410 lines · cpp
1//===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This file contains a pass that scans a machine function to determine which10// conditional branches need more than 16 bits of displacement to reach their11// target basic block. It does this in two passes; a calculation of basic block12// positions pass, and a branch pseudo op to machine branch opcode pass. This13// pass should be run last, just before the assembly printer.14//15//===----------------------------------------------------------------------===//16 17#include "MCTargetDesc/PPCPredicates.h"18#include "PPC.h"19#include "PPCInstrInfo.h"20#include "PPCSubtarget.h"21#include "llvm/ADT/Statistic.h"22#include "llvm/CodeGen/MachineFunctionPass.h"23#include "llvm/CodeGen/MachineRegisterInfo.h"24#include "llvm/CodeGen/TargetSubtargetInfo.h"25#include "llvm/Support/MathExtras.h"26#include "llvm/Target/TargetMachine.h"27#include <algorithm>28using namespace llvm;29 30#define DEBUG_TYPE "ppc-branch-select"31 32STATISTIC(NumExpanded, "Number of branches expanded to long format");33STATISTIC(NumPrefixed, "Number of prefixed instructions");34STATISTIC(NumPrefixedAligned,35 "Number of prefixed instructions that have been aligned");36 37namespace {38 struct PPCBSel : public MachineFunctionPass {39 static char ID;40 PPCBSel() : MachineFunctionPass(ID) {}41 42 // The sizes of the basic blocks in the function (the first43 // element of the pair); the second element of the pair is the amount of the44 // size that is due to potential padding.45 std::vector<std::pair<unsigned, unsigned>> BlockSizes;46 47 // The first block number which has imprecise instruction address.48 int FirstImpreciseBlock = -1;49 50 unsigned GetAlignmentAdjustment(MachineBasicBlock &MBB, unsigned Offset);51 unsigned ComputeBlockSizes(MachineFunction &Fn);52 void modifyAdjustment(MachineFunction &Fn);53 int computeBranchSize(MachineFunction &Fn,54 const MachineBasicBlock *Src,55 const MachineBasicBlock *Dest,56 unsigned BrOffset);57 58 bool runOnMachineFunction(MachineFunction &Fn) override;59 60 MachineFunctionProperties getRequiredProperties() const override {61 return MachineFunctionProperties().setNoVRegs();62 }63 64 StringRef getPassName() const override { return "PowerPC Branch Selector"; }65 };66 char PPCBSel::ID = 0;67}68 69INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",70 false, false)71 72/// createPPCBranchSelectionPass - returns an instance of the Branch Selection73/// Pass74///75FunctionPass *llvm::createPPCBranchSelectionPass() {76 return new PPCBSel();77}78 79/// In order to make MBB aligned, we need to add an adjustment value to the80/// original Offset.81unsigned PPCBSel::GetAlignmentAdjustment(MachineBasicBlock &MBB,82 unsigned Offset) {83 const Align Alignment = MBB.getAlignment();84 if (Alignment == Align(1))85 return 0;86 87 const Align ParentAlign = MBB.getParent()->getAlignment();88 89 if (Alignment <= ParentAlign)90 return offsetToAlignment(Offset, Alignment);91 92 // The alignment of this MBB is larger than the function's alignment, so we93 // can't tell whether or not it will insert nops. Assume that it will.94 if (FirstImpreciseBlock < 0)95 FirstImpreciseBlock = MBB.getNumber();96 return Alignment.value() + offsetToAlignment(Offset, Alignment);97}98 99/// We need to be careful about the offset of the first block in the function100/// because it might not have the function's alignment. This happens because,101/// under the ELFv2 ABI, for functions which require a TOC pointer, we add a102/// two-instruction sequence to the start of the function.103/// Note: This needs to be synchronized with the check in104/// PPCLinuxAsmPrinter::EmitFunctionBodyStart.105static inline unsigned GetInitialOffset(MachineFunction &Fn) {106 unsigned InitialOffset = 0;107 if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&108 !Fn.getRegInfo().use_empty(PPC::X2))109 InitialOffset = 8;110 return InitialOffset;111}112 113/// Measure each MBB and compute a size for the entire function.114unsigned PPCBSel::ComputeBlockSizes(MachineFunction &Fn) {115 const PPCInstrInfo *TII =116 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());117 unsigned FuncSize = GetInitialOffset(Fn);118 119 for (MachineBasicBlock &MBB : Fn) {120 // The end of the previous block may have extra nops if this block has an121 // alignment requirement.122 if (MBB.getNumber() > 0) {123 unsigned AlignExtra = GetAlignmentAdjustment(MBB, FuncSize);124 125 auto &BS = BlockSizes[MBB.getNumber()-1];126 BS.first += AlignExtra;127 BS.second = AlignExtra;128 129 FuncSize += AlignExtra;130 }131 132 unsigned BlockSize = 0;133 unsigned UnalignedBytesRemaining = 0;134 for (MachineInstr &MI : MBB) {135 unsigned MINumBytes = TII->getInstSizeInBytes(MI);136 if (MI.isInlineAsm() && (FirstImpreciseBlock < 0))137 FirstImpreciseBlock = MBB.getNumber();138 if (TII->isPrefixed(MI.getOpcode())) {139 NumPrefixed++;140 141 // All 8 byte