LLVM  16.0.0git
PPCBranchSelector.cpp
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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-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file contains a pass that scans a machine function to determine which
10 // conditional branches need more than 16 bits of displacement to reach their
11 // target basic block. It does this in two passes; a calculation of basic block
12 // positions pass, and a branch pseudo op to machine branch opcode pass. This
13 // pass should be run last, just before the assembly printer.
14 //
15 //===----------------------------------------------------------------------===//
16 
18 #include "PPC.h"
19 #include "PPCInstrBuilder.h"
20 #include "PPCInstrInfo.h"
21 #include "PPCSubtarget.h"
22 #include "llvm/ADT/Statistic.h"
28 #include <algorithm>
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "ppc-branch-select"
32 
33 STATISTIC(NumExpanded, "Number of branches expanded to long format");
34 STATISTIC(NumPrefixed, "Number of prefixed instructions");
35 STATISTIC(NumPrefixedAligned,
36  "Number of prefixed instructions that have been aligned");
37 
38 namespace {
39  struct PPCBSel : public MachineFunctionPass {
40  static char ID;
41  PPCBSel() : MachineFunctionPass(ID) {
43  }
44 
45  // The sizes of the basic blocks in the function (the first
46  // element of the pair); the second element of the pair is the amount of the
47  // size that is due to potential padding.
48  std::vector<std::pair<unsigned, unsigned>> BlockSizes;
49 
50  // The first block number which has imprecise instruction address.
51  int FirstImpreciseBlock = -1;
52 
53  unsigned GetAlignmentAdjustment(MachineBasicBlock &MBB, unsigned Offset);
54  unsigned ComputeBlockSizes(MachineFunction &Fn);
55  void modifyAdjustment(MachineFunction &Fn);
56  int computeBranchSize(MachineFunction &Fn,
57  const MachineBasicBlock *Src,
58  const MachineBasicBlock *Dest,
59  unsigned BrOffset);
60 
61  bool runOnMachineFunction(MachineFunction &Fn) override;
62 
63  MachineFunctionProperties getRequiredProperties() const override {
66  }
67 
68  StringRef getPassName() const override { return "PowerPC Branch Selector"; }
69  };
70  char PPCBSel::ID = 0;
71 }
72 
73 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
74  false, false)
75 
76 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
77 /// Pass
78 ///
80  return new PPCBSel();
81 }
82 
83 /// In order to make MBB aligned, we need to add an adjustment value to the
84 /// original Offset.
85 unsigned PPCBSel::GetAlignmentAdjustment(MachineBasicBlock &MBB,
86  unsigned Offset) {
87  const Align Alignment = MBB.getAlignment();
88  if (Alignment == Align(1))
89  return 0;
90 
91  const Align ParentAlign = MBB.getParent()->getAlignment();
92 
93  if (Alignment <= ParentAlign)
94  return offsetToAlignment(Offset, Alignment);
95 
96  // The alignment of this MBB is larger than the function's alignment, so we
97  // can't tell whether or not it will insert nops. Assume that it will.
98  if (FirstImpreciseBlock < 0)
99  FirstImpreciseBlock = MBB.getNumber();
100  return Alignment.value() + offsetToAlignment(Offset, Alignment);
101 }
102 
103 /// We need to be careful about the offset of the first block in the function
104 /// because it might not have the function's alignment. This happens because,
105 /// under the ELFv2 ABI, for functions which require a TOC pointer, we add a
106 /// two-instruction sequence to the start of the function.
107 /// Note: This needs to be synchronized with the check in
108 /// PPCLinuxAsmPrinter::EmitFunctionBodyStart.
109 static inline unsigned GetInitialOffset(MachineFunction &Fn) {
110  unsigned InitialOffset = 0;
111  if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
112  !Fn.getRegInfo().use_empty(PPC::X2))
113  InitialOffset = 8;
114  return InitialOffset;
115 }
116 
117 /// Measure each MBB and compute a size for the entire function.
118 unsigned PPCBSel::ComputeBlockSizes(MachineFunction &Fn) {
119  const PPCInstrInfo *TII =
120  static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
121  unsigned FuncSize = GetInitialOffset(Fn);
122 
123  for (MachineBasicBlock &MBB : Fn) {
124  // The end of the previous block may have extra nops if this block has an
125  // alignment requirement.
126  if (MBB.getNumber() > 0) {
127  unsigned AlignExtra = GetAlignmentAdjustment(MBB, FuncSize);
128 
129  auto &BS = BlockSizes[MBB.getNumber()-1];
130  BS.first += AlignExtra;
131  BS.second = AlignExtra;
132 
133  FuncSize += AlignExtra;
134  }
135 
136  unsigned BlockSize = 0;
137  unsigned UnalignedBytesRemaining = 0;
138  for (MachineInstr &MI : MBB) {
139  unsigned MINumBytes = TII->getInstSizeInBytes(MI);
140  if (MI.isInlineAsm() && (FirstImpreciseBlock < 0))
141  FirstImpreciseBlock = MBB.getNumber();
142  if (TII->isPrefixed(MI.getOpcode())) {
143  NumPrefixed++;
144 
145  // All 8 byte instructions may require alignment. Each 8 byte
146  // instruction may be aligned by another 4 bytes.
