SystemZLongBranch.cpp
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//===-- SystemZLongBranch.cpp - Branch lengthening for SystemZ ------------===//
//
// 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 makes sure that all branches are in range. There are several ways
// in which this could be done. One aggressive approach is to assume that all
// branches are in range and successively replace those that turn out not
// to be in range with a longer form (branch relaxation). A simple
// implementation is to continually walk through the function relaxing
// branches until no more changes are needed and a fixed point is reached.
// However, in the pathological worst case, this implementation is
// quadratic in the number of blocks; relaxing branch N can make branch N-1
// go out of range, which in turn can make branch N-2 go out of range,
// and so on.
//
// An alternative approach is to assume that all branches must be
// converted to their long forms, then reinstate the short forms of
// branches that, even under this pessimistic assumption, turn out to be
// in range (branch shortening). This too can be implemented as a function
// walk that is repeated until a fixed point is reached. In general,
// the result of shortening is not as good as that of relaxation, and
// shortening is also quadratic in the worst case; shortening branch N
// can bring branch N-1 in range of the short form, which in turn can do
// the same for branch N-2, and so on. The main advantage of shortening
// is that each walk through the function produces valid code, so it is
// possible to stop at any point after the first walk. The quadraticness
// could therefore be handled with a maximum pass count, although the
// question then becomes: what maximum count should be used?
//
// On SystemZ, long branches are only needed for functions bigger than 64k,
// which are relatively rare to begin with, and the long branch sequences
// are actually relatively cheap. It therefore doesn't seem worth spending
// much compilation time on the problem. Instead, the approach we take is:
//
// (1) Work out the address that each block would have if no branches
// need relaxing. Exit the pass early if all branches are in range
// according to this assumption.
//
// (2) Work out the address that each block would have if all branches
// need relaxing.
//
// (3) Walk through the block calculating the final address of each instruction
// and relaxing those that need to be relaxed. For backward branches,
// this check uses the final address of the target block, as calculated
// earlier in the walk. For forward branches, this check uses the
// address of the target block that was calculated in (2). Both checks
// give a conservatively-correct range.
//
//===----------------------------------------------------------------------===//
#include "SystemZ.h"
#include "SystemZInstrInfo.h"
#include "SystemZTargetMachine.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/Support/ErrorHandling.h"
#include <cassert>
#include <cstdint>
using namespace llvm;
#define DEBUG_TYPE "systemz-long-branch"
STATISTIC(LongBranches, "Number of long branches.");
namespace {
// Represents positional information about a basic block.
struct MBBInfo {
// The address that we currently assume the block has.
uint64_t Address = 0;
// The size of the block in bytes, excluding terminators.
// This value never changes.
uint64_t Size = 0;
// The minimum alignment of the block.
// This value never changes.
Align Alignment;
// The number of terminators in this block. This value never changes.
unsigned NumTerminators = 0;
MBBInfo() = default;
};
// Represents the state of a block terminator.
struct TerminatorInfo {
// If this terminator is a relaxable branch, this points to the branch
// instruction, otherwise it is null.
MachineInstr *Branch = nullptr;
// The address that we currently assume the terminator has.
uint64_t Address = 0;
// The current size of the terminator in bytes.
uint64_t Size = 0;
// If Branch is nonnull, this is the number of the target block,
// otherwise it is unused.
unsigned TargetBlock = 0;
// If Branch is nonnull, this is the length of the longest relaxed form,
// otherwise it is zero.
unsigned ExtraRelaxSize = 0;
TerminatorInfo() = default;
};
// Used to keep track of the current position while iterating over the blocks.
struct BlockPosition {
// The address that we assume this position has.
uint64_t Address = 0;
// The number of low bits in Address that are known to be the same
// as the runtime address.