instructions may require alignment. Each 8 byte142 // instruction may be aligned by another 4 bytes.143 // This means that an 8 byte instruction may require 12 bytes144 // (8 for the instruction itself and 4 for the alignment nop).145 // This will happen if an 8 byte instruction can be aligned to 64 bytes146 // by only adding a 4 byte nop.147 // We don't know the alignment at this point in the code so we have to148 // adopt a more pessimistic approach. If an instruction may need149 // alignment we assume that it does need alignment and add 4 bytes to150 // it. As a result we may end up with more long branches than before151 // but we are in the safe position where if we need a long branch we152 // have one.153 // The if statement checks to make sure that two 8 byte instructions154 // are at least 64 bytes away from each other. It is not possible for155 // two instructions that both need alignment to be within 64 bytes of156 // each other.157 if (!UnalignedBytesRemaining) {158 BlockSize += 4;159 UnalignedBytesRemaining = 60;160 NumPrefixedAligned++;161 }162 }163 UnalignedBytesRemaining -= std::min(UnalignedBytesRemaining, MINumBytes);164 BlockSize += MINumBytes;165 }166 167 BlockSizes[MBB.getNumber()].first = BlockSize;168 FuncSize += BlockSize;169 }170 171 return FuncSize;172}173 174/// Modify the basic block align adjustment.175void PPCBSel::modifyAdjustment(MachineFunction &Fn) {176 unsigned Offset = GetInitialOffset(Fn);177 for (MachineBasicBlock &MBB : Fn) {178 if (MBB.getNumber() > 0) {179 auto &BS = BlockSizes[MBB.getNumber()-1];180 BS.first -= BS.second;181 Offset -= BS.second;182 183 unsigned AlignExtra = GetAlignmentAdjustment(MBB, Offset);184 185 BS.first += AlignExtra;186 BS.second = AlignExtra;187 188 Offset += AlignExtra;189 }190 191 Offset += BlockSizes[MBB.getNumber()].first;192 }193}194 195/// Determine the offset from the branch in Src block to the Dest block.196/// BrOffset is the offset of the branch instruction inside Src block.197int PPCBSel::computeBranchSize(MachineFunction &Fn,198 const MachineBasicBlock *Src,199 const MachineBasicBlock *Dest,200 unsigned BrOffset) {201 int BranchSize;202 Align MaxAlign = Align(4);203 bool NeedExtraAdjustment = false;204 if (Dest->getNumber() <= Src->getNumber()) {205 // If this is a backwards branch, the delta is the offset from the206 // start of this block to this branch, plus the sizes of all blocks207 // from this block to the dest.208 BranchSize = BrOffset;209 MaxAlign = std::max(MaxAlign, Src->getAlignment());210 211 int DestBlock = Dest->getNumber();212 BranchSize += BlockSizes[DestBlock].first;213 for (unsigned i = DestBlock+1, e = Src->getNumber(); i < e; ++i) {214 BranchSize += BlockSizes[i].first;215 MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());216 }217 218 NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&219 (DestBlock >= FirstImpreciseBlock);220 } else {221 // Otherwise, add the size of the blocks between this block and the222 // dest to the number of bytes left in this block.223 unsigned StartBlock = Src->getNumber();224 BranchSize = BlockSizes[StartBlock].first - BrOffset;225 226 MaxAlign = std::max(MaxAlign, Dest->getAlignment());227 for (unsigned i = StartBlock+1, e = Dest->getNumber(); i != e; ++i) {228 BranchSize += BlockSizes[i].first;229 MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());230 }231 232 NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&233 (Src->getNumber() >= FirstImpreciseBlock);234 }235 236 // We tend to over estimate code size due to large alignment and237 // inline assembly. Usually it causes larger computed branch offset.238 // But sometimes it may also causes smaller computed branch offset239 // than actual branch offset. If the offset is close to the limit of240 // encoding, it may cause problem at run time.241 // Following is a simplified example.242 //243 // actual estimated244 // address address245 // ...246 // bne Far 100 10c247 // .p2align 4248 // Near: 110 110249 // ...250 // Far: 8108 8108251 //252 // Actual offset: 0x8108 - 0x100 = 0x8008253 // Computed offset: 0x8108 - 0x10c = 0x7ffc254 //255 // This example also shows when we can get the largest gap between256 // estimated offset and actual offset. If there is an aligned block257 // ABB between branch and target, assume its alignment is <align>258 // bits. Now consider the accumulated function size FSIZE till the end259 // of previous block PBB. If the estimated FSIZE is multiple of260 // 2^<align>, we don't need any padding for the estimated address of261 // ABB. If actual FSIZE at the end of PBB is 4 bytes more than262 // multiple of 2^<align>, then we need (2^<align> - 4) bytes of263 // padding. It also means the actual branch offset is (2^<align> - 4)264 // larger than computed offset. Other actual FSIZE needs less padding265 // bytes, so causes smaller gap between actual and computed offset.266 //267 // On the other hand, if the inline asm or large