147  // This means that an 8 byte instruction may require 12 bytes
148  // (8 for the instruction itself and 4 for the alignment nop).
149  // This will happen if an 8 byte instruction can be aligned to 64 bytes
150  // by only adding a 4 byte nop.
151  // We don't know the alignment at this point in the code so we have to
152  // adopt a more pessimistic approach. If an instruction may need
153  // alignment we assume that it does need alignment and add 4 bytes to
154  // it. As a result we may end up with more long branches than before
155  // but we are in the safe position where if we need a long branch we
156  // have one.
157  // The if statement checks to make sure that two 8 byte instructions
158  // are at least 64 bytes away from each other. It is not possible for
159  // two instructions that both need alignment to be within 64 bytes of
160  // each other.
161  if (!UnalignedBytesRemaining) {
162  BlockSize += 4;
163  UnalignedBytesRemaining = 60;
164  NumPrefixedAligned++;
165  }
166  }
167  UnalignedBytesRemaining -= std::min(UnalignedBytesRemaining, MINumBytes);
168  BlockSize += MINumBytes;
169  }
170 
171  BlockSizes[MBB.getNumber()].first = BlockSize;
172  FuncSize += BlockSize;
173  }
174 
175  return FuncSize;
176 }
177 
178 /// Modify the basic block align adjustment.
179 void PPCBSel::modifyAdjustment(MachineFunction &Fn) {
180  unsigned Offset = GetInitialOffset(Fn);
181  for (MachineBasicBlock &MBB : Fn) {
182  if (MBB.getNumber() > 0) {
183  auto &BS = BlockSizes[MBB.getNumber()-1];
184  BS.first -= BS.second;
185  Offset -= BS.second;
186 
187  unsigned AlignExtra = GetAlignmentAdjustment(MBB, Offset);
188 
189  BS.first += AlignExtra;
190  BS.second = AlignExtra;
191 
192  Offset += AlignExtra;
193  }
194 
195  Offset += BlockSizes[MBB.getNumber()].first;
196  }
197 }
198 
199 /// Determine the offset from the branch in Src block to the Dest block.
200 /// BrOffset is the offset of the branch instruction inside Src block.
201 int PPCBSel::computeBranchSize(MachineFunction &Fn,
202  const MachineBasicBlock *Src,
203  const MachineBasicBlock *Dest,
204  unsigned BrOffset) {
205  int BranchSize;
206  Align MaxAlign = Align(4);
207  bool NeedExtraAdjustment = false;
208  if (Dest->getNumber() <= Src->getNumber()) {
209  // If this is a backwards branch, the delta is the offset from the
210  // start of this block to this branch, plus the sizes of all blocks
211  // from this block to the dest.
212  BranchSize = BrOffset;
213  MaxAlign = std::max(MaxAlign, Src->getAlignment());
214 
215  int DestBlock = Dest->getNumber();
216  BranchSize += BlockSizes[DestBlock].first;
217  for (unsigned i = DestBlock+1, e = Src->getNumber(); i < e; ++i) {
218  BranchSize += BlockSizes[i].first;
219  MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());
220  }
221 
222  NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&
223  (DestBlock >= FirstImpreciseBlock);
224  } else {
225  // Otherwise, add the size of the blocks between this block and the
226  // dest to the number of bytes left in this block.
227  unsigned StartBlock = Src->getNumber();
228  BranchSize = BlockSizes[StartBlock].first - BrOffset;
229 
230  MaxAlign = std::max(MaxAlign, Dest->getAlignment());
231  for (unsigned i = StartBlock+1, e = Dest->getNumber(); i != e; ++i) {
232  BranchSize += BlockSizes[i].first;
233  MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());
234  }
235 
236  NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&
237  (Src->getNumber() >= FirstImpreciseBlock);
238  }
239 
240  // We tend to over estimate code size due to large alignment and
241  // inline assembly. Usually it causes larger computed branch offset.
242  // But sometimes it may also causes smaller computed branch offset
243  // than actual branch offset. If the offset is close to the limit of
244  // encoding, it may cause problem at run time.
245  // Following is a simplified example.
246  //
247  // actual estimated
248  // address address
249  // ...
250  // bne Far 100 10c
251  // .p2align 4
252  // Near: 110 110
253  // ...
254  // Far: 8108 8108
255  //
256  // Actual offset: 0x8108 - 0x100 = 0x8008
257  // Computed offset: 0x8108 - 0x10c = 0x7ffc
258  //
259  // This example also shows when we can get the largest gap between
260  // estimated offset and actual offset. If there is an aligned block
261  // ABB between branch and target, assume its alignment is <align>
262  // bits. Now consider the accumulated function size FSIZE till the end
263  // of previous block PBB. If the estimated FSIZE is multiple of
264  // 2^<align>, we don't need any padding for the estimated address of
265  // ABB. If actual FSIZE at the end of PBB is 4 bytes more than
266  // multiple of 2^<align>, then we need (2^<align> - 4) bytes of
267  // padding. It also means the actual branch offset is (2^<align> - 4)
268  // larger than computed offset. Other actual FSIZE needs less padding
269  // bytes, so causes smaller gap between actual and computed offset.