unsigned KnownBits;
BlockPosition(unsigned InitialLogAlignment)
: KnownBits(InitialLogAlignment) {}
};
class SystemZLongBranch : public MachineFunctionPass {
public:
static char ID;
SystemZLongBranch(const SystemZTargetMachine &tm)
: MachineFunctionPass(ID) {}
StringRef getPassName() const override { return "SystemZ Long Branch"; }
bool runOnMachineFunction(MachineFunction &F) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
private:
void skipNonTerminators(BlockPosition &Position, MBBInfo &Block);
void skipTerminator(BlockPosition &Position, TerminatorInfo &Terminator,
bool AssumeRelaxed);
TerminatorInfo describeTerminator(MachineInstr &MI);
uint64_t initMBBInfo();
bool mustRelaxBranch(const TerminatorInfo &Terminator, uint64_t Address);
bool mustRelaxABranch();
void setWorstCaseAddresses();
void splitBranchOnCount(MachineInstr *MI, unsigned AddOpcode);
void splitCompareBranch(MachineInstr *MI, unsigned CompareOpcode);
void relaxBranch(TerminatorInfo &Terminator);
void relaxBranches();
const SystemZInstrInfo *TII = nullptr;
MachineFunction *MF = nullptr;
SmallVector<MBBInfo, 16> MBBs;
SmallVector<TerminatorInfo, 16> Terminators;
};
char SystemZLongBranch::ID = 0;
const uint64_t MaxBackwardRange = 0x10000;
const uint64_t MaxForwardRange = 0xfffe;
} // end anonymous namespace
// Position describes the state immediately before Block. Update Block
// accordingly and move Position to the end of the block's non-terminator
// instructions.
void SystemZLongBranch::skipNonTerminators(BlockPosition &Position,
MBBInfo &Block) {
if (Log2(Block.Alignment) > Position.KnownBits) {
// When calculating the address of Block, we need to conservatively
// assume that Block had the worst possible misalignment.
Position.Address +=
(Block.Alignment.value() - (uint64_t(1) << Position.KnownBits));
Position.KnownBits = Log2(Block.Alignment);
}
// Align the addresses.
Position.Address = alignTo(Position.Address, Block.Alignment);
// Record the block's position.
Block.Address = Position.Address;
// Move past the non-terminators in the block.
Position.Address += Block.Size;
}
// Position describes the state immediately before Terminator.
// Update Terminator accordingly and move Position past it.
// Assume that Terminator will be relaxed if AssumeRelaxed.
void SystemZLongBranch::skipTerminator(BlockPosition &Position,
TerminatorInfo &Terminator,
bool AssumeRelaxed) {
Terminator.Address = Position.Address;
Position.Address += Terminator.Size;
if (AssumeRelaxed)
Position.Address += Terminator.ExtraRelaxSize;
}
// Return a description of terminator instruction MI.
TerminatorInfo SystemZLongBranch::describeTerminator(MachineInstr &MI) {
TerminatorInfo Terminator;
Terminator.Size = TII->getInstSizeInBytes(MI);
if (MI.isConditionalBranch() || MI.isUnconditionalBranch()) {
switch (MI.getOpcode()) {
case SystemZ::J:
// Relaxes to JG, which is 2 bytes longer.
Terminator.ExtraRelaxSize = 2;
break;
case SystemZ::BRC:
// Relaxes to BRCL, which is 2 bytes longer.
Terminator.ExtraRelaxSize = 2;
break;
case SystemZ::BRCT:
case SystemZ::BRCTG:
// Relaxes to A(G)HI and BRCL, which is 6 bytes longer.
Terminator.ExtraRelaxSize = 6;
break;
case SystemZ::BRCTH:
// Never needs to be relaxed.
Terminator.ExtraRelaxSize = 0;
break;
case SystemZ::CRJ:
case SystemZ::CLRJ:
// Relaxes to a C(L)R/BRCL sequence, which is 2 bytes longer.
Terminator.ExtraRelaxSize = 2;
break;
case SystemZ::CGRJ:
case SystemZ::CLGRJ:
// Relaxes to a C(L)GR/BRCL sequence, which is 4 bytes longer.
Terminator.ExtraRelaxSize = 4;
break;
case SystemZ::CIJ:
case SystemZ::CGIJ:
// Relaxes to a C(G)HI/BRCL sequence, which is 4 bytes longer.