alignment occurs268 // between the branch block and destination block, the estimated address269 // can be <delta> larger than actual address. If padding bytes are270 // needed for a later aligned block, the actual number of padding bytes271 // is at most <delta> more than estimated padding bytes. So the actual272 // aligned block address is less than or equal to the estimated aligned273 // block address. So the actual branch offset is less than or equal to274 // computed branch offset.275 //276 // The computed offset is at most ((1 << alignment) - 4) bytes smaller277 // than actual offset. So we add this number to the offset for safety.278 if (NeedExtraAdjustment)279 BranchSize += MaxAlign.value() - 4;280 281 return BranchSize;282}283 284bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {285 const PPCInstrInfo *TII =286 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());287 // Give the blocks of the function a dense, in-order, numbering.288 Fn.RenumberBlocks();289 BlockSizes.resize(Fn.getNumBlockIDs());290 FirstImpreciseBlock = -1;291 292 // Measure each MBB and compute a size for the entire function.293 unsigned FuncSize = ComputeBlockSizes(Fn);294 295 // If the entire function is smaller than the displacement of a branch field,296 // we know we don't need to shrink any branches in this function. This is a297 // common case.298 if (FuncSize < (1 << 15)) {299 BlockSizes.clear();300 return false;301 }302 303 // For each conditional branch, if the offset to its destination is larger304 // than the offset field allows, transform it into a long branch sequence305 // like this:306 // short branch:307 // bCC MBB308 // long branch:309 // b!CC $PC+8310 // b MBB311 //312 bool MadeChange = true;313 bool EverMadeChange = false;314 while (MadeChange) {315 // Iteratively expand branches until we reach a fixed point.316 MadeChange = false;317 318 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;319 ++MFI) {320 MachineBasicBlock &MBB = *MFI;321 unsigned MBBStartOffset = 0;322 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();323 I != E; ++I) {324 MachineBasicBlock *Dest = nullptr;325 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())326 Dest = I->getOperand(2).getMBB();327 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&328 !I->getOperand(1).isImm())329 Dest = I->getOperand(1).getMBB();330 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||331 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&332 !I->getOperand(0).isImm())333 Dest = I->getOperand(0).getMBB();334 335 if (!Dest) {336 MBBStartOffset += TII->getInstSizeInBytes(*I);337 continue;338 }339 340 // Determine the offset from the current branch to the destination341 // block.342 int BranchSize = computeBranchSize(Fn, &MBB, Dest, MBBStartOffset);343 344 // If this branch is in range, ignore it.345 if (isInt<16>(BranchSize)) {346 MBBStartOffset += 4;347 continue;348 }349 350 // Otherwise, we have to expand it to a long branch.351 MachineInstr &OldBranch = *I;352 DebugLoc dl = OldBranch.getDebugLoc();353 354 if (I->getOpcode() == PPC::BCC) {355 // The BCC operands are:356 // 0. PPC branch predicate357 // 1. CR register358 // 2. Target MBB359 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();360 Register CRReg = I->getOperand(1).getReg();361 362 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.363 BuildMI(MBB, I, dl, TII->get(PPC::BCC))364 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);365 } else if (I->getOpcode() == PPC::BC) {366 Register CRBit = I->getOperand(0).getReg();367 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);368 } else if (I->getOpcode() == PPC::BCn) {369 Register CRBit = I->getOperand(0).getReg();370 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);371 } else if (I->getOpcode() == PPC::BDNZ) {372 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);373 } else if (I->getOpcode() == PPC::BDNZ8) {374 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);375 } else if (I->getOpcode() == PPC::BDZ) {376 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);377 } else if (I->getOpcode() == PPC::BDZ8) {378 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);379 } else {380 llvm_unreachable("Unhandled branch type!");381 }382 383 // Uncond branch to the real destination.384 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);385 386 // Remove the old branch from the function.387 OldBranch.eraseFromParent();388 389 // Remember that this instruction is 8-bytes, increase the size of the390 // block by 4, remember to iterate.391 BlockSizes[MBB.getNumber()].first += 4;392 MBBStartOffset += 8;393 ++NumExpanded;394 MadeChange = true;395 }396 }397 398 if (MadeChange) {399 // If we're going to iterate again, make sure we've updated our400 // padding-based contributions to the block sizes.401 modifyAdjustment(Fn);402 }403 404 EverMadeChange |= MadeChange;405 }406 407 BlockSizes.clear();408 return EverMadeChange;409}410