270  //
271  // On the other hand, if the inline asm or large alignment occurs
272  // between the branch block and destination block, the estimated address
273  // can be <delta> larger than actual address. If padding bytes are
274  // needed for a later aligned block, the actual number of padding bytes
275  // is at most <delta> more than estimated padding bytes. So the actual
276  // aligned block address is less than or equal to the estimated aligned
277  // block address. So the actual branch offset is less than or equal to
278  // computed branch offset.
279  //
280  // The computed offset is at most ((1 << alignment) - 4) bytes smaller
281  // than actual offset. So we add this number to the offset for safety.
282  if (NeedExtraAdjustment)
283  BranchSize += MaxAlign.value() - 4;
284 
285  return BranchSize;
286 }
287 
288 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
289  const PPCInstrInfo *TII =
290  static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
291  // Give the blocks of the function a dense, in-order, numbering.
292  Fn.RenumberBlocks();
293  BlockSizes.resize(Fn.getNumBlockIDs());
294  FirstImpreciseBlock = -1;
295 
296  // Measure each MBB and compute a size for the entire function.
297  unsigned FuncSize = ComputeBlockSizes(Fn);
298 
299  // If the entire function is smaller than the displacement of a branch field,
300  // we know we don't need to shrink any branches in this function. This is a
301  // common case.
302  if (FuncSize < (1 << 15)) {
303  BlockSizes.clear();
304  return false;
305  }
306 
307  // For each conditional branch, if the offset to its destination is larger
308  // than the offset field allows, transform it into a long branch sequence
309  // like this:
310  // short branch:
311  // bCC MBB
312  // long branch:
313  // b!CC $PC+8
314  // b MBB
315  //
316  bool MadeChange = true;
317  bool EverMadeChange = false;
318  while (MadeChange) {
319  // Iteratively expand branches until we reach a fixed point.
320  MadeChange = false;
321 
322  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
323  ++MFI) {
324  MachineBasicBlock &MBB = *MFI;
325  unsigned MBBStartOffset = 0;
327  I != E; ++I) {
328  MachineBasicBlock *Dest = nullptr;
329  if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
330  Dest = I->getOperand(2).getMBB();
331  else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
332  !I->getOperand(1).isImm())
333  Dest = I->getOperand(1).getMBB();
334  else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
335  I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
336  !I->getOperand(0).isImm())
337  Dest = I->getOperand(0).getMBB();
338 
339  if (!Dest) {
340  MBBStartOffset += TII->getInstSizeInBytes(*I);
341  continue;
342  }
343 
344  // Determine the offset from the current branch to the destination
345  // block.
346  int BranchSize = computeBranchSize(Fn, &MBB, Dest, MBBStartOffset);
347 
348  // If this branch is in range, ignore it.
349  if (isInt<16>(BranchSize)) {
350  MBBStartOffset += 4;
351  continue;
352  }
353 
354  // Otherwise, we have to expand it to a long branch.
355  MachineInstr &OldBranch = *I;
356  DebugLoc dl = OldBranch.getDebugLoc();
357 
358  if (I->getOpcode() == PPC::BCC) {
359  // The BCC operands are:
360  // 0. PPC branch predicate
361  // 1. CR register
362  // 2. Target MBB
363  PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
364  Register CRReg = I->getOperand(1).getReg();
365 
366  // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
367  BuildMI(MBB, I, dl, TII->get(PPC::BCC))
368  .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
369  } else if (I->getOpcode() == PPC::BC) {
370  Register CRBit = I->getOperand(0).getReg();
371  BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
372  } else if (I->getOpcode() == PPC::BCn) {
373  Register CRBit = I->getOperand(0).getReg();
374  BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
375  } else if (I->getOpcode() == PPC::BDNZ) {
376  BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
377  } else if (I->getOpcode() == PPC::BDNZ8) {
378  BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
379  } else if (I->getOpcode() == PPC::BDZ) {
380  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
381  } else if (I->getOpcode() == PPC::BDZ8) {
382  BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
383  } else {
384  llvm_unreachable("Unhandled branch type!");
385  }
386 
387  // Uncond branch to the real destination.
388  I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
389 
390  // Remove the old branch from the function.
391  OldBranch.eraseFromParent();
392 
393  // Remember that this instruction is 8-bytes, increase the size of the
394  // block by 4, remember to iterate.
395  BlockSizes[MBB.getNumber()].first += 4;
396  MBBStartOffset += 8;
397  ++NumExpanded;
398  MadeChange = true;
399  }
400  }
401 
402  if (MadeChange) {
403  // If we're going to iterate again, make sure we've updated our
404  // padding-based contributions to the block sizes.
405  modifyAdjustment(Fn);
406  }
407 
408  EverMadeChange |= MadeChange;
409  }
410 
411  BlockSizes.clear();
412  return EverMadeChange;
413 }
i
i
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