Terminator.ExtraRelaxSize = 4;
break;
case SystemZ::CLIJ:
case SystemZ::CLGIJ:
// Relaxes to a CL(G)FI/BRCL sequence, which is 6 bytes longer.
Terminator.ExtraRelaxSize = 6;
break;
default:
llvm_unreachable("Unrecognized branch instruction");
}
Terminator.Branch = &MI;
Terminator.TargetBlock =
TII->getBranchInfo(MI).getMBBTarget()->getNumber();
}
return Terminator;
}
// Fill MBBs and Terminators, setting the addresses on the assumption
// that no branches need relaxation. Return the size of the function under
// this assumption.
uint64_t SystemZLongBranch::initMBBInfo() {
MF->RenumberBlocks();
unsigned NumBlocks = MF->size();
MBBs.clear();
MBBs.resize(NumBlocks);
Terminators.clear();
Terminators.reserve(NumBlocks);
BlockPosition Position(Log2(MF->getAlignment()));
for (unsigned I = 0; I < NumBlocks; ++I) {
MachineBasicBlock *MBB = MF->getBlockNumbered(I);
MBBInfo &Block = MBBs[I];
// Record the alignment, for quick access.
Block.Alignment = MBB->getAlignment();
// Calculate the size of the fixed part of the block.
MachineBasicBlock::iterator MI = MBB->begin();
MachineBasicBlock::iterator End = MBB->end();
while (MI != End && !MI->isTerminator()) {
Block.Size += TII->getInstSizeInBytes(*MI);
++MI;
}
skipNonTerminators(Position, Block);
// Add the terminators.
while (MI != End) {
if (!MI->isDebugInstr()) {
assert(MI->isTerminator() && "Terminator followed by non-terminator");
Terminators.push_back(describeTerminator(*MI));
skipTerminator(Position, Terminators.back(), false);
++Block.NumTerminators;
}
++MI;
}
}
return Position.Address;
}
// Return true if, under current assumptions, Terminator would need to be
// relaxed if it were placed at address Address.
bool SystemZLongBranch::mustRelaxBranch(const TerminatorInfo &Terminator,
uint64_t Address) {
if (!Terminator.Branch || Terminator.ExtraRelaxSize == 0)
return false;
const MBBInfo &Target = MBBs[Terminator.TargetBlock];
if (Address >= Target.Address) {
if (Address - Target.Address <= MaxBackwardRange)
return false;
} else {
if (Target.Address - Address <= MaxForwardRange)
return false;
}
return true;
}
// Return true if, under current assumptions, any terminator needs
// to be relaxed.
bool SystemZLongBranch::mustRelaxABranch() {
for (auto &Terminator : Terminators)
if (mustRelaxBranch(Terminator, Terminator.Address))
return true;
return false;
}
// Set the address of each block on the assumption that all branches
// must be long.
void SystemZLongBranch::setWorstCaseAddresses() {
SmallVector<TerminatorInfo, 16>::iterator TI = Terminators.begin();
BlockPosition Position(Log2(MF->getAlignment()));
for (auto &Block : MBBs) {
skipNonTerminators(Position, Block);
for (unsigned BTI = 0, BTE = Block.NumTerminators; BTI != BTE; ++BTI) {
skipTerminator(Position, *TI, true);
++TI;
}
}
}
// Split BRANCH ON COUNT MI into the addition given by AddOpcode followed
// by a BRCL on the result.
void SystemZLongBranch::splitBranchOnCount(MachineInstr *MI,
unsigned AddOpcode) {
MachineBasicBlock *MBB = MI->getParent();
DebugLoc DL = MI->getDebugLoc();
BuildMI(*MBB, MI, DL, TII->get(AddOpcode))
.add(MI->getOperand(0))
.add(MI->getOperand(1))
.addImm(-1);
MachineInstr *BRCL = BuildMI(*MBB, MI, DL, TII->get(SystemZ::BRCL))
.addImm(SystemZ::CCMASK_ICMP)
.addImm(SystemZ::CCMASK_CMP_NE)
.add(MI->getOperand(2));
// The implicit use of CC is a killing use.
