachineOperand> Cond, AArch64CC::CondCode &CC) {
// A normal br.cond simply has the condition code.
if (Cond[0].getImm() != -1) {
assert(Cond.size() == 1 && "Unknown Cond array format");
CC = (AArch64CC::CondCode)(int)Cond[0].getImm();
return true;
}
return false;
}
// Adjusts one cmp instruction to another one if result of adjustment will allow
// CSE. Returns true if compare instruction was changed, otherwise false is
// returned.
bool AArch64ConditionOptimizer::adjustTo(MachineInstr *CmpMI,
AArch64CC::CondCode Cmp, MachineInstr *To, int ToImm)
{
CmpInfo Info = adjustCmp(CmpMI, Cmp);
if (std::get<0>(Info) == ToImm && std::get<1>(Info) == To->getOpcode()) {
modifyCmp(CmpMI, Info);
return true;
}
return false;
}
bool AArch64ConditionOptimizer::runOnMachineFunction(MachineFunction &MF) {
DEBUG(dbgs() << "********** AArch64 Conditional Compares **********\n"
<< "********** Function: " << MF.getName() << '\n');
if (skipFunction(*MF.getFunction()))
return false;
TII = MF.getSubtarget().getInstrInfo();
DomTree = &getAnalysis<MachineDominatorTree>();
MRI = &MF.getRegInfo();
bool Changed = false;
// Visit blocks in dominator tree pre-order. The pre-order enables multiple
// cmp-conversions from the same head block.
// Note that updateDomTree() modifies the children of the DomTree node
// currently being visited. The df_iterator supports that; it doesn't look at
// child_begin() / child_end() until after a node has been visited.
for (MachineDomTreeNode *I : depth_first(DomTree)) {
MachineBasicBlock *HBB = I->getBlock();
SmallVector<MachineOperand, 4> HeadCond;
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
if (TII->analyzeBranch(*HBB, TBB, FBB, HeadCond)) {
continue;
}
// Equivalence check is to skip loops.
if (!TBB || TBB == HBB) {
continue;
}
SmallVector<MachineOperand, 4> TrueCond;
MachineBasicBlock *TBB_TBB = nullptr, *TBB_FBB = nullptr;
if (TII->analyzeBranch(*TBB, TBB_TBB, TBB_FBB, TrueCond)) {
continue;
}
MachineInstr *HeadCmpMI = findSuitableCompare(HBB);
if (!HeadCmpMI) {
continue;
}
MachineInstr *TrueCmpMI = findSuitableCompare(TBB);
if (!TrueCmpMI) {
continue;
}
AArch64CC::CondCode HeadCmp;
if (HeadCond.empty() || !parseCond(HeadCond, HeadCmp)) {
continue;
}
AArch64CC::CondCode TrueCmp;
if (TrueCond.empty() || !parseCond(TrueCond, TrueCmp)) {
continue;
}
const int HeadImm = (int)HeadCmpMI->getOperand(2).getImm();
const int TrueImm = (int)TrueCmpMI->getOperand(2).getImm();
DEBUG(dbgs() << "Head branch:\n");
DEBUG(dbgs() << "\tcondition: "
<< AArch64CC::getCondCodeName(HeadCmp) << '\n');
DEBUG(dbgs() << "\timmediate: " << HeadImm << '\n');
DEBUG(dbgs() << "True branch:\n");
DEBUG(dbgs() << "\tcondition: "
<< AArch64CC::getCondCodeName(TrueCmp) << '\n');
DEBUG(dbgs() << "\timmediate: " << TrueImm << '\n');
if (((HeadCmp == AArch64CC::GT && TrueCmp == AArch64CC::LT) ||
(HeadCmp == AArch64CC::LT && TrueCmp == AArch64CC::GT)) &&
std::abs(TrueImm - HeadImm) == 2) {
// This branch transforms machine instructions that correspond to
//
// 1) (a > {TrueImm} && ...) || (a < {HeadImm} && ...)
// 2) (a < {TrueImm} && ...) || (a > {HeadImm} && ...)
//
// into
//
// 1) (a >= {NewImm} && ...) || (a <= {NewImm} && ...)
// 2) (a <= {NewImm} && ...) || (a >= {NewImm} && ...)
CmpInfo HeadCmpInfo = adjustCmp(HeadCmpMI, HeadCmp);
CmpInfo TrueCmpInfo = adjustCmp(TrueCmpMI, TrueCmp);
if (std::get<0>(HeadCmpInfo) == std::get<0>(TrueCmpInfo) &&
std::get<1>(HeadCmpInfo) == std::get<1>(TrueCmpInfo)) {
modifyCmp(HeadCmpMI, HeadCmpInfo);
modifyCmp(TrueCmpMI, TrueCmpInfo);
Changed = true;
}
} else if (((HeadCmp == AArch64CC::GT && TrueCmp == AArch64CC::GT) ||
(HeadCmp == AArch64CC::LT && TrueCmp == AArch64CC::LT)) &&
std::abs(TrueImm - HeadImm) == 1) {
// This branch transforms machine instructions that correspond to
//
// 1) (a > {TrueImm} && ...) || (a > {HeadImm} && ...)
// 2) (a < {TrueImm} && ...) || (a < {HeadImm} && ...)
//
// into
//
// 1) (a <= {NewImm} && ...) || (a > {NewImm} && ...)
// 2) (a < {NewImm} && ...) || (a >= {NewImm} && ...)
// GT -> GE transformation increases immediate value, so picking the
// smaller one; LT -> LE decreases immediate value so invert the choice.
bool adjustHeadCond = (HeadImm < TrueImm);
if (HeadCmp == AArch64CC::LT) {
adjustHeadCond = !adjustHeadCond;
}
if (adjustHeadCond) {
Changed |= adjustTo(HeadCmpMI, HeadCmp, TrueCmpMI, TrueImm);
} else {
Changed |= adjustTo(TrueCmpMI, TrueCmp, HeadCmpMI, HeadImm);
}
}
// Other transformation cases almost never occur due to generation of < or >
// comparisons instead of <= and >=.
}
return Changed;
}
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