BRCL->addRegisterKilled(SystemZ::CC, &TII->getRegisterInfo());
MI->eraseFromParent();
}
// Split MI into the comparison given by CompareOpcode followed
// a BRCL on the result.
void SystemZLongBranch::splitCompareBranch(MachineInstr *MI,
unsigned CompareOpcode) {
MachineBasicBlock *MBB = MI->getParent();
DebugLoc DL = MI->getDebugLoc();
BuildMI(*MBB, MI, DL, TII->get(CompareOpcode))
.add(MI->getOperand(0))
.add(MI->getOperand(1));
MachineInstr *BRCL = BuildMI(*MBB, MI, DL, TII->get(SystemZ::BRCL))
.addImm(SystemZ::CCMASK_ICMP)
.add(MI->getOperand(2))
.add(MI->getOperand(3));
// The implicit use of CC is a killing use.
BRCL->addRegisterKilled(SystemZ::CC, &TII->getRegisterInfo());
MI->eraseFromParent();
}
// Relax the branch described by Terminator.
void SystemZLongBranch::relaxBranch(TerminatorInfo &Terminator) {
MachineInstr *Branch = Terminator.Branch;
switch (Branch->getOpcode()) {
case SystemZ::J:
Branch->setDesc(TII->get(SystemZ::JG));
break;
case SystemZ::BRC:
Branch->setDesc(TII->get(SystemZ::BRCL));
break;
case SystemZ::BRCT:
splitBranchOnCount(Branch, SystemZ::AHI);
break;
case SystemZ::BRCTG:
splitBranchOnCount(Branch, SystemZ::AGHI);
break;
case SystemZ::CRJ:
splitCompareBranch(Branch, SystemZ::CR);
break;
case SystemZ::CGRJ:
splitCompareBranch(Branch, SystemZ::CGR);
break;
case SystemZ::CIJ:
splitCompareBranch(Branch, SystemZ::CHI);
break;
case SystemZ::CGIJ:
splitCompareBranch(Branch, SystemZ::CGHI);
break;
case SystemZ::CLRJ:
splitCompareBranch(Branch, SystemZ::CLR);
break;
case SystemZ::CLGRJ:
splitCompareBranch(Branch, SystemZ::CLGR);
break;
case SystemZ::CLIJ:
splitCompareBranch(Branch, SystemZ::CLFI);
break;
case SystemZ::CLGIJ:
splitCompareBranch(Branch, SystemZ::CLGFI);
break;
default:
llvm_unreachable("Unrecognized branch");
}
Terminator.Size += Terminator.ExtraRelaxSize;
Terminator.ExtraRelaxSize = 0;
Terminator.Branch = nullptr;
++LongBranches;
}
// Run a shortening pass and relax any branches that need to be relaxed.
void SystemZLongBranch::relaxBranches() {
SmallVector<TerminatorInfo, 16>::iterator TI = Terminators.begin();
BlockPosition Position(Log2(MF->getAlignment()));
for (auto &Block : MBBs) {
skipNonTerminators(Position, Block);
for (unsigned BTI = 0, BTE = Block.NumTerminators; BTI != BTE; ++BTI) {
assert(Position.Address <= TI->Address &&
"Addresses shouldn't go forwards");
if (mustRelaxBranch(*TI, Position.Address))
relaxBranch(*TI);
skipTerminator(Position, *TI, false);
++TI;
}
}
}
bool SystemZLongBranch::runOnMachineFunction(MachineFunction &F) {
TII = static_cast<const SystemZInstrInfo *>(F.getSubtarget().getInstrInfo());
MF = &F;
uint64_t Size = initMBBInfo();
if (Size <= MaxForwardRange || !mustRelaxABranch())
return false;
setWorstCaseAddresses();
relaxBranches();
return true;
}
FunctionPass *llvm::createSystemZLongBranchPass(SystemZTargetMachine &TM) {
return new SystemZLongBranch(TM);
}